fleet oceanographic and acoustic
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sonarTRANSCRIPT
Prepared under the authority ofCommanderNaval Meteorology andOceanography Command
Naval Oceanographic OfficeStennis Space Reference PublicationCenter RP 33MS 39522-5001 July 1986, Revised April 1999
RP 33
Distribution limited to DOD and DOD contractors only;administrative/operational use; April 1999. Otherrequests for this document shall be referred toCommanding Officer, Naval Oceanographic Office.
FLEET OCEANOGRAPHIC AND ACOUSTICREFERENCE MANUAL
Littoral - defined as the region whichhorizontally encompasses the land/watermassinterface from fifty (50) statute miles ashore totwo hundred (200) nautical miles at sea;extends vertically from the bottom of theocean to the top of the atmosphere and fromthe land surface to the top of the atmosphere.
FOREWORD
This Naval Oceanographic Office (NAVOCEANO) publication supercedes ReferencePublication RP 33, Fleet Oceanographic and Acoustic Reference Manual, dated June 1992. It isa reference manual covering the basic acoustic, geologic, and physical structure of the deep andshallow ocean environment.
It is designed to provide a basic knowledge of the ocean environment for fleet users sothat they may effectively apply Naval Meteorology and Oceanography Command instructions,procedures, and products.
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First edition in July 1986 with revisions in March 1989, June 1992, and April 1999. The inclusion of names of any specificproduct, commodity, or service in this publication is for information purposes only and does not imply endorsement by the Navy,NAVOCEANO, or COMNAVMETOCCOM.
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13. ABSTRACT (Maximum 200 words)
This publication is designed for use by the meteorology and oceanography (METOC) community and Fleet operators tofamiliarize themselves with acoustic and oceanographic information for application to naval operations. Specific subjects arecovered by chapter with references, definitions, and acronyms provided in appendices.
14. SUBJECT TERMS
Acoustics, Underwater Sound, Sound Speed Profile, Propagation Loss Curve, Ambient Noise,Topographic Noise Stripping, Submerged Convergence Zone, Fronts, Eddies, Marine Geology,Bathythermograph, Figure of Merit, Wind, Waves, Cutoff Frequency, Wavelength, Secondary SoundChannel, USW, ASW, Half Channel, Diffraction, Surface Duct, Bioluminescence, Sound Intensity,Sonar, Littoral Water, Shallow Water.
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Standard Form 298 (Rev. 2-89)Prescribed by ANSI Std. 239-18
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iii
Table of Contents
Page
Foreword
Report Documentation Page
Chapter 1 The Nature of Underwater Sound ............................................................1
1.1 Elementary Aspects of Sound..................................................................1
1.1.1 Wave Motion............................................................................................1
1.1.2 Acoustic Energy vs. Electromagnetic Energy ..........................................2
1.1.3 Speed of Sound .......................................................................................2
1.1.4 Refraction ................................................................................................4
1.1.4.1 Ray Paths ................................................................................................4
1.1.4.2 Snell's Law...............................................................................................5
1.1.4.3 Sound-Speed Gradient ............................................................................6
1.1.4.3.1 Positive Sound-Speed Gradient...............................................................7
1.1.4.3.2 Negative Sound-Speed Gradient .............................................................7
1.1.4.3.3 Isospeed ..................................................................................................7
1.1.4.3.4 Acoustic Reciprocity ................................................................................8
Chapter 2 Propagation Loss.....................................................................................9
2.1 Introduction..............................................................................................9
2.2 Spreading Loss......................................................................................10
2.2.1 Spherical Spreading ..............................................................................10
2.2.2 Cylindrical Spreading.............................................................................11
2.2.3 Dipolar Spreading ..................................................................................12
iv
2.3 Absorption..............................................................................................13
2.4 Scattering (Reverberation).....................................................................14
2.4.1 Surface Reverberation...........................................................................14
2.4.2 Volume Reverberation ..........................................................................15
2.4.3 Bottom Reverberation............................................................................16
2.5 Bottom Loss...........................................................................................16
2.5.1 Bottom Interaction..................................................................................16
2.5.2 Factors of Frequency and Grazing Angle ..............................................17
2.5.3 Bottom-Loss Data Bases .......................................................................17
Chapter 3 Background Noise .................................................................................19
3.1 General ..................................................................................................19
3.2 Ambient Noise .......................................................................................19
3.2.1 Surface-Ship Traffic Noise.....................................................................19
3.2.2 Sea-State Noise.....................................................................................20
3.2.2.1 Wind-Generated Noise ..........................................................................24
3.2.2.2 Sea-State Noise Levels .........................................................................24
3.2.3 Other Ambient-Noise Sources ...............................................................24
3.2.3.1 Precipitation ...........................................................................................24
3.2.3.2 Ice..........................................................................................................24
3.2.3.3 Biologics ................................................................................................24
3.2.3.3.1 Marine Mammals ...................................................................................25
3.3 Self-Noise ..............................................................................................28
3.3.1 Machinery Noise ....................................................................................28
v
3.3.2 Propeller Noise ......................................................................................28
3.3.3 Hydrodynamic Noise..............................................................................28
3.3.4 Aircraft Noise .........................................................................................29
3.3.5 Circuit Noise ..........................................................................................29
Chapter 4 Marine Geology and Bathymetry ...........................................................30
4.1 Introduction............................................................................................30
4.2 Bottom Topography ...............................................................................30
4.2.1 Continental Shelf ...................................................................................31
4.2.2 Continental Slope ..................................................................................32
4.2.3 Continental Rise ....................................................................................32
4.2.4 Ocean Basin ..........................................................................................32
4.2.5 Submarine Ridges .................................................................................32
4.2.6 Seamounts.............................................................................................33
4.2.7 Abyssal Hills and Oceanic Rises ...........................................................33
4.2.8 Trenches................................................................................................33
4.3 Bottom Composition ..............................................................................34
4.4 Bathymetry.............................................................................................35
4.4.1 Corrected Bottom Depth ........................................................................35
4.4.2 Active Sensors.......................................................................................37
4.4.3 Convergence Zones ..............................................................................37
4.4.4 Bottom Bounce ......................................................................................37
4.4.5 Bathymetric Interference........................................................................37
Chapter 5 Water Masses, Currents, and Basic Oceanographic Analyses..............39
vi
5.1 General ..................................................................................................39
5.2 Sea-Surface Temperature (SST) Charts ...............................................39
5.3 Water Masses........................................................................................39
5.3.1 Ocean Fronts .........................................................................................42
5.3.1.1 Typical Location of World Fronts ...........................................................43
5.3.1.2 Acoustic Effects from Fronts ..................................................................47
5.3.1.3 Determining Frontal Locations from Satellite Data.................................48
5.3.1.4 Frontal Gradients ...................................................................................49
5.3.2 Eddies....................................................................................................49
5.3.2.1 Warm Eddies .........................................................................................51
5.3.2.2 Cold Eddies ...........................................................................................51
5.3.3 Fronts and Eddies in Shallow Water......................................................54
5.3.4 Internal Waves.......................................................................................56
5.4 Currents .................................................................................................56
5.5 Variability of the Ocean Environment.....................................................60
5.5.1 General ..................................................................................................60
5.5.2 Scale of Variability .................................................................................60
5.5.3 Detection-Range Calculations ...............................................................61
Chapter 6 Bathythermograph Observations ...........................................................62
6.1 General ..................................................................................................62
6.2 Expendable Bathythermographs............................................................63
6.3 Bathythermograph Encoding Procedures ..............................................63
6.3.1 Bathythermograph Log ..........................................................................64
vii
6.3.2 Quality Control of XBT Data...................................................................64
6.4 Bathythermograph Interpretation ...........................................................65
6.4.1 Mixed-Layer Depth (MLD)......................................................................66
6.4.1.1 MLD Computation..................................................................................66
6.4.2 Temperature Gradient ...........................................................................68
6.4.3 Sound Channels ....................................................................................69
6.4.4 Convergence-Zone (CZ) Prediction .......................................................69
Chapter 7 Environmental Effects Upon Sound Propagation in the Deep Ocean ....70
7.1 Depth and Seasonal Effects ..................................................................70
7.1.1 SSPs......................................................................................................72
7.1.1.1 SSP Construction ..................................................................................72
7.1.2 Horizontal Sound-Speed Gradients .......................................................73
7.2 Sound Propagation Paths......................................................................74
7.2.1 Direct Path .............................................................................................74
7.2.2 Surface Duct ..........................................................................................75
7.2.2.1 Shadow Zone.........................................................................................75
7.2.2.2 SLD........................................................................................................76
7.2.2.3 Gradient in the Layer (In-Layer Gradient) ..............................................76
7.2.2.4 Low-Frequency Cutoff ...........................................................................76
7.2.2.5 Wind-Wave Effects on Layer Depth.......................................................78
7.2.2.6 Seasonal Effects on SLD.......................................................................79
7.2.2.7 Gradient Below the Layer ......................................................................80
7.2.3 Half Channel ..........................................................................................80
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7.2.3.1 Arctic and Half-Channel Propagation.....................................................80
7.2.3.2 Propagation in Arctic Waters vs. Ice-Free Waters .................................81
7.2.4 Sound Channels ....................................................................................82
7.2.4.1 Secondary Sound Channels ..................................................................82
7.2.4.1.1 Locations ...............................................................................................83
7.2.4.1.2 Cutoff Frequency ...................................................................................85
7.2.4.2 The Deep Sound Channel .....................................................................85
7.2.5 Bottom Bounce ......................................................................................88
7.2.6 Convergence Zones ..............................................................................89
7.2.6.1 Convergence Zone Range.....................................................................91
7.2.6.2 Convergence Zone Width ......................................................................92
Chapter 8 Environmental Effects Upon Sound Propagation in Shallow Water .......93
8.1 Introduction............................................................................................93
8.2 Environmental Factors...........................................................................93
8.2.1 Sea Surface Temperature (SST) ...........................................................93
8.2.2 Salinity ...................................................................................................93
8.2.3 Layer Depths .........................................................................................94
8.2.4 Sound Channels ....................................................................................94
8.2.5 Water Depth...........................................................................................94
8.2.6 Bottom ...................................................................................................94
8.2.7 Shallow-Water Acoustics .......................................................................94
8.2.8 Shallow-Water Ambient Noise ...............................................................94
8.2.9 Sea-Ice Shallow-Water Ambient Noise..................................................95
ix
8.2.10 Biological Noise .....................................................................................96
8.2.11 Environmental Factor Variability ............................................................96
8.3 Environmental Characteristics of Shallow Water ...................................97
8.4 Propagation Paths .................................................................................98
8.5 Tactical Considerations and Search Planning .......................................98
8.5.1 Slope Enhancement ..............................................................................98
8.5.2 Topographic Shading...........................................................................100
8.5.3 Topographic Noise Stripping (TNS) .....................................................102
8.6 Sensors ...............................................................................................105
8.7 Acoustic Applications...........................................................................105
Chapter 9 Passive Sonar......................................................................................106
9.1 General ................................................................................................106
9.2 Passive-Sonar Equation ......................................................................106
9.2.1 Signal Excess (SE) ..............................................................................107
9.2.2 Source Level (SL or LS) .......................................................................107
9.2.3 Propagation Loss (PL) .........................................................................108
9.2.4 Noise Level (NL or LN) .........................................................................108
9.2.5 Total Background Noise (LE or LE) ......................................................108
9.2.5.1 Directivity Index (DI or NDI) ..................................................................108
9.2.6 Recognition Differential (RD or NRD) ....................................................109
9.3 Figure of Merit (FOM) ..........................................................................109
9.4 Passive Sonar Performance Prediction ...............................................110
9.4.1 Variability of FOM Parameters.............................................................110
x
9.4.2 Probability of Detection vs. Range.......................................................110
Chapter 10 Active Sonar ........................................................................................112
10.1 General ................................................................................................112
10.2 Active-Sonar Equations .......................................................................112
10.2.1 Noise-Limited Case .............................................................................112
10.2.2 Reverberation-Limited Case ................................................................113
10.3 Active-Sonar Equation Parameters......................................................114
10.3.1 Signal Excess (SE) ..............................................................................114
10.3.2 Recognition Differential (RD or NRD) ....................................................114
10.3.3 Source Level (SL or LS) .......................................................................114
10.3.4 Target Strength (TS)............................................................................115
10.3.5 Noise Level (NL or LN) .........................................................................115
10.3.6 Propagation Loss (PL) .........................................................................116
10.3.7 Receiver Directivity Index (DI or NDI) ...................................................116
10.3.8 Reverberation Level (RL).....................................................................116
10.4 Active-Sonar Performance Prediction..................................................116
Appendices
A. Glossary of Terms, Acronyms, and Abbreviations ...............................118
B. Sound Levels .......................................................................................134
C. Optical Oceanography .........................................................................145
D. Bioluminescence..................................................................................158
E. Tactical Oceanography Reference Packet ..........................................160
F. FOM Terminology ................................................................................190
xi
R. References ..........................................................................................191
Distribution List ....................................................................................194
xii
List of Figures
Figure Page
1-1 Compressional Wave Train..............................................................................1
1-2 Surface Duct, Bottom Bounce, and Convergence Zone Ray Trace(Full-Path and Near-Surface Illustrations)........................................................4
1-3 Secondary Sound Channel, Bottom Bounce, and ConvergenceZone Ray Trace (Full-Path and Near-Surface Illustrations) .............................5
1-4 Snell's Law–Two Layers ..................................................................................5
1-5 Snell's Law–Multiple Layers.............................................................................6
1-6 Positive Sound-Speed Gradient.......................................................................6
1-7 Negative Sound-Speed Gradient .....................................................................7
1-8 Isospeed (Straight-Line) Gradient....................................................................7
1-9 Acoustic Reciprocity (Homogenous Ocean) ....................................................8
2-1 Spherical-Spreading Loss (Loss = 20 Log R) ................................................11
2-2 Cylindrical Spreading Loss (Loss = 10 Log R)...............................................12
2-3 Dipolar-Spreading Loss (Loss = 40 Log R)....................................................12
2-4 Comparison of Spreading Losses..................................................................13
2-5 Surface Reverberation...................................................................................14
2-6 Volume Reverberation ...................................................................................15
2-7 Volume Scattering Strength vs. Depth and Time ...........................................15
2-8 Energy Partition Due to Acoustic-Wave Interaction with Bottom....................17
2-9 Smooth Curves of Bottom Backscattering Strength vs. Grazing Anglefor Various Bottom Types ..............................................................................18
3-1 Ambient-Noise Levels–Traffic and Sea Height (Modified Wenz Curves) .......21
xiii
3-2 Potential Whale Sonar Targets (Western North Atlantic) ...............................27
4-1 Nomenclature of Undersea Geophysical Features ........................................31
4-2 Corrections to Chart Depth or Echo-Sounder Depth to ObtainTrue Depth in the Pacific................................................................................36
4-3 Bathymetric Interference................................................................................38
5-1 Sea-Surface Temperatures from the FLENUMMETOCCEN OTIS 4.0Analysis for the Gulf Stream Region..............................................................40
5-2 Temperature at 400 m Depth from the FLENUMMETOCCEN OTIS 4.0Analysis for the Gulf Stream Region..............................................................40
5-3 Major Ocean Regions of the Northern Hemisphere .......................................41
5-4 Mean Positions of Ocean Fronts in the Atlantic Ocean..................................43
5-5 Mean Positions of Ocean Fronts in the Pacific Ocean...................................45
5-6 TIROS-N Satellite Infrared (IR) Image ...........................................................48
5-7 Formation of Warm and Cold Eddies from the Gulf Stream...........................50
5-8 Vertical Cross-Section of a Warm Eddy.........................................................52
5-9 Vertical Cross-Section of a Cold Eddy...........................................................53
5-10 Observed Sound-Speed Profiles Across the Polar Front...............................55
5-11 General Surface Circulation, Mediterranean Sea and Black Sea,January through December ...........................................................................57
5-12 Averaged Worldwide Currents, Winter (January, February, March) ..............58
5-13 Averaged Worldwide Currents, Summer (July, August, September) .............59
6-1 Sample XBT Recorder Trace.........................................................................63
6-2 Sample Bathythermograph Log .....................................................................65
6-3 Mixed Layer at Surface (Depth = 0) ...............................................................67
6-4 Mixed Layer at Depth–Example A .................................................................67
xiv
6-5 Mixed Layer at Depth–Example B .................................................................68
7-1 Basic Temperature and Sound-Speed Structure of the Deep Ocean ............71
7-2 Sound-Speed Profile Variations.....................................................................72
7-3 Sound-Speed Nomogram (35‰ Salinity).......................................................73
7-4 Salinity Correction to Sound Speed ...............................................................73
7-5 Horizontal Gradient–Sonar-Bearing Area ......................................................74
7-6 Direct Path Propagation Path ........................................................................75
7-7 Surface Duct Propagation Path with Limiting Rays and Shadow Zone..........75
7-8 Layer-Depth Surface Effect upon Bounced Sound Rays ...............................76
7-9 Surface Duct Cutoff Frequency Nomograph ..................................................78
7-10 Wind-Wave Mixing Action Sequence.............................................................79
7-11 Examples of Below-Layer Negative Gradient Variations ...............................80
7-12 Half-Channel Propagation Path .....................................................................81
7-13 Sound Channel Description ...........................................................................82
7-14 Secondary Sound-Channel Properties ..........................................................83
7-15 Worldwide Locations of Secondary Sound Channels ....................................84
7-16 Sound Channel Low-Frequency Cutoff Graph ...............................................86
7-17 Deep Sound Channel, as Displayed on Geophysical Fleet MissionProgram Library (GFMPL 8.0) .......................................................................87
7-18 Sound-Speed Profile, DSC, and Critical Depth..............................................88
7-19 Bottom Bounce Multipaths .............................................................................89
7-20a Convergence Zone (CZ) Propagation and Terminology ................................90
7-20b Convergence Zone Propagation Path, Undistorted Scale .............................90
7-21 Probability of Convergence Zone (CZ) Occurrence .......................................91
xv
8-1 Variations of Ambient Noise Near Compact Ice Edge UnderSea State 2 Conditions ..................................................................................95
8-2 Upslope Enhancement ..................................................................................99
8-3 Downslope Enhancement ..............................................................................99
8-4 Topographic Shading...................................................................................100
8-5 No Topographic Shading with Seamount at One CZ Range........................101
8-6 Topographic Shading with Seamount at One-Half CZ Range......................101
8-7 Topographic Noise Stripping........................................................................102
8-8 In-Layer Source and Critical Depth..............................................................103
8-9 Below-Layer Source, Conjugate Depth, and Resultant Depth Excess.........103
8-10 Procedure for Determining TNS Region ......................................................104
9-1 Signal Excess Probability-of-Detection Curve..............................................111
10-1 Aspect Variation of Submarine Target Strength...........................................115
10-2 Probability of Detection for Various Values of Signal Excess ......................117
B-1 Nomogram for Combining Spectrum Levels ................................................139
B-2 Bandwidth Conversion Curves.....................................................................142
B-3 Ideal Continuous Noise................................................................................143
B-4 Noise Containing Discrete Frequencies.......................................................144
C-1 Standard Relative Luminosity, or Visibility, Curve andLuminous Efficiency.....................................................................................146
C-2 Reflection and Refraction of a Linearly Polarized Light Wave with itsElectric Vector Parallel to the Plane of Incidence ........................................148
C-3 Reflectance as a Function of Angle of Incidence .........................................149
C-4 Angle of Incidence and Fraction of Light Refracted into Water as aFunction of φ2 ..............................................................................................150
xvi
C-5 Volume-Attenuation Coefficient of Typical Estuary, Coastal, and ClearOceanic Water Compared with that of Distilled Water .................................150
C-6 Volume-Attenuation Coefficient ∝ and Attenuation Length L in theVisible Spectrum for Distilled Water.............................................................151
C-7 Approximate Illumination as a Function of Depth for Several NaturalLight Sources...............................................................................................152
C-8 Geometry and Terms Used in Computing Apparent Target Contrast ..........155
C-9 Contrast as a Function of Viewing Distance for Black and White ObjectsWhen Viewed Downward, Upward, and Horizontally Against AmbientBackground Radiance .................................................................................156
C-10 Apparent Contrast of Black Marks on Diffuse White Target WhenViewed from Different Directions .................................................................157
xvii
List of Tables
Table Page
1-1 Frequency vs. Wavelength for a Sound Speed of1,500 m/sec (4,921 ft/sec) ...............................................................................2
3-1 Wind and Sea State Descriptions ..................................................................23
3-2 Characteristics of Large Whales Occurring in the Western North Atlantic .....26
5-1 Names of Ocean Fronts in the Atlantic and Indian Oceans ...........................44
5-2 Names of Ocean Fronts in the Pacific and Indian Oceans ............................46
5-3 Classification of Ocean Fronts .......................................................................49
5-4 Scale of Variability .........................................................................................60
6-1 Negative Temperature Gradients Required to Compensate for Depth ..........66
7-1 Location and Depths of Secondary Sound Channels ....................................84
7-2 Typical Convergence Zone Ranges...............................................................92
8-1 Environmental Factors Affecting Shallow-Water Variability ...........................96
8-2 Aspects of the Shallow-Water Environment...................................................97
8-3 Aspects of Shallow-Water Acoustics .............................................................97
8-4 Aspects of Shallow-Water Operations ...........................................................97
B-1 Sound-Pressure Level Conversion Factors .................................................135
B-2 Common Decibel Equivalents......................................................................136
B-3 Sound-Pressure Levels of Common Noises ................................................140
B-4 Bandwidth as Percentages and Selected Conversions ...............................143
C-1 Ground-Level Illumination from Several Common Sources .........................146
1
Chapter 1
The Nature of Underwater Sound
1.1 Elementary Aspects of Sound
All sound, whether produced by a cowbell or by a complicated electronic device,behaves in much the same manner. Sound originates as a wave motion produced by avibrating source and requires an elastic medium such as air or water for itstransmission. For example, consider the action of a vibrating piston located at one endof a rigid pipe containing water. Because water is elastic, the motion initiated by thepiston is communicated to adjacent particles, causing changes in pressure, in the formof alternate compressions and rarefactions, as illustrated in figure 1-1. This series ofcompressions and rarefactions constitutes a wave train, which is propagated down thepipe at the speed of sound. The changes in pressure can be detected by pressure-sensitive devices such as hydrophones.
Figure 1-1. Compressional Wave Train.
1.1.1 Wave Motion. Sound waves in water are longitudinal waves, because theparticles transmitting the wave move back and forth in the direction of the propagationof the wave. When the motion of the particles is perpendicular to the direction of thewave, the wave is a transverse wave, an example of which is the motion of a rope whenit is snapped like a whip.
The frequency of the sound wave is determined by the motion of the vibratingsource. For a single frequency, wavelength is defined as the distance betweensuccessive compression maxima. Frequency, wavelength, and sound speed are relatedby the following expression:
CompressionPhase
RarefactionPhase
SoundSource
2
C λ = f
where,
λ = wavelength
C = sound speed
f = frequency
In the metric (MKS) system of units, λ is expressed in meters, c in meters persecond, and f in hertz (Hz; cycles per second). The English system of feet, feet persecond, and Hz is also used for λ, c, and f in underwater acoustic applications.
Frequencies below 20 Hz and above 20 kHz are commonly referred to asinfrasonic and ultrasonic, respectively. Frequencies in the audio range are from 20 Hzto 20 kHz. At infrasonic frequencies, the wavelength is very long, whereas at ultrasonicfrequencies it is very short.
For a typical sound speed of 1,500 meters/second in water, the wavelengthwould correspond to frequency in the following manner:
Table 1-1. Frequency vs. Wavelength for a Sound Speed of 1,500 m/sec (4,921 ft/sec).
Frequency(Hz)
Wavelength(meters/feet)
Frequency (KHz) Wavelength(meters/feet)
10 150/492 1 1.5/4.950 30/98 5 0.3/1
100 15/49 10 0.15/.49500 3/10 50 0.03/.01
1.1.2 Acoustic Energy vs. Electromagnetic Energy. In a conductive medium such asseawater, electromagnetic energy in the form of light or radio waves is attenuated atabout 1.3 x 103 f2 dB for each thousand yards of transmission, where f is expressed inkHz. Maximum penetration is only a few hundred feet. (See appendix B for anexplanation of decibels [dB].)
At lower sonar frequencies, acoustic energy is attenuated at roughly 0.01 dB perthousand yards; consequently, sound waves can travel hundreds of miles underwater.Sound energy, therefore, propagates through the ocean with far greater efficiency thandoes electromagnetic energy.
1.1.3 Speed of Sound. The speed of sound in the ocean is a function of watertemperature, salinity, and pressure, all of which may vary with depth, season,
3
geographic location, and time at a specific location. The graphic representation ofvariation of sound speed with depth is called a Sound-Speed Profile (SSP) . Sound-speed profiles may be obtained either by direct measurement (CTD or XSV) or bymerging and conversion of bathythermograph (BT) temperature measurements andhistorical bathymetry and salinity database values in environmental prediction systems.
Historically, the speed of sound, a scalar quantity having magnitude only, hasbeen called the “sound velocity”; hence the term “Sound-Velocity Profile” or “SVP.” Theterm velocity, when properly used, indicates a vector quantity having magnitude anddirection; therefore the term “Sound-Speed Profile” or “SSP” is a more accuratedescription.
Measurements of sound speed in the ocean have led to empirical formulas suchas the following (Wilson, 1960):
Metric Formula: C = 1449.2 + 4.623t - 0.0546t2 + 1.391(S-35) + 0.016d where,
C = sound speed (meters/second)d = depth (meters) t = temperature (o C)S = salinity in parts per thousand (‰)
English Formula: C = 4427.2 + 11.962t - .0553t2 + 4.562(S-35) + 0.016d where,
C = sound speed (feet/second)d = depth (feet) t = temperature (o F)S = salinity in parts per thousand (‰)
In general, sound speed increases 3.2 meters/second per degree Centigrade(6.0 feet/second per degree Fahrenheit) about 1.4 meters/second (4.6 feet/second) perparts per thousand (‰) in salinity, and approximately 1.6 meters/second per 100meters (1.6 feet/second per 100 feet) in depth. Temperature is usually the mostinfluential factor in the upper portion of the profile above the point of minimum soundspeed (Deep Sound Channel Axis, DSCA) in deep ocean water. Below the DSCA, thepressure (depth) effect is dominant over temperature, which is relatively constant.
The effects of salinity in the open ocean are usually minor. (As discussed inchapter 8, salinity can be a major factor in shallow water.) Values for sound speed indeep-sea water range from less than 1,433 meters/second (4,700 feet/second) togreater than 1,554 meters/second (5,100 feet/second).
For more in-depth discussion of bathythermograph measurements and resultingsound-speed values, see chapter 6.
4
1.1.4 Refraction. If the ocean were infinite in extent and its physical properties werehomogeneous, sound would travel in straight lines and at constant speed. Soundpropagates along curved paths (rather than straight lines) when the speed of soundvaries either horizontally or vertically. This phenomenon is called refraction and isdescribed by Snell's Law. (See paragraph 1.1.4.2.)
1.1.4.1 Ray Paths. In discussing refraction, it is convenient to think in terms of soundas traveling between a pair of ray paths (rays). A ray path is a curve (a straight line inisospeed conditions) that is at each point normal to a wave front and which defines thedirection of propagation of the wave, that is, the direction in which the motion of aparticle on one wave front is passed on to the next. This geometrical interpretation ofthe propagation of sound is only approximate and cannot, at least in its traditional form,provide the sound intensity in regions in which no ray exists (shadow zones). A raydiagram presents a qualitative picture of sound propagation, as shown in figures 1-2and 1-3.
Figure 1-2. Surface Duct, Bottom Bounce, and Convergence Zone Ray Trace(Full-Path and Near-Surface Illustrations).
5
Figure 1-3. Secondary Sound Channel, Bottom Bounce, and Convergence ZoneRay Trace (Full-Path and Near-Surface Illustrations).
1.1.4.2 Snell's Law. The basic equation of ray acoustics is Snell's Law, whichdescribes the refraction of sound rays in a medium of variable sound speed. This lawstates that a ray going from a region with one speed will have a different direction in asecond region which has a different speed. The variation in sound speed is governedby the equation shown in figure 1-4. In this diagram, θ1 is the grazing angle of the ray,and C1 is the speed of the wave in the first region; θ2 is the grazing angle of the ray, andC2 is the speed of the wave in the second region; and C2 > C1. Both angles aremeasured relative to the boundary between the two regions.
Figure 1-4. Snell’s Law–Two Layers.
Snell's Law can be extended to cover multiple layers as shown in figure 1-5. Cx
is the vertex speed. This is the speed of sound in the layer at the point where the raybecomes horizontal. Snell's Law implies that a sound ray cannot enter a region wherethe sound speed is greater than the vertex speed of the ray. The ray becomeshorizontal, then is refracted towards the depth of origin. In a medium having layers ofconstant sound speed, the rays seem to consist of a series of connected straight lines.
Sound Ray
Sound Speedin Layer # 1 = C1
Sound Speedin Layer # 2 = C2
C1 = C2COS θθ1 COS θθ2
θθ1
θθ2
6
In a medium in which the speed of sound changes linearly with depth, it can be shownthat the sound rays are arcs of circles. These principles are commonly employed inanalog and digital ray-tracing computers.
LAYERLAYER SOUND
SPEED
C1 = C2 = C3 = Cn = Cx COS θθ1 COS θθ2 COS θθ3 COS θθn
Figure 1-5. Snell's Law–Multiple Layers.
1.1.4.3 Sound-Speed Gradient. A sound-speed gradient exists where there is acontinuous variation in the speed of sound as a function of a linear dimension, such asdepth. A variation in sound speed with depth is a vertical sound-speed gradient. Themagnitude of the gradient is the change in speed divided by the change in depth. Theamount of ray bending that occurs is directly related to the magnitude of the gradient.Sharp gradients will cause a greater refraction than weak gradients, and in an isospeedmedium the rays will travel in straight lines. (See figures 1-6, 1-7, and 1-8.)
D e p t h
Figure 1-6. Positive Sound-Speed Gradient.
Cx
C5
C4
C3
C2
C112
4
5
3
θθ1
θθ2
θθ4
θθ5 θθ5
θθ3
Sound Speed Range
7
1.1.4.3.1 Positive Sound-Speed Gradient. If the sound speed increases with depth,the gradient is said to be positive and will produce a ray curvature that bends upwardtoward the depth of the minimum sound speed.
D e p t h
Figure 1-7. Negative Sound-Speed Gradient.
1.1.4.3.2 Negative Sound-Speed Gradient. If the sound speed decreases with depth,the gradient is said to be negative and will produce ray curvature that bends downwardtoward the depth of the minimum sound speed.
D e p t h
Figure 1-8. Isospeed (Straight-Line) Gradient.
1.1.4.3.3 Isospeed. An isospeed layer is one within which the speed of sound is thesame at all points. In an isospeed layer, sound travels in straight lines.
Note that an isothermal (constant temperature) layer is not the same as an isospeedlayer. As paragraph 1.1.3 demonstrates, sound speed increases with pressure (depth),so that an isothermal layer will exhibit a positive sound-speed gradient. Acompensating negative temperature gradient is required for a resultant isospeed sound-speed profile to exist. A temperature decrease of 0.2oF per 100 feet, or .36oC per 100meters, of depth at a temperature of 40oF, or 4.44oC, will result in an isospeed profile.
Sound Speed Range
Sound Speed Range
8
1.1.4.3.4 Acoustic Reciprocity. Between an acoustic source that radiates equally wellin all directions and an acoustic receiver that receives equally well in all directions, thereare a number of different paths along which sound may propagate. These paths mightbe reflected from either surface or bottom, or totally refracted within the water columnby undergoing a combination of reflections and refractions. However complicated thepropagation paths may be, the source and receiver can be interchanged (as illustratedin figure 1-9), and the sound will travel the same paths but in the reverse direction. Aslong as the "radiation" and "receiving" characteristics of the source and receiver are thesame, this reciprocity holds.
Figure 1-9. Acoustic Reciprocity (Homogenous Ocean).
9
Chapter 2
Propagation Loss
2.1 Introduction
As sound travels through the ocean, the pressure associated with the wave frontdiminishes. This decrease in pressure is referred to as propagation loss (alsocommonly called transmission loss).
Sound propagation loss in water depends on the following factors:
a. Spreading Loss. The spreading of a wave front causes the energyassociated with the wave front to be distributed over an increasingly large area with aresultant decrease in intensity.
b. Absorption Loss. The conversion of some of the mechanical energy in thesound wave to heat causes energy losses referred to as absorption losses.
c. Scattering Loss. Suspended particulate matter in the water column scatterssound energy into directions other than the direction the main wave is traveling. Thisresults in a reduced sound-pressure level in the wave front.
d. Bottom Loss. When a sound wave strikes the ocean bottom, a portion of theenergy in the wave front will enter the bottom and may be strongly attenuated there.Resulting losses may prevent some bottom interacting energy from returning to thewater column. The reflected energy associated with the main wave front in the water isthereby reduced, and the sound-pressure level of the wave is decreased.
e. Surface Loss. Reflection and scattering of sound by the surface of the seacause a loss of energy from the main wave. Surface loss increases with sea state andwith frequency.
f. Diffraction Loss. Diffraction concerns the wave motion beyond an obstaclethat has cut off a portion of an advancing wave front. Gradients that result in surfaceducts and shadow zones provide such obstacles. The leakage of sound energy fromsurface ducts or into shadow zones, thus out of the main wave, is an example ofdiffraction loss.
g. Multipath Interference. The existence of multipaths results in a condition thatpermits constructive and destructive interference to occur between energy propagatingin separate paths. As one or more of the paths change with time, fluctuations inintensity are observed.
10
The preparation of propagation loss (PROPLOSS) curves or profiles is currentlyaccomplished by computerized Environmental Prediction Systems, such as theGeophysics Fleet Mission Program Library (GFMPL), Tactical Environmental SupportSystem (TESS), or Integrated Carrier ASW Prediction System (ICAPS).
2.2 Spreading Loss
Spreading loss is a geometrical effect representing the regular weakening of asound signal as it spreads outward from the source. For a homogeneous losslessmedium, without boundaries, intensity decreases with the inverse square of the range,a condition that is termed spherical spreading. Under actual environmental conditions,spherical and cylindrical spreading are the most common, while dipolar may occur atstrong boundaries of surface ducts.
2.2.1 Spherical Spreading. Spreading loss is governed by the inverse-squarespreading law. To illustrate this law, assume the source to be a point that has radiateda fixed amount of acoustic power (watts, for example) into the surrounding medium. Asthe energy travels away from the source, it spreads in the form of a spherical shell.Since this shell is enlarging as the distance from the source increases, the soundintensity (watts/meter2) must therefore decrease proportionally. The decrease inintensity is exactly proportional to the increase in the surface area of the sphere. Sincethe surface area is given by A = 4∏R2, the decrease in intensity is proportional to thesquare of the radius. When the radius of the sphere is considered to be the range, theloss in dB due to spreading between a point a yard from the source and the receiver isgiven by:
Spherical-Spreading Loss (dB) = 10 log R2 = 20 log R
where R is the range in yards between the source and receiver.
Spherical spreading occurs when refraction or reflection does not affect the raypaths. Figure 2-1 illustrates spherical spreading. When refraction effects are present,the loss can be either greater or less than that given by the spherical spreading law.
Sound intensity decreases as the square of the distance, or 6 dB per distancedoubled. (Since 20 log R - 20 log 2R = 20 log _R = 20 log 1/2 = -6 dB.)
2R
11
Figure 2-1. Spherical-Spreading Loss (Loss = 20 Log R).
2.2.2 Cylindrical Spreading. When the propagation path has upper and lower bounds,the spreading is no longer spherical because sound does not cross the boundingplanes. Surface ducts and sound channels represent cases in which spreading is lessthan spherical, but rather approaches cylindrical. In cylindrical spreading, illustrated infigure 2-2, the wave front expands in the form of a cylinder having a height that isdetermined by the thickness H of the duct or channel. Since this cylindrical shell isexpanding as the distance from the source increases, the sound intensity (watts/meter2)must decrease proportionally. The decrease in intensity is exactly proportional to theincrease in the surface area of the cylinder. Since the surface area of interest is givenby A = 2∏RH, the decrease in intensity is proportional to the radius R, which is also therange. The cylindrical-spreading loss is given by:
Cylindrical-Spreading Loss (dB) = 10 log R
Sound intensity decreases as the inverse first power of the distance, or 3 dB perdistance doubled. (Since 10 log R - 10 log 2R = 10 log _R =10 log 1/2 = -3 dB.)
2R
12
2.2.3 Dipolar Spreading. Through normal refraction, sound energy above and below asound-speed maximum is bent toward lower sound speed. Dipolar spreading mayoccur over short ranges at the Sonic Layer Depth and at sound speed maximums alongthe SSP, as shown in figure 2-3. The propagation loss due to dipolar spreading isgreater than for either spherical or cylindrical spreading (see figure 2-4). This rapidreduction in signal (noise) over a short range make the use of dipolar spreading optimalin submarine counterdetection considerations. In dipolar spreading, the sound intensitydecreases by 1/R4 as range R increases, or
Dipolar-Spreading Loss (dB) = 10 log R4 = 40 log R
Figure 2-3. Dipolar-Spreading Loss (Loss = 40 Log R).
H
Sound S peed
Depth
SLDDipolar Spreading
Dipolar Spreading
Figure 2-2. Cylindrical-Spreading Loss (Loss = 10 Log R).
2R
R
A’ 2A
13
Figure 2-4. Comparison of Spreading Losses.
2.3 Absorption
Absorption involves a process of conversion of acoustic energy to heat. As thesound wave travels through the ocean, it alternately produces compressions andrarefactions of the water. During this process, some of the acoustic energy is convertedto heat. From theory, we know that for low frequencies (5 to 40 Hz) and for highfrequencies (>1,000 Hz) the absorption is proportional to the square of the frequency.Measurements of absorption loss in the ocean generally confirm this. The amount ofabsorption loss should theoretically also depend on the temperature of the water. Asthe water temperature increases, the absorption loss should decrease. This has beenconfirmed by measurements. At intermediate frequencies, the absorption varies in acomplicated way with both frequency and temperature (Urick, 1967).
At the lower frequencies, total absorption loss over any acoustic path(determined by simply multiplying the range by the absorption coefficient), according toThorp (in Urick, 1975), is:
0.1 f2 40 f2
a = ------------- + -------------- + 2.75 x 10-4f2 + 0.003 dB/kyd 1 + f2 4100 + f2
where f is the frequency in kHz.
0 6 12 18 24 30 36 42 48 54 60 40
50
60
70
80
90
100
110
120
Cylindrical Spreading - Source is inthe Surface Duct at a frequency twicethe Cutoff Frequency.Spreading Loss = 10 Log R
Dipolar Spreading - Source is at SLD orat a sound speed maximum.Spreading Loss = 40 Log R
14
Thorp's curve is fitted to empirical data and is valid from about 0.1 to 10 kHz at atemperature of about 39oF.
2.4 Scattering (Reverberation)
Discontinuities in the physical properties of the medium intercept and reradiate aportion of the acoustic energy incident upon them. This reradiation of sound is calledscattering. Scattering losses, therefore, involve reflections of sound energy away fromthe direction in which the major portion of the sound field is traveling, so that the waveitself suffers a loss in energy and hence the intensity decreases. Scattering occurs inseveral ways. It can be caused by particles in the water such as plankton, oil droplets,bubbles, and fish, or by reflection from the ocean boundaries. Scattering loss due toreflectors suspended in the medium (volume scattering) is difficult to measure directly.Scattering loss due to surface reflections (boundary scattering) can be measureddirectly by comparing data taken under a variety of sea-surface conditions. Scatteringloss due to bottom reflection is generally not isolated as a factor, but rather is includedas part of the total bottom-reflection loss described in paragraph 2.4.3. Scatteredenergy that is reflected back to the acoustic source is called reverberation and makesup part of the interfering background in active sonar operations. Scattering is notimportant at low frequencies as a factor in the determination of propagation loss.
2.4.1 Surface Reverberation. Surface reverberation (figure 2-5) is due to surfacewaves. It is always a factor in active-sonar operations. At short ranges, the surfacescattering increases with wind speed. With higher wind speeds, an acoustic screen isformed near the surface by entrapped air bubbles, preventing a further increase in thesurface-reverberation level. Surface reverberation from ranges in excess of 1,500yards is usually lower in level than either bottom or volume reverberation. Wind speedthat correlates with sea state and, to a lesser degree, wind-speed history are the majorenvironmental factors influencing surface reverberation.
Figure 2-5. Surface Reverberation.
15
2.4.2 Volume Reverberation. Volume reverberation (figure 2-6) is caused by variousscatterers or reflectors in the ocean such as fish, other marine organisms, suspendedsolids, bubbles, and even water masses of markedly different temperatures. Volumereverberation depends upon the number and distribution of scatterers, as well as theirsize, shape, and reflectivity. If the density of these reflectors were constant, volumereverberation would decrease as the inverse square of range (20 dB for each tenfoldincrease in range). Volume reverberation is also a function of the frequency used inecho ranging and is generally greatest at night, when the scattering layer is near thesurface.
Figure 2-6. Volume Reverberation.
Volume scatterers are not uniformly distributed in depth, but tend to beconcentrated in a diffuse layer called the deep-scattering layer (DSL), depicted in figure2-7.
Figure 2-7. Volume Scattering Strength vs. Depth and Time.
This layer is from 50 to 150 meters thick and is found between 100 and 400fathoms in tropical waters. The layer or layers may undergo diurnal verticalmovements. There may be more than one scattering layer at a given location. Thetopography of the scattering layer may be affected by internal waves (see paragraph5.3.4), thermoclines, currents, etc. Scattering layers have different characteristics, suchas patchy, split, or nonmigratory, in different water masses. In many parts of theNorthern Hemisphere, the maximum volume reverberation occurs in March and theminimum in November. The intensity of the scattering is a function of frequency andthe density of the organisms causing the scattering.
16
2.4.3 Bottom Reverberation. Reverberation, regardless of source, may be consideredthe unwanted portion of a returned signal and is a problem peculiar to active-sonarsystems. In the case of bottom reverberation, the scattered component of the bottomreturn is undesirable. The desired signal in bottom-bounce ranging operations is thespecular return, or the coherently reflected echo. Bottom reverberation, orbackscattering, can severely limit active sonars operating in shallow water or in thebottom bounce or convergence zone modes.
In theory, bottom backscattering should be directly related to seafloor roughness.In practice, however, a rigorous relationship between backscattering and seafloortopography has yet to be established. Much of the theory developed for bottomreverberation has evolved from that developed for backscattering from the sea surface.Roughness parameters of the seafloor, however, are not as well known as for the seasurface, and the wavelength component of bottom roughness can range from microns(particle size) to miles (abyssal hills). Reflected signals from subbottom layers furthercomplicate backscattering measurements. Most bottom backscattering modelsconsider the ocean floor to be a volumetric scattering surface.
Reported bottom-backscatter data generally show little or no frequencydependence in the range between 0.5 and 80 kHz. However, a Russian study(Jitkovskiy and Volovova, 1965) reported instances where high-frequency and highgrazing-angle dependence were observed in the range of 1 to 30 kHz over a very roughseafloor. Conclusions drawn from the study were that when bottom roughness is largecompared to wavelength, the backscattering coefficient is independent of frequencyand when bottom roughness is small compared to wavelength, scattering strengthincreases with increasing frequency.
2.5 Bottom Loss
2.5.1 Bottom Interaction. Sound interacting with the ocean bottom will normally suffera loss in intensity. Two mechanisms are involved in the decrease in intensity:scattering and absorption. Figure 2-8 illustrates the bottom-interacting energy paths.The amount of energy that is lost into or scattered off of the ocean bottom and itsunderlying sediments will depend on the bottom roughness, the geoacoustic parameterof the bottom sediments (sound-speed and attenuation gradients of the sediment andbulk sediment density), the frequency of the sound wave, and the angle at which thesound wave strikes the bottom. Further complications occur if the lateral variability inthe ocean bottom changes along the refracted path.
17
Figure 2-8. Energy Partition Due to Acoustic-Wave Interaction with Bottom.
2.5.2 Factors of Frequency and Grazing Angle. Extreme care must be used inapplying generalizations to acoustic performance predictions. Bottom loss will tend toincrease with frequency and grazing angle. Lower frequencies of sound generallyundergo less reflection loss at the ocean-bottom interface and, when combined with therefracted energy returned to the sediment-water interface, will result in lower loss at allgrazing angles. Refer to the Naval Oceanographic Office (NAVOCEANO)Environmental Guides and Submarine Tactical Oceanographic Reference Manuals(STORMs) publications for practical applications. See figure 2-9 for illustrations ofbottom type and grazing angle effects on bottom loss.
2.5.3 Bottom-Loss Data Bases. Bottom-loss measurements have been made in asignificant number of operational areas during surveys sponsored by NAVOCEANO,the Naval Underwater Warfare Center (COMNAUNSEAWARCEN), and the Naval AirWarfare Center (NAVAIRWARCEN). These bottom-loss measurements have led to thedevelopment of bottom-loss data bases. The data bases are the High-FrequencyBottom-Loss (HFBL) data base and the Low-Frequency Bottom-Loss (LFBL) data base.For further information on bottom-loss values, refer to the STORMs or EnvironmentalGuides produced by NAVOCEANO.
18
Figure 2-9. Smooth Curves of Bottom Backscattering Strength vs. GrazingAngle for Various Bottom Types.
(Frequency range 0.5 to 100 kHz. Individual measurements show deviations averaging about 5 dB from these curves.)
19
Chapter 3
Background Noise
3.1 General
The primary goal in underwater acoustics is to distinguish sounds from the totalbackground noise. Ambient noise is that part of the total noise background not due tosome identifiable localized source. It exists in the medium independent of theobserver's activity. Interfering noise sources that are located on, or are a part of, theplatform on which a sensor is installed are sources of self-noise. Self-noise is distinctfrom ambient noise.
3.2 Ambient Noise
Deep-sea ambient-noise measurements have been made over a frequencyrange from below 1 Hz to about 100 kHz. Over this range the noise is due to a varietyof sources, each of which may be dominant in one region of the spectrum. Principalsources of ambient noise in the frequency range of about 30 Hz to 10 kHz are distantshipping and wind-generated surface agitation. Other important contributors are rain,ice, and biological activity. Under certain conditions, these latter sources of backgroundnoise can seriously interfere with detection systems; however, not enough is knownabout their occurrence to permit meaningful predictions.
Ambient-noise levels fluctuate in both time and space. Differences of as muchas 5 to 10 dB are frequently observed between readings made only a few minutesapart. In consequence, "average" noise levels cannot be expected to correspondexactly to individual measurements or to reflect actual noise conditions during anyparticular phase of a tactical exercise. Climatological data concerning the long-termmean local environment (including wind speed, sound-speed profile, and ship-trafficdistribution) are indicative of average intensities but not of the instantaneous conditionsexperienced by a sensor. Whenever precise knowledge of local ambient noise isrequired, in-situ measurements of these noise levels should be made.
Ambient-noise levels versus frequency are graphically depicted in figure 3-1 andare listed in tables in Appendix B.
3.2.1 Surface-Ship Traffic Noise. At the lower frequencies (figure 3-1), the dominantsource of ambient noise is the cumulative effect of ships that are too far away to beheard individually. The radiated noise spectrum of merchant ships peaks atapproximately 60 Hz, a frequency that corresponds to the maximum in the cavitationspectrum of typical merchants ships. The spectrum of the noise radiated from ships asobserved at great distances differs from the spectrum at close range due to the effect offrequency-dependent attenuation. The shape of the radiated noise spectrum of typicalmerchant ships, as seen at various ranges (ONR, 1968), clearly suggests that the
20
contribution of ships to the ambient-noise background depends on the distance of thereceiver from traffic lanes or from any other ships' concentrations.
3.2.2 Sea-State Noise. Sea state is a critical factor in both active and passivedetection. In active sonobuoy detection, waves 6 feet or greater will start to produce asea-state-limited situation. For shipboard sonar systems, location of the sonar dome,ship's speed, course, and relation to the sea all have an effect. The limiting situation isgenerally sea state 4 or 5. For passive detection, the noise level created by wind wavesof 10 feet or greater will result in a minimum of ASW operational effectiveness,depending on type of sensor.
Wind waves are produced by surface winds; swells are born of wind waves.These sea-state parameters are normally depicted on wind wave, swell, and combinedsea-state analyses. The action and interaction of waves and swell are complex. Theymay be generalized as reinforcing each other when crests are in conjunction with eachother and dampening when crests meet troughs. The interaction may also result inamplification when directions and wavelengths are harmonic, and in dampening whenthey are in opposition.
The highest wave that will normally be encountered under existingmeteorological conditions can be derived from the wind wave, swell, and combined sea-state analysis charts by multiplying the significant height (H 1/3) by a factor of 9/5.Table 3-1, a wind and sea-state description table, contains an explanation of sea-stateparameters for specific sea states.
21
Figure 3-1. Ambient-Noise Levels - Traffic and Sea Height(Modified Wenz Curves) (NUSC TD 8063-1, 1988).
22
NOTES ON FIGURE 3-1
1. Along the Gulf Stream and major trans-Atlantic shipping lanes, the heavytraffic predictor (Curve VI) forecasts average noise within ±2 dB at 100 and 200 Hz.Maximum values usually occur with ships closer than 10 nautical miles, and the valuesfollow the individual ship's curve (Curve VII). Minimum values vary radically but appearto group around the average traffic curve (Curves IV and V).
2. For 440 Hz, the predictor curves appear to be 2 or 3 dB too low.
3. Four or more ships closer than 30 nautical miles constitute heavy noise, withships closer than 10 nautical miles driving the noise level up to the individual ship'starget curve (Curve VII). Where the bulk of the traffic is farther than 40 nautical miles,the average traffic curves (Curves IV and V) apply.
4. Correlations of noise intensity with distance to nearest ship, with all shipspresent in the shipping lanes, were negative. For areas not immediately in a heavytraffic area, ship concentration and distance become critical.
Seasonal changes in long-range acoustic propagation can have a significanteffect on low-frequency ambient-noise levels. This phenomenon is attributed to themore favorable sonar sound reception, or deep sound-channel propagation, duringmonths of low surface temperature.
23
Table 3-1. Wind and Sea State Descriptions.
WIND ESTIMATING WIND SPEED STATE OF THE SEA
BeaufortNumber
DescriptiveTerm
MeanVelocity(knots) MPH
Effects Observedon Land
Effects Observedat Sea
WMOCode
DescriptiveTerm
Height(H 1/3) of
waves in feet0 Calm 1 1 Calm; smoke rises vertically Sea like a mirror
1 Light Air 1–3 1-3 Direction of wind shown bysmoke drift but not by windvanes
Ripples with the appearance ofscales are formed, but withoutfoam crests
0 Calm(Glassy)
0
2 LightBreeze
4-6 4-7 Wind felt on face/leavesrustle; ordinary vanes movedby wind
Small wavelets, still short butmore pronounced; crests have aglassy appearance and do notbreak
1 Calm(Rippled)
0 –1/3
3 GentleBreeze
7-10 8-12 Leaves and small twigs inconstant motion, windextends light flag
Large wavelets; crests begin tobreak; foam of glassy appearance;perhaps scattered white horses
2 Smooth(Wavelets)
1/3 – 1-2/3
4 ModerateBreeze
11-16 13-18 Raises dust and loose paper;small branches are moved
Small waves, becoming longer;fairly frequent white horses
3 Slight 1-2/3 – 4
5 FreshBreeze
17-21 19-24 Small, leafy trees begin tosway; crested wavelets formon inland waters
Moderate waves, taking a morepronounced long form; manywhite horses are formed (chanceof some spray)
4 Moderate 4 – 8
6 StrongBreeze
22-27 25-31 Large branches in motion;whistling heard in telegraphwires; umbrellas used withdifficulty
Large waves begin to form; thewhite foam crests are moreextensive everywhere (probablysome spray)
5 Rough 8 – 13
7 Near Gale 28-33 32-38 Whole trees in motion;inconvenience felt whenwalking against wind
Sea heaps up and white foam frombreaking waves begins to beblown in streaks along thedirection of wind
8 Gale 34-40 39-46 Breaks twigs off trees;generally impedes progress
Moderately high waves of greaterlength; edges of crests begin tobreak into the spin-drift; the foamis blown in well-marked streaksalong the direction of the wind
9 StrongGale
41-47 47-54 Slight structural damageoccurs (chimney pots andslates removed)
High waves; dense streaks of foamalong the direction of the wind;crests of waves begin to topple,tumble, and roll over; spray mayaffect visibility
6 VeryRough
13 – 20
10 Storm 48-55 55-63 Seldom experienced inland;trees uprooted; considerablestructural damage occurs
Very high waves with longoverhanging crests; the resultingfoam, in great patches, is blown indense white streaks along thedirection of the wind; on thewhole, the surface of the sea takesa white appearance; the tumblingof the sea becomes heavy andshocklike, visibility affected
7 High 20 – 30
11 ViolentStorm
56-63 64-72 Very rarely experienced;accompanied by widespreaddamage
Exceptionally high waves (smalland medium-sized ships might befor a time lost to view behind thewaves); the sea is completelycovered with long white patchesof foam lying along the directionof the wind; everywhere the edgesof the wave crests are blown intofroth; visibility affected
8 Very High 30 – 45
12 Hurricane 64 73and andover over
The air is filled with foam andspray; sea completely white withdriving spray; visibility veryseriously affected
9 Phenomenal over 45
24
3.2.2.1 Wind-Generated Noise. Sea-state noise generated by surface wave activity isusually the primary component over a range of frequencies from 300 Hz to 5 kHz. It maybe considered to be one of the most critical variables in active and passive detection. Seastate is a factor that normally cannot be measured directly with either precision or accuracy.It is primarily correlated with wind speed, which can be measured and predicted. Eachvalue of sea state represents a range of conditions, with the boundaries between theseconditions, usually defined in terms of wave height. It has been found possible to deducethe sea state, and hence, to a rough approximation, the wave height, from the observedvalue of wind velocity. Sea-state noise will vary with wind speed. Figure 3-1 relates themagnitude of water noise with wind speed and wave height for the frequency band between100 Hz and 10 kHz.
3.2.2.2 Sea-State Noise Levels. The wind-generated noise level decreases with increasingacoustic frequency (slope of -6 dB per octave) and increases with increasing sea state(approximately 6 dB for each increase in sea state). It is very important to understand thatall sound-sensor ranges are reduced by additional noise, and that there can be a 20-dBspread in background noise between various sea states.
3.2.3 Other Ambient-Noise Sources. Ambient noise is also produced by intermittent andlocal effects such as earthquakes, biologics, precipitation, ice, and breakage of waves.
3.2.3.1 Precipitation. Rain and hail will increase ambient-noise levels at some frequencies.Significant noise is produced by rain squalls over a range of frequencies from 500 Hz to 15kHz. Large storms can generate noise at frequencies as low as 100 Hz and cansubstantially affect sonar conditions at a considerable distance from the storm center.
3.2.3.2 Ice. Sea ice affects ambient-noise levels in polar regions. Its influence on water-noise levels is dependent upon the state of the ice, that is, whether it is forming, covers thesurface, or is breaking up. Provided that no mechanical or thermal pressure is beingexerted upon the ice, the noise level generally is relatively low during the growth of ice.According to investigations carried out in the Bering Sea, the noise level should not exceedthat for sea state 2, even for winds over 35 knots. The same investigation established thatthe intensity of the ice noise decreases with increasing frequency during the time the ice isgrowing. An exception to this period of relatively low noise level is the extremely noisycondition (due to entrapped air) resulting from the deformation and temporary breakup ofthe ice cover during growth.
3.2.3.3 Biologics. Biological noise may contribute significantly to ambient noise in manyareas of the ocean. Because of the habits, distribution, and sonic characteristics of thevarious sound producers, certain areas of the ocean are more intensely insonified thanothers. The effect of biological activity on overall noise levels is more pronounced inshallow coastal waters than in the open sea. It is more pronounced in the tropics and intemperate zones than in colder waters.
Although many marine animals produce noise of some sort, certain forms are sodominant that the study of only these few is a key to the prediction of the intensity, spaceand time distribution, and spectrum of significant ambient noise originated by marineanimals. By far the most intense and widespread noises from animal sources in shallow
25
water observed are those produced by croakers (representative of a variety of fish classifiedas drumfish) and snapping shrimp. Fish, more than crustaceans (crabs, lobsters, andshrimp), are the source of biological noise in most of the oceans. In addition to croakersand snapping shrimp, other varieties of noise producers include sea robins, toadfish, grunts,porpoises, certain whales, and others that are of only local importance. Sound-producingfishes and crustaceans are restricted almost entirely to bays, reefs, and coastal waters. Inoceanic waters, whales and porpoises are the principal contributors to biological noise.
In order to predict the ambient noise due to marine animals in any one location, oneof two techniques can be used. Either (a) observations of the actual noise can be madeover a period of time sufficient to determine cyclic variations, or (b) a general study of noise-producing animals can be correlated with a knowledge of the environment to givereasonable conclusions as to the type and variation of the sounds.
3.2.3.3.1 Marine Mammals. Mammal sounds include a much greater range of frequenciesthan do the sounds of either crustaceans or fishes. They have been recorded at as low as19 Hz and possibly lower (whale sounds), and as high as 196 kHz (porpoise sounds),although the principal frequencies are in the audible range. During echo-rangingoperations, porpoises have often been heard over equipment responsive only to a narrowband of ultrasonic frequencies.
Whales produce a variety of sounds, up to 189 dB//µPa, in a frequency range from20 Hz to 36 kHz. These marine animals resemble submarines in speed, acousticcharacteristics, and certain modes of behavior. A summary of the characteristics of largewhales compiled from a NAVOCEANO study in the Western North Atlantic is included astable 3-2. Figure 3-2 shows the potential whale sonar targets in the Western North Atlanticduring the month of September.
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Table 3-2. Characteristics of Large Whales Occurring in the Western North Atlantic(Levenson, 1969).
SPEED(Knots)
DIVE/SURFACECYCLE (Minutes) ACOUSTIC CHARACTERISTICS
SPECIESADULT
SIZE(Feet) Avg. Max. Sub-
mergedSur-faced
Freq.Range(Hz)
Prin.Freq.(Hz)
SourceLevel
dB//µPa
TargetStrengthdB//µPa
REMARKS
12.5-150Hz
24 Hz 180-189BlueBalaenopteramusculus
70-100 10 20 5-50 2-5
19-36 kHz 25 kHz 159
no data Occurs single or in groups of 2or 3; dives deep; avoids coastalwaters; high vertical blow
FinBalaenopteraphysalus
50-80 8 22 4-15 2-5 20-200 Hz 20-40 Hz 170-180 no data Occurs singly or in smallgroups, primarily oceanic,occasionally entering coastwaters; dives to at least 650 ft;high vertical blow
SeiBalaenopteraborealis
40-55 7 26 4-12 0.5-2 no data no data no data no data Usually occurs singly or inpairs; may congregate in groupsof 50 or more when feeding;small vertical blow
MinkeBalaenopteraacutorostrata
25-33 5 nodata
3-6 <0.5 4-8 kHz 6 kHz 154 no data Occurs near shore, usuallyalone, less often in pairs, rarelyin groups; small blow
RightEubalaenaglacialis
45-55 4 8 10-20 4-6 100-750Hz
100-150Hz
no data no data Occurs singly or in groups of 2or 3; high V-shaped doubleblow
HumpbackMegapteranovaeangliae
35-55 7 15 2-23 2-5 0.2-8.0kHz
200-1600Hz
154-160max. 184
95-108 (1/3octavebands
centered at15, 1.0 & 1-
6 kHz)
Occurs singly or in smallgroups; coastal, except whenmigrating; spouts at eachsurfacing; length of time atsurface dependent onsucceeding dive; low bushyblow
Sperm Physetercatadon
30-65 5 10 15-60 10-15 0.2-32 kHz 3-5 kHz 170-177 92.7-110 Occurs alone or in looselyscattered schools; changescourse often while feeding;when frightened will usuallyflee to windward; dives to 3,500ft; low bushy blow, directedforward
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Figure 3-2. Potential Whale Sonar Targets (Western North Atlantic)(Levenson, 1969).
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3.3 Self-Noise
If the ocean environment were completely free of noise, the detection of anacoustic signal would still be difficult because of (a) the noise inherent in the soundequipment itself and in the platform on which it is mounted, and/or (b) the noise causedby the motion of the platform. Even when the sound equipment is towed separately,noise is generated by the water moving past the unit and the supporting cable. Thisnoise is known as self-noise and is present in submarines, surface vessels, and aircraft.Self-noise may result from (a) circuit noise arising from relay contacts and othercomponents, (b) transducer noise caused by water turbulence around the housing, (c)hull noise arising from structural parts that are loose, (d) machinery noise from basestructural parts, and/or (e) machinery noise from propulsion or auxiliary equipment. Themajor classes of self-noise are machinery noise, propeller noise, and hydrodynamicnoise.
3.3.1 Machinery Noise. Machinery noise is produced by the main propulsion plant,reduction gears, propeller shafts, auxiliary machinery, and various underwaterdischarges from ships. Sounds include whines, squeaks, or grumbles of variousdiscrete frequencies and broadband noise components. The spectrum of noise createdby a typical piece of machinery contains a series of tonal components of high level,superimposed on a continuous background. This noise is of the greatest importance atlow speeds because it is then concentrated in the low-frequency range and undergoesless attenuation than does noise of high frequencies. Rigid adherence to the ship’squiet bill, which results in the use of the quietest equipment during acoustic operations,reduces noise in this area significantly.
3.3.2 Propeller Noise. The primary source of propeller noise is cavitation. High-speedmovement of underwater propeller blades causes cavitation noise. In this case,cavitation results from the separation between the propeller and the surrounding waterdue to the rapid movement of the propeller blades. The propeller motion preventswater from immediately closing in behind its blades. As a result of the low-pressureregion being formed, a stream of bubbles is continually being formed. These bubblescollapse, and the noise produced by a great many of these collapsing bubbles iscavitation noise. It has a continuous spectrum, which can dominate the high-frequencyend of the spectrum of ship noise. For a submarine, propeller noise is affected not onlyby the speed but also by the depth of the submarine. Since the hydrostatic pressure ofthe water around the propeller increases with depth, a deeply submerged submarinemay operate at greater speed without cavitation than might a submarine operating at ashallow depth.
3.3.3 Hydrodynamic Noise. Hydrodynamic noise results from the flow of water past thehydrophone, its supports, and the hull structure of the platform. In a submarine,hydrodynamic noise includes turbulent pressures upon the hydrophones from floweddies, as well as rattles and vibration from the submarine's plating and sonar gear.The water flow around the sonar dome sometimes creates the major portion of self-noise. Flow noise is characterized by its dependence upon speed. This noise
29
increases as the fifth or sixth power of speed and is independent of the operating depthof the submarine. This latter characteristic distinguishes flow noise from cavitationnoise. Flow noise has a continuous spectrum, peaking in the low-frequency range andincreasing in intensity as the speed increases. Low-frequency, long-range listeningfrom a submarine-mounted hydrophone may be seriously hampered at speeds greaterthan 5 knots.
Hydrodynamic noise also affects the detection capabilities of sonobuoys. Twoconditions which affect sonobuoys in high seas are water flow past the deployedhydrophone and cable strumming.
In a surface ship, hydrodynamic noise is caused by the movement of the shipthrough the water and is predominant at speeds above 12 knots. This noise has threemain components: flow noise, flow excitation, and cavitation around the sonar dome.Flow noise is caused by turbulent flow around the underwater hull, which causespressure fluctuations at the face of the transducer. When the ship's speed is fastenough, a bow-mounted or hull-mounted sonar dome will cavitate and noise will begenerated. Moreover, if the ship's bottom is not clean, any appendage will cavitatewhen the ship's speed is sufficiently high. Regular inspection and cleaning of the ship'sbottom are essential if the ship is to obtain optimum quietness at higher speeds.
3.3.4 Aircraft Noise. Noise developed by aircraft does not appreciably affect theeffectiveness of the ASW sensors. The aircraft does produce an artifact line on thepassive sonobuoy readout. Also, when an aircraft passes over a sonobuoy, a Dopplershift can be observed on the gram in the same frequency range. The artifact line iscaused by the revolution of the propellers.
3.3.5 Circuit Noise. Circuit noise is generated primarily in sonar-scanning switches,preamplifiers, connections, relay contacts, and power pack. With proper maintenance,this source of noise should not seriously affect sonar performance.
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Chapter 4
Marine Geology and Bathymetry
4.1 Introduction
Knowledge of marine geology is important for all phases of naval operations andis particularly so in undersea warfare. Specifically, this knowledge is vital whenconsidering problems in
a. transmission of underwater sound,
b. concealment of submarines,
c. false sonar targets, and
d. submarine navigation.
Bathymetry bottom types and bottom loss of major oceanic provinces areincluded in "Oceanographic Outlooks" prepared by Naval Meteorology andOceanography Command Centers. (These "Outlooks" are described inNAVMETOCCOMINST C3140.1K.) Environmental briefings generally include adiscussion on bathymetry and bottom types for the operating areas. In addition,METOC activities can prepare bottom-composition and bathymetry charts tailored tolocal operational requirements.
4.2 Bottom Topography
The ocean bottom is considered to consist of four major physiographic ormorphological provinces:
a. the Continental Shelf
b. the Continental Slope and Rise
c. the Ocean Basin, and
d. Mid-Ocean Ridges (e.g., Submarine Ridges).
In addition, many other features (for example, ridges, trenches, seamounts, andguyots) are found within these major provinces. Figure 4-1 presents nomenclature usedto identify undersea geological features.
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Figure 4-1. Nomenclature of Undersea Geophysical Features (NAVOCEANO, 1966).
4.2.1 Continental Shelf. The Continental Shelf province contains gently slopingseafloor areas extending seaward from the shoreline into water depths of 60 fathoms(110 meters) to 100 fathoms (183 meters). The seaward termination of the ContinentalShelf is the Shelf Break, an abrupt increase in the angle of the seafloor marking achange from the nearly flat shelf gradient of <1.7 meters/kilometers (slope angle =<0.1º), to the more steeply dipping (3-6º) Continental Slope below. Although theContinental Shelf is relatively flat and gently sloping, submarine hills, ridges, terraces,depressions, and steep-walled submarine canyons may be found within the shelfprovince.
1) Coastal Plain 10) Continental Rise 19) Ridge2) Estuary 11) Sea Mount 20) Rise3) Continental Shelf 12) Island Arc 21) Rift Valley4) Submarine Canyon 13) Deep Trench 22) Guyots5) Plateau 14) Deep Sea Channel 23) Volcanic Island6) Continental Slope 15) Fracture Zone 24) Metamorphic Rock7) Continental Borderland 16) Abyssal Plain 25) Sedimentary Rock8) Fault Scarp 17) Atoll 26) Basaltic Rock9) Deep Sea Fans 18) Abyssal Hills
26
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4.2.2 Continental Slope. Beyond the seaward edge of the Continental Shelf is themore steeply inclined Continental Slope. Within the Continental Slope, depth increasesrapidly from shallow water shelf areas less than 100 fathoms (183 meters) downslopeinto Ocean Basins deeper than 1,500 fathoms (2,745 meters). Continental Slopes havegradients between 1:5 (slope angle = 11.5º) and 1:25 (2.3º). Off mountainous coasts(such as the Pacific coast of North America) the Continental Slope dips at a gradient ofabout 1:20 (2.9º), but off coasts with wide, well-drained plains (such as the Atlanticcoast of North America), the slope inclines at around 1:30 (1.9º). Extreme slopes (suchas those off volcanic islands or mid-ocean ridges) may slant as much as 1:2 (30º).Extreme reverberation occurs with inclines greater than 1:10 (slope angles = > 5.7º).
4.2.3 Continental Rise. At the base of Continental Slopes the slope angle graduallydecreases as a result of sediment accumulation in the Continental Rise at the foot of theslope. The Continental Rise is typified by gradients of 1:40 (1.4º) to 1:1,000 (0.06º), andthe Continental Slope/Rise transition is usually pronounced on echo sounder records,being found at depths of >500 fathoms (915 meters). Because of its low slope angle,the Rise exhibits good bottom-bounce characteristics. The base of the Rise is markedby another marked boundary in seaward gradient, as the Rise gives way to the nearlyflat Abyssal Plains in those Ocean Basins that are not limited by trenches. The bottomof the Continental Rise is defined as the point where the seaward gradient drops below1:40 (slope angles = ≤1.4º).
4.2.4 Ocean Basin. Pacific Ocean Basins and Atlantic-type Ocean Basins differ insediment types and locations. Trenches prevent sediments of continental origin fromreaching the ocean basin floor. The Ocean Basin province includes 76 percent of theseafloor, with depths ranging from 1,500 fathoms (2,745 meters) to 3,000 fathoms(5,490 meters). Generally, Ocean Basins have average inclines of no more than 1:90(0.6°), and Abyssal Plains within the Ocean Basins have gradients of <1:1000 (slopes<0.06°). Although on the average the Ocean Basins have little vertical relief overconsiderable distances, the relief superimposed on this average incline may be at leastas rugged as the larger topographic features found on land. Present in all ocean basinsare submarine volcanoes, seamounts, and submarine mountain ridges which may risehundreds to thousands of meters above the adjacent Abyssal Plains.
4.2.5 Submarine Ridges. Great submarine mountain ranges occur in all oceans,extending for a total of some forty-two thousand miles. Submarine Ridges rise fromabyssal depths of 2,500 fathoms (4,575 meters) to depths less than 1,500 fathoms(2,745 meters) at the crest of the ridge, which may extend for thousands of milesthrough the ocean basin. In every ocean except the Pacific, these submarine ridges arefound near the center of the ocean basin, and divide the oceans into eastern andwestern basins. In the Pacific Ocean, the submarine ridge is offset closer to North andSouth America and is called the East Pacific Rise. One of the more prominentsubmarine ridges is the Mid-Atlantic Ridge, which extends from north of Icelandsouthward across the Equator until it intersects the Indian Ridge south of Africa. Inseveral places this ridge rises above sea level to form islands such as St. Peter and St.Paul Rocks, Ascension, and Tristan de Cunha.
33
There are spreading ridges and non-spreading ridges. The Hawaiian Arc is anexample of a non-spreading submarine ridge. Along the center of the Mid-AtlanticRidge and those ridges characterized by seafloor spreading at rates of less than 4-6centimeters/year is a V-shaped, steep-walled Rift Valley. Spreading ridges in thePacific Ocean extend/expand at rates of over 10 centimeters/year and do not have axialV-shaped rift valleys.
4.2.6 Seamounts. Seamounts, isolated submarine volcanoes which rise 500 fathoms(915 meters) or more above the adjacent seafloor, are present in all ocean basins.Some of these mountains have flat tops, and are called guyots or tablemounts. Atolls,round to oval islands or coral and sand surrounding a central lagoon, are found atopmany guyots in the Pacific and Indian Oceans.
4.2.7 Abyssal Hills and Oceanic Rises. Abyssal Hills are smaller submarine featureswhich rise to heights from 20-40 meters (10-20 fathoms) to a few hundred meters abovethe seafloor. In most ocean basins, Abyssal Hills flank the Mid-Ocean Ridge and otherSubmarine Ridges. Abyssal Hills may be found in other regions within ocean basins.
Oceanic Rises, areas hundreds of kilometers wide over which the surface risesseveral hundred meters above the surrounding seafloor, are found in those oceanbasins where sediments have not covered them, such as in the Pacific Basin. Oceanicrises appear to be similar to several hundred closely spaced abyssal hills in a localarea.
4.2.8 Trenches. Submarine Trenches comprise the deepest parts of the oceans.These narrow (40-120 kilometers wide), steep-sided depressions extend in curving arcs500-4,500 kilometers long near the margins of ocean basins, and the bottom oftrenches may include depths of 4,100-4,920 fathoms (7,500-9,000 meters). A "deep" isthe deepest part of any trench, but the term "deep" is reserved for water depths greaterthan 3,000 fathoms (5,490 meters). The Atlantic Ocean contains only three smalltrenches--two near Puerto Rico and the West Indies, and one east of Cape Horn nearthe Falkland Islands of the South Atlantic. The Pacific Ocean is ringed by submarinetrenches, including the deepest trench (the Marianas Trench, near Guam) and thelongest trench (the Peru-Chile Trench, which extends for 5,900 kilometers along thewestern margin of South America). In the Western Pacific, some of these trenches forma nearly continuous north-south depression from the Arctic to the Antarctic. Examplesof the depths of some of these Pacific trenches are: Kuril Trench - 34,020 feet (10,542meters); Japan Trench - 27,950 feet (9,810 meters); Mindanao Trench - 34,428 feet(10,494 meters); Challenger Deep of the Marianas Trench - 35,800 feet (10,915meters); and Tonga Trench - 35,430 feet (10,882 meters).
It should be noted that trenches appear when oceanic crust (density of 3.3 - 3.4)slides under (subduction) another oceanic crust or a continental crust of a lower density(density of 2.7). Subduction is caused by seafloor spreading, which generates thedynamic force.
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4.3 Bottom Composition
It must be understood that sediments on the seafloor are in different layers,which represent separate types such as a sandy layer overlying a clay layer. Thedifferent layers will have different geoacoustic characteristics (velocity of compressionaland shear waves, density, sound attenuation, etc.). The genesis of the different layersis the study of sedimentology, a sizable topic beyond the scope of this reference.
The geologic composition of the ocean bottom has an extremely significant effectupon the final strength of bottom-reflected sound. Depending on composition, suchinterrelated effects as reflection, absorption, scattering, attenuation, and reverberationcome into play.
The different types of bottom sediments have various effects upon soundpropagation. Factors that increase sound reflectivity are (a) an increase in the calciumcarbonate content of the sediments, (b) a decrease in sediment porosity and resultingcompaction, (c) an increase in the mean diameter of sediment particles, (d) an increasein the degree of cementation or lithification (increase in sediment rigidity), and (e) anincrease in the temperature of the sediments from one location to another.
Energy loss into the bottom depends primarily upon bottom composition,frequency of the sound transmitted, and the angle of incidence of the sound ray at thesea bottom.
Classified Marine Geophysical Survey (MGS) bottom-loss charts are producedby NAVOCEANO, upon request. Each chart delineates various acoustic provinces fromgood (#1) to very poor (#9). MGS survey data are input parameters in active- andpassive-sonar environmental service products. Bottom-type losses are for variousbottom types and are discussed in NAVMETOCCOMINST C3140.1K, U.S. NavyOceanographic and Meteorological Support Systems Manual (September 26, 1996).
A second data base designated as Low-Frequency Bottom Loss (LFBL) supportslow-frequency performance prediction capability for sonar application (50-1,000 Hz).The LFBL data base is comprised of 803 LFBL provinces, each of which have 15geoacoustic parameters. The fundamental characteristic of LFBL is that there is asingle velocity verses depth to describe acoustic propagation in the seafloor. Theseparameters describe the reflective and refractive characteristics of the ocean bottom.The LFBL implementation uses geoacoustic parameters of sediment sound speed,attenuation, density, and sediment thickness to derive bottom loss for input intoperformance prediction models. LFBL models work most accurately in abyssal plains.The use of geoacoustic parameters as the data base and a model which calculatesbottom loss ensures that future updates and expansions can occur in a routine mannerwith a minimal impact on Fleet operations.
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A third data base designated as the Geophysical Data Base (GDB) also supportslow-frequency performance prediction capabilities of sonar applications in the frequencyrange of 10-1,000 Hz. The basic refinement in data base preparation is that a GDBattempts to define the different layers within the "earth beneath the sea" marked bydifferent velocities. Unconsolidated sediments (those sediments one could mold withtheir hands) have compressional wave velocities from 1,425 meters/second. to perhaps2,000 meters/second. Once sediments are consolidated, their characteristic velocitiesrange from 2,200 meters/second. to 2,600 meters/second. Other step velocityincreases with depth are found, which the GDB recognizes in an effort to be morerealistic.
GDBs are found along continental margins such as the Barents Sea, theMediterranean Sea, the Arabian Gulf, and the north Gulf of Oman. Other areas arepresently in preparation.
4.4 Bathymetry
An understanding of the importance of bathymetry and of the use of bathymetriccharts and information is necessary for efficient ASW operations.
• Considerable detail concerning depth is available on some Dead Reckoning Tracer(DRT) plotting charts. If more specific requirements for bathymetry exist, additionaldata can be obtained from NAVOCEANO.
• DMA Magnetic Anomaly Detection (MAD) Operational Effectiveness Charts alsodepict bathymetry.
• Recommendations for the application of bathymetry to ASW operations are offeredin the following paragraphs.
4.4.1 Corrected Bottom Depth. U.S. Naval echo sounders are calibrated for a meanvertical sound speed in the water column (top to bottom) of 4,800 feet per second(1,483 meters per second). The bathymetry data shown on charts are in agreement withecho-sounder readings. Because both the echo-sounder calibration and navalbathymetric charts assume a theoretical value for sound speed in water, both areequally in error. An active or passive bottom-bounce signal will travel at the actualspeed of sound encountered at a specific depth rather than at the 4,800 feet-per-secondspeed used in echo-sounder calculations. Similar considerations apply to convergencezone calculations.
For example, a bottom-bounce range error of 732 meters (800 yards) is possiblewhen using an uncorrected depth of 4,572 meters (5,000 yards) and a target is at arange of 18,200 meters (20,000 yards). This error assumes no refraction (refraction indeep water results in longer apparent ranges) and a flat bottom.
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Correction tables, such as Matthews' tables (Matthews, 1939), or more recently,Carter's tables (Carter, 1980), and graphics (the nomogram depicted in figure 4-2) areavailable to estimate true depth for various oceanic domains.
Figure 4-2. Corrections to Chart Depth or Echo-Sounder Depth to Obtain True Depth inthe Pacific.
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4.4.2 Active Sensors. Active detection can be "bottom-limited." If the bottom depth isless than 1,000 fathoms, bottom reverberations can dominate the backgroundcontinuously. In those areas in which the bottom is hard or irregular, the rangesobtained are somewhat less than twice the water depth. An exception to this ruleoccurs in shallow water. In shallow water, extreme ranges (10,000 yards) are possible;however, with an irregular rock bottom present, reverberation usually produces signalsat ranges exceeding 5,000 yards (4,572 meters).
4.4.3 Convergence Zones. Convergence zone detection is unlikely in warm ormoderately warm water in depths of less than 1,200 fathoms (2,196 meters) except inthe Mediterranean Sea.
Seamounts, guyots, islands, and other bottom features will disrupt convergence-zone activity and cause larger shadow zones.
4.4.4 Bottom Bounce. Where the bottom slope is greater than 1:10 (5.7º),reverberation due to roughness of the slopes is so intense and complex that bottombounce is essentially useless. If possible, slopes exceeding 1:20 (2.9º) should beavoided by a vessel operating active sonar in the bottom-bounce mode.
4.4.5 Bathymetric Interference. It is important to consider bathymetric interference ofthe deep sound channel (DSC) since low-frequency acoustic energy can be transmittedover long ranges via this sound propagation path (figure 4-3). Although the principle isthe same for each season, the effect is more pronounced in the summer when the DSCdeepens.
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Figure 4-3. Bathymetric Interference.
39
Chapter 5
Water Masses, Currents, and Basic Oceanographic Analyses
5.1 General
Water masses, oceanic fronts, cold- and warm-core eddies, internal waves, andcurrents are physical features of the oceans having a significant effect on ASWoperations. Oceanographic analysesfor example, sea-surface temperature (SST) andmixed-layer depth (MLD)are major inputs in the construction of other environmentalproducts. A brief discussion of these basic oceanographic products, as well as variousenvironmental features of the oceans, is presented in this chapter.
5.2 Sea-Surface Temperature (SST) Charts
The daily SST chart is the most accurate of the various existing oceanographicanalyses because of its greater amount of input data.
The SST chart portrays the average temperature pattern for a 24-hour period.Daily charts show little day-to-day variation in absolute value. It is not unusual for asea-surface temperature chart to remain essentially unchanged for as long as 5 days.Longer term changes in the ocean thermal structure are gradual, being affected mainlyby the revolution of the earth around the sun and by significant meteorological changessuch as the passage of a storm or prolonged periods of abnormal weather.
The distribution of sea-surface temperature is controlled by three major factors:currents, seasonal effects, and latitude. Temperatures in the vicinity of a major currentare influenced more by the current than by either seasonal or latitudinal factors.Seasonal and latitudinal factors include the influences of convective mixing, mechanicalmixing, surface heating, precipitation, evaporation, and sea-ice distribution.
5.3 Water Masses
Water masses are formed in source regions and acquire specific temperatureand salinity (thermohaline) characteristics. As a water mass spreads or moves into anew area, it retains many of its original characteristics but is also modified by surfaceheating/cooling (combined with vertical mixing), evaporation, and mixing with otherwater masses. Classical analysis of water masses enables oceanographers to identifywater masses in areas other than their source area; this is important in determining thelarge-scale world ocean circulation. Thermohaline properties are fairly homogeneoushorizontally within classical water masses; however, there are often weak gradientsacross most water masses.
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Figures 5-1 and 5-2 are samples of computer-produced SST analyses.
Figure 5.1. Sea-Surface Temperatures from the FLENUMMETOCCEN OTIS 4.0Analysis for the Gulf Stream Region.
Figure 5.2. Temperatures at 400 m Depth from the FLENUMMETOCCEN OTIS 4.0Analysis for the Gulf Stream Region.
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For Navy applications, water masses are sometimes defined differently thanclassical water masses to relate the water characteristics to acoustics better. Classicalwater mass definitions depend heavily on the relationship between temperature andsalinity (T-S). Acoustic applications usually emphasize temperature characteristicsalone for water mass definition, especially in deep water. Between two water massesthere is a transition zone known as a front. One depiction of classical water masses isshown in figure 5-3.
NOTE: Transition Zones are Hatched Regions.
Figure 5-3. Major Ocean Regions of the Northern Hemisphere.
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Water masses for Navy applications can be inferred from figures 5-4 and 5-5.These two figures actually denote mean positions of ocean fronts; water masses existbetween the fronts. For example, in figure 5-4, Sargasso Water lies between fronts 03(Gulf Stream South Wall) and 06 (Subtropical Convergence). Between fronts 02 (GulfStream North Wall) and 11 (Shelf/Slope Front) lies Slope Water. In the case of majorwestern boundary currents such as the Gulf Stream, the water between the North andSouth walls (fronts 02 and 03) is actually the warm core, a feature that exists to about150-200 meters.
5.3.1 Ocean Fronts
An ocean front is the interface between two water masses having differentphysical characteristics. Usually fronts show strong horizontal gradients of temperatureand/or salinity, with resulting density variation and current shear. Some fronts whichhave weak horizontal gradients at the surface have strong gradients below the surface.In some cases, gradients are weak at all levels, but variability across the front, asreflected by the shape of the thermal profile, is sufficient to complicate or influencesound transmission.
A useful definition for the purpose of naval operations can be stated as follows: Atactically significant front is any discontinuity in the ocean which significantly alters thepattern of sound transmission and propagation loss. Thus, a rapid change in the depthof the sound channel, a difference in the sonic-layer depth, or a temperature inversionwould denote the presence of a front.
In figures 5-4 and 5-5, the fronts have a "beginning" and an "end." These areregions where significant mixing has occurred between the water masses on either sideof each front. There is no precise definition of where a front begins or ends along its"downstream/upstream" axis; horizontal gradients across a front gradually decrease asincreased mixing occurs across the front. In summer, the seasonal thermoclinedecreases the temperature difference across fronts in the near-surface layer. In warmerbasins such as the Gulf of Mexico during summer, it is difficult to determine the positionof major fronts (Loop Current) from satellite IR imagery, which can detect only surfacegradients.
The important fronts in the northwest Atlantic include the North wall of the GulfStream and the Shelf/Slope front along the U.S. coast north of Cape Hatteras. In thenorthwest Pacific, major fronts are found along the north side of the Kuroshio and alongthe south side of the Oyashio currents. Both temperature and salinity dynamics areimportant to the formation and location of these stronger fronts; however, it is thetemperature differences/gradients that are important to acoustic applications for theNavy. Typical horizontal temperature gradients across the Gulf Stream are 0.2o to1.0oC (0.4o to 2.0oF) per nautical mile.
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5.3.1.1 Typical Location of World Fronts
Figures 5-4 and 5-5 illustrate approximate positions for ocean fronts around theworld. Tables 5-1 and 5-2 provide names of ocean fronts shown in figures 5-4 and 5-5,respectively. The keys in figures 5-4 and 5-5 show the relative strengths of the fronts.
Figure 5-4. Mean Positions of Ocean Fronts in the Atlantic Ocean.
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Table 5-1. Names of Ocean Fronts in the Atlantic and Indian Oceans.
Atlantic Ocean Mediterranean Sea01 Loop Current (Gulf of Mexico) 41 Huelva Front02 Gulf Stream North Wall 42 Alboran Sea Front03 Gulf Stream South Wall 43 Almeria-Oran Front04 North Atlantic Current 44 Tyrrhenian Divergence
(North Polar Front) North Wall 45 Maltese Front05 North Atlantic Current 46 Ionian Front
(North Polar Front) South Wall 47 Aegean Outflow06 Sargasso Sea Front 48 Levantine Basin Front
(Subtropical Convergence) 49 Balearic Front07 Azores Front 50 North African Current08 Guiana Current 51 Strait of Sicily Front09 Northwest African Upwelling 52 Ligurian Sea Front10 Gulf of Guiana Front11 Shelf/Slope Front12 Labrador Front13 West Greenland Front Indian Ocean14 Denmark Strait Front 53 Arabian Upwelling15 East Greenland Front 54 Somali Upwelling16 East Icelandic Front 55 Equatorial Countercurrent Front17 Iceland-Faeroe Front 56 --------18 Jan Mayen Front 57 Australian Subantarctic Front19 Greenland-Norwegian Sea Front20 Norwegian Coastal Front21 North Cape Current22 Murman Coastal Current23 -------- (Future Use)24 Pechora Current25 Persey Current26 Bear Island Front27 West Spitzbergen Front28 East Spitzbergen Front29 Benguela Upwelling30 South Atlantic Subtropical
Convergence31 Antarctic Convergence (South
Polar Front)32 Antarctic Convergence33 Canary Current34 North Equatorial Current35 Gulf Stream Extension
45
Figure 5-5. Mean Positions of Ocean Fronts in the Pacific Ocean.
46
Table 5-2. Names of Ocean Fronts in the Pacific and Indian Oceans.
Pacific Ocean64 East Pacific Equatorial Front65 North Pacific Tropical Convergence66 East China Coastal Current67 Yellow Sea Warm Current67 East Korean Warm Current69 Limon Front70 Maritime Province Cold Current71 Tsugaru Front72 Tsushima Front73 North Wall Kuroshio Front74 South Wall Kuroshio Front75 North Pacific North Subtropical Front76 North Pacific South Subtropical Front77 Subarctic/Subtropical Transition78 South Oyashio Front79 North Oyashio Front80 North Pacific North Subarctic Front81 North Pacific South Subarctic Front82 California Front83 Alaskan Stream North Wall84 Alaskan Stream South Wall85 Soya Front86 Kuril Front87 Bering Sea Front88 South Pacific Tropical Convergence89 Mid-Tasman Convergence90 South Pacific Subtropical Front91 Sakhalin Front92 West Kamchatka Front93 East Kamchatka Front94 East Subarctic/Subtropical Transition
Indian Ocean55 Equatorial Countercurrent Front56 West Australian Front57 Australian Subantarctic Front
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5.3.1.2 Acoustic Effects from Fronts
In terms of acoustics, the following changes can be of significant importance asa front is crossed.
1. Near-surface sound speed can change by as much as 100 feet/second. Thisis due to the combined effect of changing temperature and salinity, with temperatureusually being the dominant factor. Shallow-water seasonal salinity changes cansometimes dominate.
2. Sonic-Layer Depth (SLD) can change by as much as 1,000 feet from one sideof a front to the other during certain seasons.
3. A change of the in-layer and below-layer gradient usually accompanies achange in surface sound speed and SLD.
4. Depth of the Deep Sound Channel Axis (DSCA) can change by as much as2,500 feet when crossing from one water mass to another.
5. Increased biological activity generally found along a front will increasereverberation and ambient noise.
6. Sea-air interaction along a frontal zone can cause a dramatic change in seastate and thus increase ambient noise levels.
7. Changes in the vertical arrival angle of sound rays as they pass through afront can cause towed-array bearing errors.
It is clear that any one of these effects can have a significant impact on USWoperations. Together they determine the mode and range of sound propagation andthus control the effectiveness of both short-range and long-range acoustic systems.The combined effect of these characteristics is so complex that it is not always possibleto develop simple rules for utilizing ocean fronts for USW tactics. For example, thewarm core of the Gulf Stream south of Newfoundland will bend sound rays downwardinto the deep sound channel, thereby enhancing the receiving capability of a deepreceiver. The same situation with a slightly shallower bottom south of Maine maycreate a bottom-limited situation, and the receiving capability of the same sensor will beimpeded. In view of the above, the acoustic effects of a front must be determined foreach particular situation by using multiprofile (range-dependent environment) acousticmodels. The input of these models can come from detailed oceanographicmeasurements or from historical data in combination with surface frontal positionsobtained from satellites.
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5.3.1.3 Determining Frontal Locations from Satellite Data
Most fronts exhibit surface-temperature signatures that can be detected bysatellite infrared (IR) sensors and used in determining frontal locations. Figure 5-6 is anexample of a satellite IR image obtained by the TIROS-N satellite (#N-14) showing theinflux of warm water from the Kuroshio Current flowing into the Sea of Japan betweenSouth Korea and Japan (Tsushima Strait/Korean Strait region). This is the beginning ofthe Tsushima Warm Current. In the upper right-hand corner of the image the coldwaters of the Liman Current (North Korean Cold Current) are outlined. Between the twocontrasting water masses, at approximately 39N, lies the transitional water mass.Because surface-temperature gradients are not always reliable indicators of thesubsurface front, satellite images must be interpreted by a skilled analyst, preferably incombination with data from other sources (such as XBTs).
Automatic interpretation of satellite data is being developed using techniquesgenerally known as Automatic Imagery-Pattern Recognition or Artificial Intelligence.Satellite IR sensors can read only the upper few millimeters of the water mass, whichallows cloud cover to prohibit surface-temperature observations.
Figure 5-6. TIROS-N Satellite Infrared (IR) Image.
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5.3.1.4 Frontal Gradients
The strength of a front may be indicated by the measured temperature gradient,or horizontal temperature difference over a particular distance. This gradient isreported by the NAVOCEANO Warfighting Support Center in degrees F per 10 nauticalmiles. All frontal temperature gradients in RAINFORM GOLD messages produced byNAVOCEANO use this standard.
Table 5-3 divides the classification of the fronts into four categories. Weakfronts are represented by dashed lines and may not be strong enough to be significantin USW operations. Moderate fronts are shown as solid lines and may sometimesinfluence USW operations. Strong fronts are expressed as heavy lines and usuallyhave significant effects on USW operations. Very strong fronts (usually only GulfStream Current, Kuroshio Current, and Oyashio Current regions) are shown as heavylines and will have significant effects on USW operations in their regions.
Table 5-3. Classification of Ocean Fronts.
RelativeStrength
Maximum Changein Sound Speed
(ft/sec)Change in SLD
(ft)
SST Gradient(Deg F/10 nmi) Depth (ft) Persistence
Very Strong >100 >500 >8 >3,000 Year-roundStrong 70-100 250-500 5-8 1,200-3,000 Year-roundModerate 50-70 100-250 3-4 300-1,200 Year-roundWeak <50 100 ≤2 300 Seasonal
5.3.2 Eddies
An "eddy" in oceanography is a large rotating mass of warm or cold water. Theycan be considered circular fronts with water trapped inside having different physicalproperties from the surrounding waters. Eddies can range from 60 to 200 nauticalmiles in diameter and can extend to depths of 800 meters (3,000 feet) or more. Largereddies are found on both sides of major ocean fronts, particularly those involving majorcurrents such as the Gulf Stream in the Atlantic and the Kuroshio in the Pacific. Theseeddies are caused by the breaking off of a large meander from the current (figure 5-7),similar to the way an oxbow lake is generated by the cutoff of a river meander. Notethat the water inside the cold eddy was "captured" from the relatively cold Slope watermass during a Gulf Stream meander and is surrounded by warmer Sargasso water.Warm eddies (figure 5-8), which occur north (to the left) of the Gulf Stream, contain"captured" Sargasso water and are surrounded by Slope water.
Eddies rotate relative to the surrounding watermass. Some eddies arestationary, but most drift in a direction opposite to the direction of their source current.Eddies can separate from the source current and drift until they are absorbed into thesurrounding watermass, or they may recombine with the source current. Surface ductsoutside an eddy will pinch out at the boundaries.
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Figure 5-7. Formation of Warm and Cold Eddies from the Gulf Stream.
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5.3.2.1 Warm Eddies
Warm eddies form on the cold side of the watermass interface and contain waterfrom the warm side. They seldom last longer than six to eight months, due to heattransfer to the atmosphere and to the surrounding watermass. Figure 5-8 illustrates atypical sound-speed structure of a warm eddy.
Within a warm eddy, the SLD and DSC are usually deeper than that of thesurrounding water. During winter, a deep SLD can occur in the eddy center, providinggood surface duct propagation. During summer, a negative gradient can dominate theeddy and produce strong downward refraction.
When entering a warm eddy and moving toward the center, expect improvingsurface duct quality, improving active ranges, increasing ranges to the first CZ,decreasing depth excess, decreasing ability to access the DSC, and an improving BBpropagation quality. Warm eddies are considered a submarine haven throughout theentire eddy.
5.3.2.2 Cold Eddies
Cold eddies form on the warm side of the watermass interface and contain acore of cold water from the cold side. They may last up to two years before beingabsorbed into the surrounding watermass. Cold eddies sink at a non-uniform rate,which removes them from a surface environment and atmospheric effects. While nolonger detectable from satellite imaging, cold eddies are present in the watermass andcan influence acoustic propagation. Figure 5-9 provides an example of a cold eddysound- speed structure.
The characteristics of a cold eddy include shallower SLD than the surroundingwatermass, a weaker below-layer gradient, poor surface ducting, good cross-layercoupling, better CZ conditions, and better DSC coupling.
Within a cold eddy, the CZ range will be shorter than that for the surroundingwatermass and improve acoustic conditions for both detection and counterdetection.Cold eddies are considered a submarine haven only at the boundaries.
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Figure 5-8. Vertical Cross-Section of a Warm Eddy.
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Figure 5-9. Vertical Cross-Section of a Cold Eddy.
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5.3.3 Fronts and Eddies in Shallow Water
When fronts occur in water depths less than several hundred meters,bathymetric effects play a large role in the frontal characteristics. In these regions,fronts tend to be "locked" into the bottom; the envelope of observed surface paths ismuch wider than the envelope of the bottom paths. For example, the bottom of theIceland-Faeroes front intersects the top of the ridge between Iceland and the FaeroeIslands (water depths of 400-600 meters) and slopes upward toward the northeast. TheShelf/Slope front off the U.S. east coast intersects the bottom along the shelf break (70-120 meters). Meanders of the topographically locked fronts are much smaller then themeanders of the "classical" fronts such as the Gulf Stream. The eddies formed bythese shallow-water fronts are also much smaller and often stay embedded in the frontinstead of breaking off.
Even major fronts can be affected by the bathymetry. The path of the GulfStream south of Cape Hatteras conforms generally to the shelf break; large "fold-back"meanders do not occur, but small eddies are formed during gentle offshore meandersand stay embedded in the shoreward side of the front. These frontal eddies aredynamically different than the cutoff eddies north of Cape Hatteras, in that they are coldcore and on the shoreward side of the front.
Typical locations where topographically locked fronts occur are along continentalshelf breaks/slopes and ridges that act as a deep barrier between two water masses. Insome cases, shelf break fronts separate relatively cooler, fresher shelf water fromwarmer, saltier offshore water. This causes a temperature inversion; the colder shelfwater lies over the warmer offshore water with density compensated by salinity. Inthese situations, strong acoustic upbending occurs, such as across the Shelf/Slopefront off the U.S. east coast. Because density gradients across these fronts are oftensmall, interleaving occurs where "fingers" of alternating layers of warm and cold water inthe vertical are formed. These frontal structures are very complex and difficult to modelor predict.
Fronts that are locked into the bottom are often stronger at depth than at thesurface, especially in summer, when seasonal thermoclines tend to wash acrossdifferent water masses. Figure 5-10 shows sound-speed profiles (constructed fromobserved temperature/salinity profiles) on both sides of the Polar front in the BarentsSea during August. This front generally follows the 250-meter isobath and separatesthe colder Polar water to the northeast from the warmer Norwegian or Atlantic water tothe southwest. Note that at the surface the historical profiles from both water massesoverlap; at depth there is significant difference in sound speed between the two watertypes. More importantly, the Polar water will produce upbending at depth, whereas theAtlantic water will generally produce downbending.
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Figure 5-10. Observed Sound-Speed Profiles Across the Polar Front(Curved path on location chart is mean position of the front).
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5.3.4 Internal Waves
The thermocline is an interface between colder, denser water below, andwarmer, less dense water above. Because it is a density interface, gravity waves canpropagate along it, just as surface waves propagate along the density interfacebetween the air and water. We call these waves which move along the thermoclineinternal waves because they propagate inside the ocean. Internal waves can be foundmoving along other vertical density gradients such as the strong salinity gradients foundin both the Baltic and Black Seas. Although these "salinity gradient" waves areimportant to other maritime professions such as shipping and fisheries, the internalwaves that concern naval operators are those which have different temperatures and,therefore, different sound speeds above and below the waves.
The main effect of internal waves on acoustics is the oscillation of the depth ofthe thermocline, either the main (permanent) thermocline or the seasonal thermocline.This causes fluctuations in the sonic-layer depth and produces changes in the trappingfrequency and propagation loss in the surface duct. Internal waves travel more slowlythan surface gravity waves and generally have much greater periods, lengths, andheights (amplitudes). Internal waves in deep water may have periods of tens ofminutes to hours, wavelengths of kilometers, and amplitudes of tens of meters. Inshallow water these values are, respectively, minutes, kilometers, and meters. Internalwaves in shallow water may also cause significant current oscillations superimposed onthe existing background currents. Most ocean basins have internal waves that oscillateat tidal frequencies and are called "internal tides."
5.4 Currents
Major currents in large ocean basins are mainly forced by large-scale winds andhorizontal density differences (thermohaline circulation) and are modified by the Coriolisforce and bathymetry. A simplified depiction of currents can be inferred fromtemperature contours from an SST chart. The thermal gradient (distance between theisotherms) is indicative of current strength. The maximum gradient coincides with thegreatest horizontal temperature gradient (closely packed isotherms). In reality, aninstantaneous picture of the ocean surface circulation is a complex combination oflarge- and small-scale eddy and frontal features. General ocean circulation is illustratedin figures 5-11, 5-12, and 5-13; these atlas-type depictions do not represent the truehorizontal resolution or time-varying nature of the surface currents.
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58
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Forcing mechanisms for currents in shallow water or restricted channels includewind, density differences, tides, freshwater runoff, and atmospheric pressuredifferences; bathymetry and Coriolis force modify the flows. Currents in these regionscan be driven both locally and non-locally. For example, currents on a continental shelfdriven by local winds may be somewhat predictable by measuring or modeling the localwind field; however, large-scale, wind-driven circulation from the deep ocean mayimpinge upon the shelf and mask the locally driven currents. Another complexity ofshallow-water currents is that the driving mechanisms are more variable than theirdeep-water counterparts; thus, climatological values of winds and freshwater runoff areless useful for prediction purposes.
Currents are important for a variety of reasons for ASW and submarineapplications, including planning sonobuoy fields (drift), mine drift, submarine drift, andsubsurface rescue operations. Currents are extremely important for numerous MineWarfare and Special Warfare applications.
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5.5 Variability of the Ocean Environment
5.5.1 General
Variability within ocean areas can be classified as:
a. Geographic Variability - ocean fronts and eddies, bathymetry, or bottomtopography.
b. Temporal Variability - changes in the temperature caused by cloud cover,storm passages, and meandering of ocean currents.
c. Cyclic Variability - seasonal influences.
d. Geophysical Variability - anomalous temperature distribution caused byintermittent influx of water from another area (e.g., El Nino).
5.5.2 Scale of Variability
Different types of ocean features have different size scales, as shown in table5-4.
Table 5-4. Scale of Variability.
Feature Example SizeLarge-scale oceaniccirculation systems
Current as seen in climatological oceanplot charts
10,000 – 5,000 km(5,000 – 2,500 nmi)
Major ocean currents Gulf Stream, Kuroshio 5,000 – 1,000 km(2,500 – 500 nmi)
Medium-scale ormesoscale features
Warm and Cold Eddies 1,000 – 100 km(500 – 50 nmi)
Small-scale features River runoff, wind-influencedconvergences and divergences
100 – 1 km(50 – 0.5 nmi)
Fine-scale features Sonic-layer depth, in-layer and below-layer gradients, sound channels
1 km – 1 m(0.5 nmi – 1 yd)
Microscale features Turbulence, diffusion of heat and salt 1 m – 1 mm(3 ft – .04 in)
Medium-scale and fine-scale variability are prevalent in approximately 60 percentof the world's oceans and includes bottom topography, transition zones, and dailyheating and cooling. Variations of vertical fine-scale features, such as sea-surfacetemperature, sonic-layer depth, gradient below the layer, sound-speed profile, and seastate, are all considered in detection range calculations. However, thebathythermograph observations that are the basis for most of these parameters do not
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measure medium-scale horizontal changes and therefore cannot be extendedindiscriminately in all directions about the point at which they are taken. They are takenat point "A" along a transit route. Sonar-performance prediction based on thisinformation must take this into account.
5.5.3 Detection-Range Calculations
Range-independent detection-range calculations, through use of an on-scenebathythermograph (BT) or by a computer using either the on-scene BT or arepresentative BT, do not consider medium-scale features. These features are bestaccounted for through knowledgeable analysis by the oceanographer/acoustician, bythree-dimensional ocean numerical models, and by use of range-dependentpropagation loss models.
Experience shows that a detection-range calculation in a highly variable area(such as a shallow-water/littoral environment) has a reduced temporal and spatialvalidity. Rapid fluctuations in detection range are common. Range calculations do notconsider medium-scale features that are inherent in the ocean environment. Variabilityin the environment is both vertical and horizontal, and it causes a correspondingvariability in detection ranges and inaccuracies in detection-range calculations.
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Chapter 6
Bathythermograph Observations
6.1 General
Bathythermograph (BT) observations provide operational forces with accuratethermal information for specific operating areas at specific times. The observations givea recording of temperature versus depth. The recording presents the vertical thermalstructure of the water column at the specific location where the observation was taken.The thermal structure can be used to determine the mixed-layer depth (MLD) andtemperature gradients below and in the mixed-layer (surface layer) and providetemperature/depth data for environmental prediction systems in construction of sound-speed profiles.
BT observations further provide a vital input to the production of oceanographicand acoustic support products described in NAVMETOCCOMINST 3140.1K, U.S. NavyOceanographic and Meteorological Support Systems Manual (Rev 9/96).
A BT observation may be representative of conditions for a large area in somehomogeneous regions, whereas an observation taken near the northern edge of theGulf Stream, the Kuroshio Current, or in the vicinity of the Azores may be valid only fora small area. Frequent bathythermograph observations should be made during USWoperations to obtain an adequate sample size in order to define the subsurface thermalstructure. Monitoring of several environmental parameters will aid in determining whenthe environment has changed enough to warrant dropping a new BT.
The first step in measuring the subsurface vertical water-mass environment islaunching an accurate BT. Any change of equal or greater value in the listedparameters should be followed by a new environmental/acoustic range prediction.
Sea State Any ChangeWind Speed 5 knotsSea Surface Temperature 2o FSonic Layer Depth (SLD) 50 feetGradient Below Layer 0.5o F/100 feetAmbient Noise 2 dBWater Depth 100 fathomsBottom Province Any ChangeBiologics Low to High/High to LowOwn Ship's Speed 3 knots
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6.2 Expendable Bathythermographs
Expendable bathythermographs (XBT) are available to the operational forces.These particular BTs utilize a probe containing a thermistor connected to a fine spool ofwire. The wire is unreeled from the spool as the probe sinks vertically through the waterat a known rate of descent. As the probe descends, a chart-type recorder converts timeand thermistor resistance into depth (meters or feet) and temperature (Fahrenheit orCelsius). The maximum depth of the temperature recording used in the Fleet is beingstandardized to 2,500 feet for all the launching platforms (aircraft, ship, submarine, orhelicopter).
6.3 Bathythermogram Encoding Procedures
Bathythermograph recorders provide continuous traces of temperature versusdepth. These traces are essentially a connected series of short line segments. Pointsto be digitized are those points where the profile slope or gradient makes a significantchange. These points are called inflection points . The inflection points which mustalways be recorded are the sea-surface temperature (SST), mixed layer depth (MLD),and the deepest useable point of the trace.
0
50
100
150
200
250
300
350
400
450
Temperature, oC
Figure 6-1. Sample XBT Recorder Trace.
0 5 10 15 20 25 30
D e p t h
(m)
00 , 17.827 , 17.0
60 , 16.8100 , 15.7
117 , 15.8131 , 14.3
180 , 15.0230 , 14.4
250 , 12.5261 , 09.0
357 , 08.0
450 , 07.5
Ship: USS STETHEM(DDG 63)
Cruise: 16 - 4Latitude: 4314NLongitude: 17124WTime: 1600 GMTDay/Mo/Yr: 21/08/98Observation #: 25Btm. Depth (m): 1910
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6.3.1 Bathythermograph Log. Figure 6-1 is an illustration of a typical bathythermogramtrace. Instructions for preparing the bathythermograph log are provided with eachpackage of Form CNMOC 3167/2 (Rev. 5/96). Each package contains detailedinstructions on:
a. how to mark the XBT recorder chart,
b. encoding XBT data on the bathythermograph log sheet,
c. preparing a bathythermograph message for transmission using thebathythermograph log (figure 6-2), and
d. how to obtain additional log sheets.
Instructions are also contained in NAVMETOCCOMINST 3140.1K (Rev. 9/96), U.S.Navy Oceanographic and Meteorological Support System Manual.
6.3.2 Quality Control of XBT Data. A proper quality control of XBT data is veryimportant in making reliable acoustic predictions. Observers need to be familiar withproper procedures for the handling and storage of XBT probes and the commonmechanical and electrical causes of XBT malfunctions. Probes should be stored in theiroriginal containers in a vertical position with the protective cap down and should be keptaway from extremes of temperature and humidity. Failure to follow these proceduresmay result in damage to the very thin copper wire and its insulation. NavalOceanographic Office Reference Publication 21, Guide to Common ShipboardExpendable Bathythermograph (SXBT) Recording Malfunctions, contains a thoroughdiscussion of the various causes of erroneous data such as wire stretch, wire leak, wirebreakage, improper recorder calibration, improper launcher grounding, andelectromagnetic interference. When operating in areas of high oceanic variability, XBTusers should be familiar with the oceanographic conditions to be expected in the area.For example, subsurface temperature inversions are quite common in the slope frontnorth of the Gulf Stream. Such inversions may be associated with tactically significantsound channels yet may be easily mistaken for an XBT malfunction. Significantchanges in the XBT trace should be carefully evaluated and encoded.
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Figure 6-2. Sample Bathythermograph Log.
Proper care must also be taken when digitizing an XBT trace. Each time a probeis launched, the date, time (GMT), latitude and longitude should be noted and recorded,even if the trace looks like a bad BT. It is also useful to note and record the depth to thebottom. When recording depth-temperature pairs, record only those points where theslope of the trace changes. Do not record depth-temperature pairs at even depthssuch as 100, 200, 300, or 400 meters unless an inflection point or slope changeoccurs at such a depth. (This procedure is referred to as Standard Depths and isinaccurate.) Particular care must be taken in digitizing and recording SST and mixed-layer depth.
6.4 Bathythermograph Interpretation
Interpretation of the BT trace by personnel onboard the platform taking the BTobservations can provide real-time environmental information which could be significantto their operations. Procedures to derive this information from a BT trace are presentedin subsequent paragraphs. Several applicable reference documents are available for
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training in digitization and evaluation of BT traces. Evaluating and EncodingBathythermograph (BT) Data, METOC 60-1T-9701 (Rev 5/97), Naval OceanographicOffice; Guide to Common Shipboard Expendable Bathythermograph (SXBT) RecordingMalfunctions, RP 21, Naval Oceanographic Office, February 1981; and SomeGuidelines for the Submarine-Launched Expendable Bathythermograph (SSXBT)System, RP 39, Naval Oceanographic Office, October 1981 are examples of theavailable documentation.
6.4.1 Mixed-Layer Depth (MLD). The MLD is defined as the point of maximum nearsurface temperature. The MLD is normally located in the seasonal thermocline.
A negative temperature gradient (temperature decreasing with depth), withincertain limits, will compensate for the increase in sound speed with depth due topressure; this results in a constant sound speed with depth. These limits per 100-feet(30-meter) increments of depth are as follows:
Table 6-1. Negative Temperature Gradients Required to Compensate for Depth.
oF oC oF oCa. 0.2o 0.1o at 40o 4.4o
b. 0.3o 0.17o at 55o 12.8o
c. 0.4o 0.22o at 65o 18.3o
A more sharply negative temperature gradient will result in decreasing sound speedwith depth, while any gradient more positive than the preceding limits will result in anincreasing sound speed with depth. Sound speed increases with depth when the watertemperature is constant with depth (isothermal).
Reliable, on-scene measurements of mixed-layer depth are difficult to obtain.Large variations may be encountered in the amount of transient heating and internalwaves present near current boundaries and related oceanic fronts. Caution is thereforeurged in selecting a representative value of MLD. Frequent BT measurements will haveto be taken to obtain this value.
6.4.1.1 MLD Computation. The MLD may be determined from a BT trace. The SLD iscorrectly determined from the SSP derived from a BT trace entered into theenvironmental prediction system.
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a. If the maximum temperature is at the surface, the MLD is zero (figure 6-3).
Figure 6-3. Mixed Layer at Surface (Depth = 0).
b. If the trace is isothermal or has a slight negative gradient (less than thepreviously stated limits), the depth of the mixed layer is that point at which the gradientbecomes more negative than the limits stated (figure 6-4).
Figure 6-4. Mixed Layer at Depth - Example A.
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c. Except when the temperature gradient beneath the maximum temperaturedepth is less than the stated limits, the mixed layer is at the deepest point at which themaximum temperature occurs (figure 6-5).
Figure 6-5. Mixed Layer at Depth - Example B.
6.4.2 Temperature Gradient. To compute the gradient below the layer, in the layer, orfor a uniform segment of a BT trace:
a. the temperature and depth of the beginning point are labeled T1 and D1,respectively;
b. the temperature and depth of the segment termination point are labeled T2
and D2, respectively; and
c. using these values and the following formula, the gradient for the segment iscomputed in degrees Fahrenheit per 100-foot increments (or Celsius per 30 meters).The formula is
100(T1 - T2) = gradient in oF/100 feet (D1 - D2)
30(T1 - T2)_ = gradient in oC/30 meters (D1 - D2)
The sign of the result indicates whether the gradient is positive or negative. Forexample, to compute the temperature for a segment, that is, 340 feet (Temp 58.0oF) to540 feet (Temp 46.0oF), use the formula stated above. Substitute in the equation withthe following values:
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D1 = 340; D2 = 540; T1 = 58.0; and T2 = 46.0
100(58.0 - 46.0) = 100(12.0) = 1200 = -6 = -6oF/100 feet (340 - 540) -200 -200 1
Thus, the gradient for the segment is a negative gradient of 6 degrees per 100 feet.
6.4.3 Sound Channels. The existence, depth, thickness, and relative strength of deepand secondary sound channels should not be inferred from BT traces. A BT trace doesnot reflect sound speed values needed to predict acoustic range propagationaccurately. Always evaluate the existence of and measure any and all soundchannel strength, thickness, and structure points from the SSP developed on anenvironmental prediction system. Do not use the BQH-7 SVP readout for accuratemeasurements of sound speed, as its salinity function is not variable with either locationor season. The refractive processes that create a sound channel are discussed insection 7.2.4.
6.4.4 Convergence-Zone (CZ) Prediction. Accurate CZ predictions are based on aknowledge of the environment, usually obtained by dropping a BT and entering thetemperature/depth pairs into onboard computerized environmental prediction systems.The applicable models and data bases use a combination of in-situ and historical datato predict the acoustic structure of the environment.
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Chapter 7
Environmental Effects Upon Sound Propagation in the Deep Ocean
7.1 Depth and Seasonal Effects
Temperature is the dominant variable affecting the speed of sound in deep-ocean areas. At depth, however, temperature changes are slight, and changes inpressure due to depth are the primary influence. Over much of the deep-ocean areathe temperature structure can be divided into three parts:
a. relatively warm surface layer, often showing isothermal structure,
b. main thermocline in which the temperature decreases at a moderately rapidrate with depth, and
c. the colder deep-water layers in which the temperature decreases slowly withdepth.
The surface layer is affected by meteorological changes and sea-roughnesschanges. Through wind action, the surface layer often shows complete mixing; thesound-speed structure is then an isothermal channel paralleling the ocean surface. Thesurface layer changes in a daily, seasonal, and areal manner. The deep-water layers,to a lesser degree, also show variation only with respect to areal changes (Officer,1958). Representative temperature and SSPs for the deep ocean are shown in figure7-1.
These profiles consist of three basic parts:
a. a varying, seasonally dependent portion (surface layer),
b. an underlying more stable portion (main thermocline layer) extending to mid-depths, and
c. the deep-water layer.
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Figure 7-1. Basic Temperature and Sound-Speed Structure of the Deep Ocean.
The seasonally dependent, near-surface region may, at any point in time, consistof an isothermal (constant temperature) layer in the upper part that varies in thicknesswith the season. A negative gradient is below. Pressure produces a slightly positivesound-speed gradient in the isothermal layer. In summer, the isothermal layer is,typically, about 15 meters (50 feet) deep in tropical waters. The abrupt negativetemperature gradient below this depth is called the seasonal thermocline. In the winter,the greater wind action and cooling produce deeper mixing and increase the depth ofthe isothermal layer to several hundred feet. In addition, the temperature and soundspeed at the surface decrease with the colder air temperature encountered. Figure 7-2gives examples of hourly, weekly, seasonal, and geographic sound-speed profilevariations resulting from temperature changes. Hourly variations can occur near thesurface of the sound-speed profile due to diurnal heating. Deeper variations, but still inthe upper portion of the profile, can occur on a weekly basis, due to weather frontalpassages, extended periods of precipitation, overcast skies, or high winds. Seasonalvariations occur throughout the main thermocline. Geographic variations extend to thedeep-water layer.
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Figure 7-2. Sound-Speed Profile Variations.
The deeper, semipermanent profile consists of a negative temperature gradientfrom the base of the season thermocline to a depth of about 900 meters (3,000 feet).Below this main thermocline, the temperature decreases only slightly with an increase indepth. Temperature decrease is so slight that a steady sound-speed increase isproduced due to the increase in pressure. The resultant sound-speed structure belowthe permanent thermocline consists of a positive gradient of about 0.017 sec-1.
7.1.1 SSPs. A number of distinct advantages are gained by using an SSP instead of aBT. Significant features available in an SSP include the surface sound speed, sonic-layer depth, and deep sound-channel axis depth. An SSP further provides the user witha more accurate means of evaluating the following for passive detections.
a. The presence, quality, and usefulness of surface duct signals.
b. The presence, quality, and usability of sound channels in the operating area.
c. The probability, horizontal range, and annulus of convergence zones in theoperating area.
SSP overlays can be constructed to read an SSP directly from an XBT trace(Huff, n.d.). An SSP can also be constructed by computing the sound speed atsignificant and mandatory points in the bathythermogram.
7.1.1.1 SSP Construction. The sound speed as a function of depth, temperature, andsalinity can be determined from the nomograms in figures 7-3 and 7-4. An SSP canthen be constructed. Figure 7-3 gives accurate values of sound speed for anycombination of depth and temperature encountered in the ocean for the average open-sea salinity of 35 O (O = ppt = parts per thousand).
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Figure 7-3. Sound-Speed Nomogram (35 O Salinity) (NOLTR, 1963).
Figure 7-4. Salinity Correction to Sound Speed (Urick, 1979).
7.1.2 Horizontal Sound-Speed Gradients. While the major changes in sound speedoccur with depth, horizontal changes also occur. Variations in the horizontal planecontribute to sonar bearing errors, particularly with passive detection systems wherelonger detection ranges are expected. Horizontal gradients are especially significant inthe Strait of Gibraltar and in the northern edges of both the Gulf Stream and KuroshioCurrent. These gradients can be expected in areas of strong currents and ocean fronts,internal ocean waves (paragraph 5.3.4), and uneven heating and cooling of the oceansurfaces. Figure 7-5 illustrates a typical refraction situation where the maximum bearing
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error (θ) is 5 degrees. The sound rays bend toward colder water, resulting in a range-bearing error of 10 percent (for example, a 2-mile error when the range is 20 nauticalmiles).
Figure 7-5. Horizontal Gradient - Sonar-Bearing Area (Hanssen, 1967).
7.2 Sound Propagation Paths
Sound propagation paths may be divided into several types:
a. Direct Path
b. Surface Duct
c. Half Channel
d. Sound Channel
e. Bottom Bounce
f. Convergence Zone
7.2.1 Direct Path. Direct path is the first portion of all the propagation paths. It isdefined as a short range propagation where there is approximately a straight-line pathbetween the source and receiver, with no reflection from the surface or bottom and onlyone change of direction due to refraction. In this case, propagation loss equalsspherical spreading loss plus attenuation loss. An example of direct-path propagation isshown in figure 7-6.
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Figure 7-6. Direct Path Propagation Path.
7.2.2 Surface Duct. It was previously stated that one of the usual characteristics of thethermal structure of the ocean is the existence of a mixed surface layer. Surface layers(ducts) exhibit a slightly positive sound-speed gradient, and sound rays emitted from asource within the layer, or trapped in the layer, will be refracted upward and surface-reflected, as shown in figure 7-7. The limiting ray is characterized by the fact that itsvertex sound speed is equal to the maximum sound speed in the near-surface profile.The depth at which this sound speed occurs is defined as the SLD, which in most casesis equal to the MLD. MLD may be read from a BT trace, whereas the SLD must be readfrom an SSP trace to assure accuracy. As a general rule, surface-duct propagation willimprove as the layer depth increases.
Figure 7-7. Surface Duct Propagation Path with Limiting Rays and Shadow Zone.
7.2.2.1 Shadow Zone. A shadow zone is depicted in figure 7-7 beneath the layer atranges beyond the direct or close-in sound field. The limiting ray becomes horizontal atthe base of the layer; rays leaving the source at greater angles than the limiting ray aresent downward. Ray theory indicates that no energy should penetrate the shadowzone, but this is not the case. Some sound energy does enter the shadow zone. It isgenerally attributed to scattered sound from the sea surface and to leakage of soundout of the channel due to the frequency-dependent trapping qualities of the duct
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(paragraph 7.2.2.4). At low frequencies, diffraction is important. The diffraction(leakage), in turn, depends upon the sharpness of the discontinuity between the mixedlayer and the thermocline, as well as upon the sound-speed gradients in the layer andbelow the layer in the thermocline, called the "below-layer gradient" (Urick, 1967).
The intensity of sound within a shadow zone decreases exponentially withdistance from the limiting ray. The exponent is a function of both source frequency andthe magnitude of the negative gradient, increasing slowly with both. Thus, as frequencyand the magnitude of the below-layer gradient increase, the intensity of sound within theshadow zone decreases.
7.2.2.2 SLD. The SLD is defined as the depth of the maximum near-surface soundspeed above the deep sound channel axis. The sound field in a layer depends greatlyupon the layer depth, as illustrated in figure 7-8. The deeper the layer, the farther thesound can travel without having to reflect off the surface, and the greater the amount ofenergy initially trapped. Each contact with the surface tends to scatter sound energyout of the surface layer unless the sea surface is perfectly smooth. On the average,sound can reach any given range beyond the direct path zone with fewer bouncesunder deep-layer conditions than under shallow-layer conditions. With both the sourceand the receiver in the layer, the deeper the layer is, the better the propagation.
Figure 7-8. Layer-Depth Surface Effect Upon Bounced Sound Rays (Bell, 1966).
7.2.2.3 Gradient in the Layer (In-Layer Gradient). Weak temperature gradients in thelayer can play a major role in the determination of the amount and strength of soundtrapped in the surface duct. In order to have a usable duct, the sound-speed gradientmust be positive.
7.2.2.4 Low-Frequency Cutoff. At low frequencies, sound energy will not be trapped inthe surface duct. This occurs when the frequency is low enough that its correspondingwavelength is too large for all the energy to fit within the duct.
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The cutoff frequency formula applies to variable conditions of temperature,salinity, and depth within the surface duct.
**fc = 0.3978 x Co1.5
Zld x (∆ C).5
where fc = cutoff frequency in Hz,
∆C = Cld - Co,
Cld = sound speed at sonic layer depth, and
Co = surface sound speed,
Zld = sonic layer depth,
0.3978 = constant, independent of measuring system (feet or meters).
**Reference: "Submarine Tactics (U)" Vol. 7, No. 2, pp. 2-6, September 1986,COMSUBDEVRON 12. CONFIDENTIAL
For the important special case of an isothermal, isohaline surface duct, thisequation becomes
*fc = 1.06 x 106
Zld 3/2
whereZld = sonic layer depth, in feet
fc = frequency, in Hz.
For example, in a surface duct 100 feet thick, the lowest trapped frequency, or fc,is 1,100 Hz. The cutoff is not sharp, but the energy in frequencies lower than fc will notbe ducted.
*Reference: Urick, Principles of Underwater Sound
Utilizing figure 7-9, the surface duct cutoff frequency can be derived for either thesingle-variable formula or the three-variable formula, as previously given in paragraph7.2.2.4. Comparison to, or verification of, the Geophysical Fleet Mission ProgramLibrary (GFMPL) Acoustic Range Prediction System, based on either MS-DOS Version
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8.0 or NT Version 2.1, is possible. The single-variable formula, isothermal and isohalinewater conditions, relates to the MS-DOS version, while the three-variable formularelates to the newer NT version. For figure 7-9, the surface sound speed was keptconstant at 5,000 fps with variations in sonic layer depth and ∆C. Interpolation must beapplied when the surface sound speed differs from 5,000 fps, with higher cutofffrequencies for higher surface sound speeds and lower cutoff frequencies for lowersurface sound speeds.
7.2.2.5 Wind-Wave Effects on Layer Depth. The depth of the isothermal layer is greatlyinfluenced by the mixing action of ocean waves. As illustrated in figure 7-10, if themixing due to wave action is sufficiently high and persistent, a relatively thick layer ofwell-mixed, and therefore isothermal, water will develop (profiles B and C). If there is
Figure 7-9. Surface Duct Cutoff Frequency Nomograph.
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little or no wave action, heating from the sun's rays will tend to warm the surface,resulting in the gradual production of a negative gradient, which will result in a "zerolayer depth" (profile A).
Figure 7-10. Wind-Wave Mixing Action Sequence (Bell, 1966).
Profiles D and E show the development of transient thermoclines caused bycontinued heating of the ocean surface. This situation occurs frequently on summerafternoons at lower latitudes and is referred to as the "afternoon effect." Transientthermoclines are generally not assumed to be of major significance in VP/VS ASWoperations, but their magnitude may severely limit the effectiveness of hull-mountedsonar systems. The effectiveness of shipboard hull-mounted systems is considerablyreduced if the transient is deeper than 25 feet and its magnitude is greater than-0.3°C/30 meters (-0.6°F/100 feet). The "afternoon effect" is not normally included inSLD prognoses, but the possibility of the existence of transient thermoclines can bededuced from weather conditions.
Transient thermoclines usually result in a minor sound channel existing within thesurface duct. These short-term fluctuations in the general thermal structure can causeconsiderable fluctuation of the sound field, especially below the transient thermocline. Ifthe SLD fluctuations are pronounced (greater than 35 feet), there may be aconsiderable fluctuation of the sound field at the convergence zone due to fluctuationsin the depth excess.
7.2.2.6 Seasonal Effects on SLD. In the winter months, strong winds, storms, highseas, weak solar radiation, and a great amount of mechanical mixing produce thedeepest layer depths: 120 to >275 meters (400 to >900 feet) in cold-water areas. Inthe summer, the converse is generally true. In warm water, layer depths vary from 30 to60 meters (100 to 200 feet) in winter and summer. The shallowest layer depths arefound in the tropics.
The transitional seasons (spring and autumn) produce a complex vertical sound-speed gradient. In the spring, cool water moving southward will become heated at the
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surface, producing shallow layer depths. The sharp vertical temperature gradients inthe cold waters north of the Gulf Stream are modified less rapidly than those in the coolmasses to the south, where less pronounced upper-layer gradients exist. As a result,deeper layers will appear in the cool waters south of the Gulf Stream before they appearin the cold-water masses of more northerly latitudes. Transitional season charts includetypical features of both summer and winter charts.
7.2.2.7 Gradient Below the Layer. The gradient of the profile just below the SLD is amajor factor in sensor placement decisions, as is illustrated in figure 7-11. A strongnegative gradient will refract sound energy sharply downward, forcing it into therelatively short-range bottom bounce propagation path. Placing sensors deep will allowthem a greater probability of detection. A weak negative gradient will refract soundenergy in a less vertical manner, possibly allowing it to enter into sound channel or CZpropagation paths.
Figure 7-11. Examples of Below-Layer Negative Gradient Variations.
7.2.3 Half Channel. Half-channel conditions exist where the water is essentiallyisothermal from surface to bottom, so that sound speed increases continuously withincreasing depth. Under these conditions, the greatest sound speed is at the bottom ofthe ocean, and sound energy will be refracted upward, then reflected downward at thesurface, and refracted upward again. The effect is similar to a strong surface duct, andlong ranges are possible. Half-channel propagation is common during winter in theMediterranean and will almost always occur under the ice in polar regions.
7.2.3.1 Arctic and Half-Channel Propagation. In the Arctic Ocean region, the lack ofsolar heating prevents the formation of the main thermocline evident in the lowerlatitude oceans. A positive sound speed gradient extends up to shallow depths in thesummer and all the way to the ice boundary in the winter.
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In the summer, in open water, a thin surface duct (normally ≤ 100 feet) can occur.Strong salinity-generated positive sound speed gradients can occur in the surfaceregion due to melting ice or fresh water effluent from rivers in near coastal regions;thereby removing any solar-generated negative gradients.
With the positive sound speed gradient being constant and dominant throughseasonal and diurnal variations, the sound speed profile tends to be relatively constantover long ranges. However, with ambient noise a function of ice coverage, wind speed,temperature, and location with respect to the Marginal Ice Zone (MIZ), there is noguarantee of extended acoustic detection ranges.
The interaction of upward-refracted energy with the under-ice surface isdependent upon the roughness of the ice, which serves as the major cause ofattenuation. Due to the upward refraction of the energy and the dominant effect of theice cover on attenuation, bottom bounce, or interaction with the seafloor, is a minorsource of propagation loss in the Arctic region.
7.2.3.2 Propagation in Arctic Waters vs. Ice-Free Waters. As compared to non-Arcticacoustic propagation, Arctic half-channel may be expressed in general as:
Propagation: Better - due to the dominant upward refraction of the positivegradient (dependent upon ice roughness).
Less Variable - due to more constant meteorologicalconditions.
Ambient Noise: Lower - under a continuous ice cover with rising temperature
Higher - in a broken ice cover (as in the MIZ) or in fallingtemperatures.
Surface Scattering: Higher - due to under-ice surface roughness.
Volume Scattering: Lower - due to lower occurrence of marine life.
SOUND SPEED
DEPTH
RANGE
Figure 7-12. Half-Channel Propagation Path.
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7.2.4 Sound Channels. A Sound Channel is defined as a region in the water columnwhere sound speed first decreases with depth to a minimum value, and then increases(figure 7-13). Above the depth of minimum value, the sound-speed gradient is negative,and sound rays are bent downward. Below the depth of minimum value, the sound-speed gradient is positive, and sound rays are bent upward. Sound rays within thechannel having the proper frequency and angle may be trapped.
Figure 7-13. Sound Channel Description (Lehmann, 1998).
7.2.4.1 Secondary Sound Channels. Secondary sound channels occur in the upperlevels of the water column in the thermocline, within and below the surface layer. To beconsidered useful, a secondary sound-channel must be within the depth capabilities ofthe applicable tactical sensor; its thickness (∆Z) must be at least 100 feet; and itsstrength (∆C), or difference in sound-speed between the boundaries and the axis, mustbe at least 2.5 feet/second. Secondary sound channels are important and useful in bothactive and passive detection through a range of depths and frequencies. Threeparameters are used to describe a secondary sound channel:
a. depth of the axis (SSCA = Secondary Sound Channel Axis),
b. thickness of the channel (∆Z), determined by the difference in depth betweenthe upper and lower boundaries (thickness = ∆Z = Z2 - Z1), and
c. strength of the channel (∆C), determined by the difference in sound speedbetween the axis and the boundary (strength = ∆C = Cb - Ca).
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Figure 7-14. Secondary Sound-Channel Properties (Lehmann, 1998).
7.2.4.1.1 Locations. Secondary sound channels are found in numerous regions of theworld’s oceans. They are observed in the vicinity of strong ocean fronts. The depths ofthese secondary channels have a wide variation from within the surface layer to greaterthan 1,000 feet. General worldwide locations of secondary sound channels are shownin figure 7-15, with example depths illustrated in table 7-1.
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Figure 7-15. Worldwide Locations of Secondary Sound Channels.
Table 7-1. Location and Depths of Secondary Sound Channels.
Area Location When FoundDepths where SSCs maybe encountered (in feet)
A Norwegian Sea spring through fall 300 to 700Barents Sea spring 200 to 400North Cape year-round 600 to 800
B Baffin Bay fall 250C Davis Strait summer 300D WESTLANT Gulf Stream spring through fall 200
WESTLANT Sargasso Sea year-round 450 to 1,100E EASTLANT Near Gibraltar year-round 400 to 1,100
EASTLANT Other Areas spring through fall 300 to 500F MIDLANT spring 300 to 400G Gulf of Alaska spring through fall 250 to 450H Off Japan Kuroshio Current summer 450 to 750I Arabian Sea year-round 1,100 to 1,700J South Indian year-round 450 to 500 west to 800 to
1,300 east
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Secondary sound channels may be formed by two different methods. First, alayer of cold, low-salinity water intrudes between layers of warmer, more saline water.The depth of the intrusion is determined by the density of the colder water. This mayoccur in regions of fronts and eddies. Second, a depressed sound channel may formwhen the decrease in sound speed from a weak negative temperature gradient (nearisothermal water) is more than compensated for by the effect of increasing depth.Examples of the variety in channel depths are shown in figure 7-16.
7.2.4.1.2 Cutoff Frequency. An equation for cutoff frequencies, fc, in secondary soundchannels is
** fc = 0.2652 x Ca 1.5
∆Ζ x (∆C) .5
where,fc = cutoff frequency in Hz.
∆Z = sound channel thickness,
∆C = Cb - Ca,
Ca = sound speed at sound channel axis depth,
Cb = sound speed at channel boundaries.
0.2652 = constant, independent of measuring system (feet or meters).
**Reference: "Submarine Tactics (U)" Vol. 7, No. 2, pp. 2-6, September 1986,COMSUBDEVRON 12.
An easy method of determining the cutoff frequency fc of a secondary soundchannel is to use the nomogram illustrated in figure 7-16.
7.2.4.2 The Deep Sound Channel. The Deep Sound Channel (DSC) is sometimesreferred to as the Primary Sound Channel and has been well-known since World War II,when the earliest investigations were made and a Sound Fixing and Ranging (SOFAR)network was established in the Pacific. In later years it has provided the necessarylong-range propagation paths for investigations of the attenuation coefficient in the seaat low frequencies. Today the DSC remains the best natural non-radio channel for long-distance communication, should such communication become necessary. The soundfrom a small (1-2 lb) explosion can be heard above background at distances ofthousands of miles.
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The DSC is caused by the fact that the deep sea is warm on the surface and coldbelow. The surface-warming effect is not sufficient to extend all the way to the bottomand is limited to the upper part of the water column, where it forms the mainthermocline. Below it, the sea is nearly isothermal (near 40°F) and therefore has apositive velocity gradient (figure 7-17).
Accordingly, a depth of minimum sound-speed exists, called the "axis" of theDeep Sound Channel Axis (DSCA), toward which sound rays are continuously bent byrefraction (figure 7-17). This minimum sound-speed depth varies from around 4,000feet (1,225 meters) in mid-latitudes to near the surface in polar regions. However, notall propagation paths in the DSC are entirely refracted paths. When the source orreceiver, or both, lies beyond the limits of the channel, only reflected paths thatencounter either surface or bottom, or both, are possible. Refracted Surface Reflected(RSR) paths are reflected above by the surface, and refracted below by the sound-speed gradient. Refracted Bottom Reflected (RBR) paths are refracted above andreflected below by the bottom.
Figure 7-16. Sound Channel Low-Frequency Cutoff Graph.
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Attenuation coefficients for RSR paths have been measured and found to behigher by a factor of 1.5 than those for entirely refracted paths, probably because oflosses at or near the sea surface. Doubly reflected paths from both surface and bottom,together with scattering from these boundaries, form the weak, rapidly decreasingsound signal "tail" that extends beyond the time of sudden cessation of long-rangeSOFAR signals.
The upper and lower limits of the DSC are determined by the SSP and the waterdepth. The highest values of sound speed in the profile usually occur in the near-surface region. If the water depth is sufficient and the water mass is structuredproperly, there is a depth below the DSCA where the sound speed increases to thesame value as that at the top of the DSC (often the SLD). This depth, referred to as theCritical Depth, forms the lower boundary of the DSC. When the water depth is less thanthat required for Critical Depth, a near-surface source will not be within the DSC and,therefore, will not propagate sound for long ranges via the DSC. The concepts of DSC,Critical Depth, and water depth are illustrated in figure 7-18.
(DSCA)
DeepSoundChannelAxis
Figure 7-17. Deep Sound Channel, as displayed on Geophysical FleetMission Program Library (GFMPL 8.0).
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In Profile 1, the water depth equals the Critical Depth, and the entire watercolumn forms the DSC. In Profile 2, the water depth is less than Critical Depth;therefore, long-range DSC propagation is lost. In Profile 3, the water depth is greaterthan Critical Depth, allowing a Depth Excess below the lower DSC boundary. In Profile3, the DSC extends from Point A to Point B. Depth Excess is required for anyprobability of CZ propagation.
7.2.5 Bottom Bounce. Reflections from the ocean bottom can extend propagationranges. At low frequencies, refraction within the bottom is the predominant mechanismfor returning energy. The effect of bottom bounce is to return to the depth of thetransducer sound energy that has been carried downward by the depression angle ofthe transmitted pulse or by refraction. In figure 7-19, bottom-bounce rays arerepresented as straight lines, and refraction effects have been ignored.
Major factors affecting bottom-bounce transmission include water depth, angle ofincidence, frequency, bottom composition, and bottom roughness. A flat ocean bottomproduces the greatest accuracy in estimating range and bearing in the bottom-bouncemode. In active detection, the bottom-bounce transmission mode can produceextended ranges with fewer shadow zones. More than one bottom-reflected path existsbetween the sonar and target. Figure 7-19 shows the four major paths that involve asingle bottom reflection. With the existence of these paths, multipath addition canincrease the received signal level.
Figure 7-18. Sound-Speed Profile, DSC, and Critical Depth.
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7.2.6 Convergence Zones. Convergence Zones (CZ) are regions at or near the oceansurface in which focusing of sound rays occurs, resulting in higher sound levels. Theexistence with a positive gradient below, and at least 200 fathoms of depth excessbelow the of convergence zones requires a negative sound-speed gradient at or nearthe surface, Critical Depth for a 50-percent probability of CZ occurrence, as shown infigure 7-20a. For example, sound rays leaving the near-surface region due todownward refraction at shorter ranges are refracted back to the surface because of thepositive sound-speed gradient produced by greater pressure at increased oceandepths. These deep-refracted rays often become concentrated at or near the surfacethrough the combined effects of downward and upward refraction. Partial focusingbegins to occur at depth when sound rays approach each other, as shown in figure 7-20a. The focusing effect produced by this convergence forms intense sound fields(caustics) that may be exploited for submarine detection. When referring to figure 7-20a, it must be remembered that it is a vertically exaggerated example ofconvergence zone propagation. The actual ray trace of the energy contained withinthe convergence zone bundle travels a path similar to that illustrated in figure 7-20b.The departure angle of the energy leaving the source usually must be near a 15° downangle or less to be retained in the convergence zone path.
Surface
Target
Bottom
Figure 7-19. Bottom Bounce Multipaths.
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Convergence zone existence is dependent upon several factors: the sound-speed at source depth, the Critical Depth, and the depth excess or sound-speed excessvalues. A minimum depth excess of 200 fathoms or a minimum sound-speed excess of22 feet/second is required for a 50-percent probability of CZ occurrence with a near-surface source. A near-surface source is at the SLD or shallower (within the layer).With a depth excess of 300 fathoms or a sound-speed excess of 33 feet/second, theprobability of CZ occurrence increases to 80 percent for a near-surface source. Figure7-21 illustrates the change in probability of CZ occurrence with change in the amount ofdepth excess or sound-speed excess.
Figure 7-20a. Convergence Zone (CZ) Propagation and Terminology (Swanson, 1974).
Figure 7-20b. Convergence Zone Propagation Path, Undistorted Scale (Lehmann, 1992).
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In the Mediterranean Sea, the bottom water is much warmer than in the AtlanticOcean, and the sound speed near the bottom is consequently higher. The Critical Depthis therefore much shallower, and the acoustic energy is refracted upward at a muchshallower depth than elsewhere. Convergence zone ranges are therefore much shorterthan those generally found in the Atlantic or Pacific oceans.
There are other factors governing convergence zone propagation. Seamounts,islands, and other features will disrupt convergence zone paths. (See chapter 8 underdiscussion of Topographic Shading.)
7.2.6.1 Convergence Zone Range. Convergence zone ranges vary widely according toseveral factors, such as water depth, surface temperature, sound-speed profile, andsource depth. Examples of typical ranges to first CZ are 60 kyds in the mid-Pacific and33 kyds in the Mediterranean, as shown in table 7-2.
Table 7-2 shows the approximate relationship between surface temperature, thewater depth in fathoms required for a usable convergence zone to be present, and therange to the first CZ. This table allows for a 200-fathom depth excess and assumes thesurface duct to be absent.
Figure 7-21. Probability of Convergence Zone (CZ) Occurrence.
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Table 7-2. Typical Convergence Zone Ranges (NUSC).
Area SurfaceTemperature
Minimum Depth for CZOperation
Range toFirst CZ
(°F) (°C) fm m ft kyd kmNorth Pacific 50 10.0 1,270 2,324 7,620 47 43
55 12.8 1,610 2,946 9,660 52 4760 15.6 1,900 3,477 11,400 56 5165 18.4 2,150 3,934 12,900 60 5570 21.1 2,400 4,392 14,400 64 5775 23.9 2,600 4,758 15,600 66 6080 26.7 2,800 5,124 16,800 69 63
North Atlantic 50 10.0 1,050 1,920 6,300 46 42Norwegian Sea 50 10.0 1,680 3,074 10,080 53 48Mediterranean Sea 67 19.4 800 1,464 4,800 33 30
7.2.6.2 Convergence Zone Width. The width of the CZ is a result of complexinterrelationships and cannot be correlated with any specific factor. In practice,however, the width of the zone is often on the order of 5 to 10 percent of the range. Itcan be determined accurately on a propagation loss curve by placing the Figure of Merit(FOM) line on top of the propagation loss curve and noting where the FOM lineintercepts the CZ inner and outer annuli. The CZ width is the difference in rangebetween these two points.
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Chapter 8
Environmental Effects on Sound Propagation in Shallow Water
8.1 Introduction
USW doctrine defines shallow water as water less than 100 fathoms deep(continental shelf). Using this general definition, 7.6 percent of the world’s oceans areshallow water. For most naval operations, the most critical strategic and tacticalsignificant shallow-water regions are those continental shelf/slope areas (includingstraits and choke points) adjacent to major land masses.
From an acoustic viewpoint, shallow water includes any water mass that cannotsupport CZ or deep sound channel (DSC) sound propagation paths. The loss of thelong-range purely refractive propagation paths forces a dependence on normally shorterdetection range non-refractive paths. When combined with the high variability in bothtemporal (time) and spatial (size/location) aspects, these factors create a much moredifficult USW environment than deep water.
The term "littoral" is defined as the region which horizontally encompasses theland/water-mass interface from 50 statute miles ashore to 200 nautical miles at sea.This littoral region extends vertically from the bottom of the ocean to the top of theatmosphere and from the land surface to the top of the atmosphere. The term "shallowwater" refers only to the vertical extent from the ocean/atmosphere interface to thebottom of the ocean. The two terms, littoral and shallow water, are often intermixed indiscussion and print, but in fact are not interchangeable. Caution should be observedwhen designing briefs or presentations to specify between the two.
8.2 Environmental Factors
Numerous environmental factors influence sound propagation in shallow water.These factors, in turn, are affected by season, geographic location, water-massstructure, frequencies of interest, biologics, and interaction with humans.
8.2.1 Sea Surface Temperature (SST). Significant horizontal variations in temperaturestructure often occur over short distances in shallow waters, and refraction in thesehorizontal gradients assumes importance seldom encountered in the open ocean,except perhaps in the vicinity of the Gulf Stream and Kuroshio/Oyashio current systems.Due to seasonal runoff, coastal water temperature and current temperature can varydramatically over an annual period.
8.2.2 Salinity. Salinity, the amount of dissolved solids in seawater, has a significanteffect on the speed of sound in shallow water. Changes in salinity values (measured inparts per thousand, O) cause changes in the acoustic properties of the water. The
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speed of sound varies by approximately 1.4 meters per second (4.6 feet per second) foreach part per thousand change in salinity.
In shallow water, salinity can become a very important acoustic factor affectingUSW under the following circumstances: freshwater intrusion from a river or fjord orfreshwater formation from ice melt. A major intrusion of freshwater into a saltwater bodycan create a salinity front. In the frontal region containing the freshwater, sound speedwill be lower within the extent of the freshwater influence. An in-situ SSP will reflect theextent of the freshwater influence.
8.2.3 Layer Depths. MLDs, and resulting sonic layer depths (SLDs), over thecontinental shelf tend to vary more from the seasonal mean than do those in deepwater. Additionally, more marked and sudden variations in both time and space are tobe expected.
8.2.4 Sound Channels. Secondary sound channels (SSCs) frequently occur in shallowwaters because of the intermixing of waters of differing temperature and salinity. Asthese waters intermingle and try to sort the mixture out according to density, they tendto resemble a poorly shuffled deck of cards. Erratic BT traces and weak, short-lived/short-extent sound channels result. These SSCs are seldom of sufficient extent orpersistence to be tactically useful to USW forces.
8.2.5 Water Depth. When the water depth/wave length ratio is less than unity, sound ofthat frequency is propagated only to short ranges. The lack of any depth excess orsufficient water depth to allow pressure to overcome the temperature influence on thesound speed gradient prevents the formation of any longer range sound propagationpaths.
8.2.6 Bottom. Shallow-water bottom composition and topography control thereflective capabilities of the bottom and the attenuation of sound energy. These factorsalso control the degree of reverberation that masks target echoes.
8.2.7 Shallow-Water Acoustics. The principal difference between shallow-water anddeep-water sound transmission is the effects of interference produced by multiplereflected transmission paths. These effects are dependent on several environmentalfactors, the more important of which are:
a. depth of the waterb. topography, composition of the bottom, and sea state, andc. the sound speed structure
8.2.8 Shallow-Water Ambient Noise. Deep-water ambient noise has well-defined levelsbased on sea state and shipping density, whereas shallow-water levels varyconsiderably. This fluctuation in shallow-water noise levels allows only roughpredictions of expected ambient noise. In situ measurements are very important in
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littoral waters. Sound-producing marine life and man-made noise (industrial andmaritime) contribute much to the variability of shallow-water noise levels, along with thedomain effect of the bottom (basins, plateaus, ridges, canyons, etc.).
In the frequency range 100 Hz to 1,000 Hz, shallow-water ambient noise levelsare about 9 dB higher than in deep water for the same sea state and shipping density.
8.2.9 Sea-Ice Shallow-Water Ambient Noise. In shallow waters that ice over, sea icecan significantly affect ambient noise levels. Its influence on noise levels dependsprimarily on the state of the ice, that is, forming, water surface covered, or breaking up.If no mechanical or thermal pressure is being exerted on the ice, the noise level isgenerally low during ice formation. The quietest condition is ice-covered water whenthe ice is neither growing, breaking up, nor ridging or hummocking. Ambient noise mayactually be attenuated by the dampening effect of the ice cover. Considerable amountof noise is generally associated with the breakup and hummocking of ice. Thecharacteristic sounds of ice under stress (moaning, screeching, scraping) create a highlevel of continuous interference to passive sonar. This noise peaks near 500 Hz atabout 70 dB and falls off 3-5 dB per octave from there. Figure 8-1 shows the effects onfrequency in the region of the ice edge.
Figure 8-1. Variations of Ambient Noise Near Compact Ice Edge Under Sea State 2Conditions (redrawn from O.I. Diachok and R.S. Winokur, 1974).
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8.2.10 Biological Noise. The effect of biological activity on overall ambient noise levelis more pronounced in shallow coastal waters than it is in the deep ocean. It is alsomore pronounced in tropic and temperate zones than it is in colder regions.
In coastal waters, snapping shrimp and certain species of fish are the maincontributors to ambient noise. Snapping shrimp generally congregate in watersshallower than 30 fathoms, and colonies inhabit areas of coral, rock, shell, andvegetation-covered bottoms. They are found between 40 degrees north and 40degrees south of the equator. Over a shrimp bed, levels as high as 86 dB have beenrecorded in frequencies ranging from 100 Hz to 10 kHz. Snapping shrimp noise variesdiurnally; usually the levels at night are about 5 dB above those of day.
Schooling fish such as croakers can increase background noise considerably incoastal waters. As with snapping shrimp, the individual contribution may not besignificant, but large numbers of these fish can effectively mask a quiet diesel-electricsubmarine. Most sonic fish are migratory; thus, noise levels in a given area mayfluctuate throughout the year. Nearly all littoral areas have some sonic species, buttemperate and tropical waters contain greater numbers of the known sound producers.Rock, coral, and sand bottoms are the preferred habitat of most sonic fish. Feeding,spawning, and migratory activity of schools of sonic fish put about 74 dB of noise intothe water at frequencies ranging from 20 Hz to 4 kHz.
Marine mammals are common inhabitants of coastal waters; examples arewhales, porpoises, seals, sea lions, walruses, and manatees. Locally, where somespecies congregate in herds, a considerable increase in background noise can beexpected. Since many of these animals are migratory, their contributions to ambientnoise in any given area may be only transitory. Marine mammals are worldwide inoccurrence and are generally more common in temperate and polar waters than intropical. Noise from porpoises has been recorded ranging from 7 Hz to 196 kHz, atlevels around 100 dB. Marine mammal noise increases slightly in the warmer months.
8.2.11 Environmental Factor Variability. Extreme variability in the water mass and seafloor typifies shallow-water regions throughout the world. An assortment ofenvironmental factors, listed in table 8-1, has a direct effect on that variability.
Table 8-1. Environmental Factors Affecting Shallow-Water Variability.
Environmental Factors1. Tides2. Deep-water intrusion3. Upwelling4. Continental runoff (freshwater from rivers, snow and ice melt, etc.)5. Increased sediment deposits6. Landmass influences on dynamic oceanographic and atmospheric forces7. Large concentrations of sea life8. Shipping activity
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8.3 Environmental Characteristics of Shallow Water
The operational and strategic roles of targets and types of targets found inshallow water typically differs from their counterparts in deep water. Not only is theshallow-water environment itself unique, the acoustic response of this environment andthe acoustic USW operational scenarios have unique features as well. Some of themany distinctive aspects of shallow water are grouped into the three general categoriesof environment, acoustics, and operations as shown in tables 8-2, 8-3, and 8-4.
Table 8-2. Aspects of the Shallow-Water Environment.
Shallow-Water Environment1. High variability in temperature and salinity that significantly affects soundspeed.2. Irregular bathymetry, including bottom debris, pinnacles, and reefs.3. Differing sediments than those found in deep water.4. Tide and current effects.5. Differing biological population and density from that of deep water.6. High levels of wind, surf, shipping noise, and possible drilling noise fromoffshore oil rigs.
Table 8-3. Aspects of Shallow-Water Acoustics.
Shallow-Water Acoustics1. Lack of CZ, Deep Sound Channel, and other long-range propagation paths.2. High reverberation levels.3. Dominating role of bottom loss.4. Repeated boundary interactions.5. Complexity of multipath structure.6. High and variable ambient noise levels.7. Currently unpredictable acoustic propagation conditions.8. Acoustic sensor system depth restrictions.
Table 8-4. Aspects of Shallow-Water Operations.
Shallow-Water Operations1. Greater likelihood of low doppler, quiet targets.2. Greater likelihood of diesel-electric targets.3. Greater likelihood of shallow-running targets.4. Targets have greater opportunity to exploit the environment.5. Targets may be operating in familiar coastal home waters.6. Targets may have limited speed and depth available.7. Surface ships may have to operate in restricted waters.8. Air assets may be airspace limited.9. Greater likelihood of land-based air support.10. Proximity of nearby land targets of general strategic importance.11. High-density shipping regions.
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8.4 Propagation Paths
Given the extreme variability of the shallow-water USW problem, acousticpropagation conditions will normally be highly unpredictable. Direct Path and BottomBounce (BB) are the dominant propagation paths, with Surface Duct, Secondary SoundChannel, and Half Channel possibly being available.
8.5 Tactical Considerations and Search Planning
The requirement for careful study of area bathymetric characteristics using thebest possible bottom contour charts is much more critical in shallow-water regimes.Water depths, seafloor slope, and canyons should be noted. Wrecks and pinnaclesshould be marked or circled. The 100-fathom curve should be highlighted for easyreference. Shallow-water search is generally performed by passive acoustic sensorscomplemented by high-frequency active sonars and nonacoustic sensors. Activesensors and preparation for an urgent attack should be primary considerations inlocalization efforts.
A thorough knowledge of threat mission and platform characteristics is asimportant as area and environment factors. Search planning against a dieselsubmarine should place emphasis on nonacoustic search modes. Passive sensorsshould be placed in high-probability areas to detect snorkel periods, or in barriersacross expected transit lanes. Active sensors would be employed during periods ofalert (e.g., following nonacoustic detection, receipt of intelligence information, datumhandoff, or in isolated tactical situations such as strategic choke points). Searchplanning against a nuclear submarine should place emphasis on acoustic sensors, butshould always be complemented by nonacoustic search.
Through studying the unique characteristics of the shallow-water environment,research has identified phenomena called environmental and slope effects. Thefollowing effects might be exploited to enhance acoustic USW operations:
a. Upslope Enhancement (USE)b. Downslope Enhancement (DSE)c. Topographic Shadingd. Topographic Noise Stripping (TNS)
8.5.1 Slope Enhancement. For slope enhancement to occur, several factors must besatisfied. The slope of the ocean bottom must fall within a limited range of degrees, andthe bottom must have a low loss coefficient for bottom interaction at the frequencies ofinterest. The deep-water environment must support the long-range refractivepropagation paths. Correct sensor placement relative to the shelf-slope breakpoint isessential. Upslope and downslope enhancement of acoustic energy increasesdetection ranges due to the phase addition of energy overcoming losses from bottom
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interactions and surface reflections. Figure 8-2 represents upslope enhancement, whileFigure 8-3 represents downslope enhancement.
For upslope enhancement, the source must be in deep water, while the receivermust be in shallow water. This geometric relationship generates an "upslope"environment for the receiving sensor. In USE, a CZ or DSC acoustic path is convertedto a BB path as the energy moves from deep water into shallow water (upslope).
Figure 8-2. Upslope Enhancement.
For DSE, the source must be in shallow water and the receiver must be in deepwater. In DSE, a BB acoustic path is converted to a CZ or DSC path as the energymoves downslope.
Figure 8-3. Downslope Enhancement.
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8.5.2 Topographic Shading. The concept of topographic shading is relatively simple; itoccurs due to the removal of required water-mass depth by seamounts, islands, andridges. The effect is the loss of the required water depth to attain the depth excess fordeep sound channel or CZ propagation. The lack of sufficient water depth will createshadow zones extending outward from the region of bottom interaction (figure 8-4).
Figure 8-4. Topographic Shading.
Knowledge of CZ annulus ranges within the water mass is important.Seamounts, islands, or ridges occurring at one-half the range to the annulus will providethe highest probability of CZ interruption. Figures 8-5 and 8-6 illustrate the bottominteraction at one CZ range (no interaction) and at one-half the CZ range (largeattenuation of the signal).
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Figure 8-5. No Topographic Shading with Seamount at One CZ Range.
Figure 8-6. Topographic Shading with Seamount at One-Half CZ Range.
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8.5.3 Topographic Noise Stripping (TNS). With TNS, ambient noise from distantshipping (greater than one CZ range) is attenuated, or stripped, via the bottom bouncepath, and the acoustic signal of interest is received via the submerged CZ propagationpath (figure 8-7). Under proper conditions, exploitation of TNS will afford an increasedsignal-to-noise ratio.
Figure 8-7. Topographic Noise Stripping.
Both the Source and Receiver must be located in the negative sound speedgradient below the Sonic Layer Depth (SLD) for this phenomenon to be exploited. Anunderstanding of TNS requires an understanding of the submerged CZ.
If an acoustic source moves from within the surface layer to a location of lowersound speed below the SLD, there will be more depth excess (or sound speed excess)available for an increased probability of CZ propagation path occurrence. The depthexcess for an “in-layer” source is measured from the Critical Depth to the bottom.Critical Depth is defined as that depth below the Deep Sound Channel Axis (DSCA) withthe same sound speed as that at the SLD (figure 8-8).
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Figure 8-8. In-Layer Source and Critical Depth.
For a “below-layer” source, the depth excess is measured from the ConjugateDepth. Conjugate Depth is defined as that depth below the DSCA with the same soundspeed as that of the Source Depth. TNS may be exploited in areas where there is littleor no depth excess for an in-layer source, but still sufficient depth excess for a below-layer source (figure 8-9).
Figure 8-9. Below-Layer Source, Conjugate Depth, and Resultant Depth Excess.
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The operator can utilize an SSP, or multiple SSPs, and a bathymetric chart todetermine if TNS is possible in the area of interest. For an in-layer source, determinethe critical depth and mark the closest depth contour on the bathymetric chart.
CZ propagation for an in-layer source is not possible in areas shallower than themarked contour and is considered “Bottom-Limited” for an in-layer source. For a below-layer source, determine the conjugate depth and add 200 or 300 fathoms and mark theclosest depth contour on the bathymetric chart. The 200-fathom value relates to a 50percent probability of CZ propagation path occurrence, whereas, the 300-fathom valuerelates to an 80-percent probability of CZ propagation path occurrence. CZ energypropagation for a below-layer source is unlikely for areas shallower than the markedcontour. Topographic Noise Stripping (TNS) can be exploited in the regionbetween the two depth contours marked on the bathymetric chart (figure 8-10).
Procedure for Determining TNS Region:
Critical Depth
Conjugate Depth
DSCA
Source DepthSLD
1. Determine Critical Depth from SSP. (equals maximum depth for TNS)
2. Determine Conjugate Depth from SSP.
3. Add 200/300 fathoms to Conjugate Depth. (equals minimum depth for TNS)
4. Ensure that Critical Depth is greater than Conjugate Depth + 200/300 fathoms.
5. Outline depth contour on Bathymetric Chart corresponding to Critical Depth.
6. Outline depth contour on Bathymetric Chart corresponding to Conjugate Depth + 200/300 fathoms.
7. TNS will occur in the region between the two outlined contours.
Figure 8-10. Procedure for Determining TNS Region.
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8.6 Sensors
High-power, low-frequency active sonars are the most effective sensor fordetection of both nuclear and diesel-electric submarines in shallow-water areas. High-power, low-frequency active sonars increase the signal-to-noise ratio by increasing thesignal output. Use processed directional transmission (PDT) or rotational directionaltransmission (RDT) mode on hull-mounted sonar systems for highest source levels.Ensure operator procedures and equipment settings are in accordance with operationalguidelines. Equipment must be aligned to peak conditions for accurate interpretation ofthe environment.
Towed array employment provides effective direct path, surface duct, andsecondary sound channel monitoring. Place end fire toward high ambient noiseregions.
8.7 Acoustic Applications
Active sensors exploit downslope enhancement to reduce bottom reverberationlevels. Use maximum power to search large areas of coverage. Use frequency shiftingto reduce the effects of reverberation and mutual interference.
Passive sensors determine the acoustic environment (predeployment and in-situmeasurements). Exploit any upslope or downslope enhancement opportunities. Placesensors below the shallow SLD to enhance detection of dominant BB path. When thesurface duct is of sufficient size, place sensors above the SLD at approximately 75percent of SLD to monitor for shallow-running diesel-electric submarines.
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Chapter 9
Passive Sonar
9.1 General
In passive-sonar detection and tracking, the sonar sensor receives a signalgenerated by the target. The detection process involves the recognition of targetsignals in the presence of interfering background noise. Thus, passive detection can bedescribed in terms of the factors that affect the received signal-to-noise ratio.
Passive-sonar prediction ranges supplied in environmental service productsinvolve equating an estimated Figure of Merit (FOM) derived from the passive-sonarequation, to propagation-loss curves. Propagation-loss profiles are representations ofthe combined effects—expressed as functions of range—of direct path, bottom bounce,surface duct, convergence zone, and sound-channel modes of sound propagation in theocean. The propagation-loss profiles, used in conjunction with the FOM, provide amethod for predicting expected range, signal excess, and probability of detection.
9.2 Passive-Sonar Equation
The passive form of the sonar equation may be written as follows:
SE = SL - PL - NL + DI – RD
or
SE = LS - PL - LN + NDI - NRD
which is also expressed as:
SE = SL - PL - LE - RD
or
SE = LS - PL - LE - NRD
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where
NUSC Urick Description Controlled By
SE = SE = Signal excess (dB) Required probability of detection
LS = SL = Source level (dB/µPa) Target design, maintenance andoperating mode
PL = PL = Propagation loss (dB) Environment
LN = NL = Noise level (dB/µPa) Environment and own platform speed
NRD = RD = Recognition differential(dB)
Sonar design, maintenance, andoperator training/fatigue
LE = LE = Total background noise Ambient noise, sea state, shippingdensity, rain, and self noise associatedwith a sonar and platform at a givenspeed, sonar design, and maintenance
NDI = DI = Directivity index (dB) Sonar design and maintenance
and dB/µPa means dB relative to 1 micropascal.
There are several different sets of symbols in use for sonar equation parameters.Aviation USW operators use the symbols from Urick (1979). Surface and submarineoperators use the notation from the NUWC operating manuals for their particularsonars. This publication will try to provide both. Definitions of the terms used in thepassive-sonar equation are presented in the following paragraphs.
9.2.1 Signal Excess (SE). Signal excess is the received signal level (in dB) in excessof that required for detection, under the probability conditions implied in the term RD.Detection occurs at a specified probability of detection (usually 50 percent) when thesignal excess is zero. The relationship between signal excess and probability ofdetection can be determined if the statistical distribution of values of signal excess isknown or assumed.
9.2.2 Source Level (SL or LS). The source level for target-radiated noise is the acousticintensity reduced to a reference distance of 1 yard from the point from which the soundappears to be radiated. SL is generally expressed as the average plane-wave acoustic
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intensity in a 1-Hz frequency band at a reference distance of 1 yard, relative to areference intensity of 1µPa.
The target-noise source level depends on the type of target and its mode ofoperation. It is a function of frequency, speed, depth, and aspect. Target-noisecharacteristics may be obtained from applicable intelligence information.
9.2.3 Propagation Loss (PL). Propagation loss (transmission loss), as a sonarparameter, is the reduction in signal intensity (in dB) between a point 1 yard from thesound source and the receiving sensor. PL may be obtained from curves provided forspecific configurations, frequencies, and environmental conditions.
9.2.4 Noise Level (NL or LN). The noise level is the acoustic intensity of the total noisebackground (ambient and self-noise) at the location of the receiving sensor, asmeasured by a non-directional (omnidirectional) hydrophone. NL, or LN, is generallyexpressed as the average plane-wave acoustic intensity in a 1-Hz bandwidth.
9.2.5 Total Background Noise (LE or LE). The Total Background Noise is the total levelof interfering noise against which a sonar system must process acoustic information inorder to detect a contact. LE, or LE, is a power summation of self-noise (Le) andambient noise (La).
LE = Le + La (POWER SUMMATION)
or
LE = Le + La (POWER SUMMATION)
Refer to Appendix B, section B.2.3, for instructions on Power Summing two dB values.
Le consists primarily of own-ship machinery noise (mechanical and electrical)and flow noise caused by water flowing past the sonar transducer or hydrophone.
La is that part of the total background beam noise that is not caused by own-shippresence in the acoustic medium and includes noise from biologics, shipping, seasurface, and fixed sources (such as oil rigs).
The LE, or LE, term accounts for any reductions in the effective background noisedue to directional processing employed by beam-formed sonar systems. LE, or LE, isanalogous to the NL - DI, or LN - NDI term, where NL, or LN, is noise level and DI, or, NDI
is directivity index.
9.2.5.1 Directivity Index (DI or NDI). The receiver directivity index is a measure of theamount by which an array, through its beam pattern, discriminates against noise in favorof a signal. This property of array directionality is highly desirable, for it enables thedirection of a signal to be determined and adjacent signals to be resolved.
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At the same time, directivity reduces noise, relative to the signal, arriving fromother directions. DI is defined as the signal-to-noise ratio (in dB) at the terminals of ahydrophone array (or directional hydrophone), relative to the signal-to-noise ratio of anondirectional hydrophone. Thus defined, DI is always a positive quantity, although itmay be determined by measuring the reduction in noise intensity observed in anisotropic noise field. The directivity of an array is a function of the dimensions of thearray, the number and spacing of elements, and the frequency of the received acousticenergy.
9.2.6 Recognition Differential (RD or NRD). Recognition differential is defined as thesignal-to-background-noise ratio required at the sonar receiver to enable an operator torecognize the presence of a signal 50 percent of the time. RD is determined for bothauditory and visual displays.
9.3 Figure of Merit (FOM)
The FOM is widely used in estimating overall sonar performance. It relatesallowed propagation loss to estimated detection range. The FOM for passive sonar isdefined as the maximum allowable one-way propagation loss (in dB) that a signal cansuffer for a system to meet a desired performance criterion under specific conditions.The performance criterion requires that the signal be detected 50 percent of the time.The FOM concept can be extended to more sophisticated detection criteria. Note thatthe FOM may also be defined as the propagation loss for which signal excess is equalto zero.
The FOM equation is as follows:
SE = SL - PL - NL + DI – RD
φ = SL - PL - NL + DI - RD
PL = SL - NL + DI - RD
FOM = SL - NL + DI - RD
or
FOM = SL - LE - RD
or
FOM = LS - LE - NRD
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9.4 Passive Sonar Performance Prediction
Predictions of passive-sonar performance by using the FOM expression involveestimates of system parameters, ambient noise, target characteristics, and sound-propagation characteristics. The accuracy of these estimates is directly related to theamount and quality of information available on each of the terms of the expression atthe time the prediction is made.
9.4.1 Variability of FOM Parameters. The value of a parameter may, at any particularinstant, be greater or less than the value used as an estimate for it in the expression forFOM. In effect, parameter estimates are the averages that would be obtained from alarge number of measurements made under fixed conditions. Experimental evidenceindicates that the frequency of values observed is distributed in a bell-shaped(Gaussian) curve, so that it is convenient to characterize each term in statistical termsby its mean and standard deviation. The estimated value of FOM is, therefore, astatistical average having a standard deviation. Standard deviations usually associatedwith the terms in the passive sonar equation have been tabulated. (See Del Santo andBell, 1962; Bell, 1963; and paragraph 9.4.2.)
9.4.2 Probability of Detection vs. Range. The passive-sonar equation and the FOMexpression may be used with acoustic support products such as propagation-loss(PROPLOSS) profiles, FOM probability-of-detection modification overlays, andprobability-detection nomograms to predict passive-sonar performance. Figure 9-1 isan example of a nomogram that relates probability of detection, signal excess, andFOM. The signal excess is derived from the difference in the determined FOM and thepropagation-loss curve. The chosen sigma value is based on the amount of knowledgeof the target and its environment. When the signal excess is applied to the selectedsigma value line, a probability of detection (%) can be determined.
To select the appropriate sigma value, use the following guidelines:
a. A sigma of 6 if ambient-noise measurements have been made and submarinespeed and type are known.
b. A sigma of 8 if ambient noise is estimated from forecasts, submarine speed isknown to within 3 knots, and type is known.
c. A sigma of 10 if ambient noise is estimated from forecasts and submarinespeed and type are uncertain.
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Standard Deviation of FOM(Sigma)
0.2 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99
Probability of Detection (%)
10 dB
8 dB
6 dB
Signal
Excess
(dB)
Figure 9-1. Signal Excess Probability-of-Detection Graph.
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Chapter 10
Active Sonar
10.1 General
Active sonar provides a means for detecting and tracking submerged or surfacedtargets; the sonar does this by "listening" to returned echoes reflected from the target.In active detection, pulses of acoustic energy generated by the sonar (or by active-acoustic circuits in the weapons themselves) are propagated through the water to thetarget. Reflected from the target, these pulses of acoustic energy travel back to thereceiver. There, range information is obtained by electronic circuitry that measures thetime interval between transmitted and received pulses.
10.2 Active-Sonar Equations
The active-sonar equations are similar to those for passive sonar. However,active-sonar performance may be either noise- or reverberation-limited, depending onwhich type of interfering background is dominant.
10.2.1 Noise-Limited Case. When the dominant background is noise, the active form ofthe sonar equation may be written as follows:
SE = SL + TS - RD - NL + DI - 2PL
or
SE = LS + TS - NRD - LN + NDI - 2PL
which is also expressed as:
SE = SL + TS - RD - LE - 2PL
or
SE = LS + TS - NRD - LE - 2PL
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where
NUSC Urick Description Controlled By
SE = SE Signal excess (echo excess)(dB)
Required probability of detection
LS = SL Source level (dB//µPa @ 1yard)
Sonar design, maintenance, andoperating mode
TS = TS Target strength (dB) Target design and aspect
LN = NL Noise level Environment and own platform speed
NRD = RD Recognition differential (dB) Sonar design/maintenance andoperating training/fatigue
LE = LE Total background noise(dB//µPa)
Environment and own ship’s speed,sonar design, and maintenance
NDI = DI Receiver directivity index(dB)
Sonar design and maintenance
PL = PL Propagation loss (dB) Environment
10.2.2 Reverberation-Limited Case. When the dominant background is reverberation,the active-sonar equation may be written as follows:
SE = SL + TS - RD - RL - 2PL
or
SE = LS + TS - NRD - RL - 2PL
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where
NUSC Urick Description Controlled By
SE = SE Signal excess (echo excess)(dB)
Required probability of detection
LS = SL Source level (dB//µPa @ 1yard)
Sonar design, maintenance, andoperating mode
TS = TS Target strength (dB) Target design and aspect
NRD = RD Recognition differential (dB) Sonar design/maintenance andoperating training/fatigue
RL = RL Reverberation level Environment and beam steeringsonar mode
PL = PL Propagation loss (dB) Environment, frequency, andgeometry
10.3 Active-Sonar Equation Parameters
The terms used in the active-sonar equations are described in the followingparagraphs.
10.3.1 Signal Excess (SE). Signal excess is the received signal level (in dB) in excessof that required for detection, under the probability conditions implied in the term RD.Detection occurs at a specified probability of detection (usually 50 percent) when thesignal excess is zero. The relationship between signal excess and probability ofdetection can be determined if the statistical distribution of values of signal excess isknown or assumed. In active-sonar systems, however, signal excess is often referredto as echo excess.
10.3.2 Recognition Differential (RD or NRD). Recognition differential is defined as thesignal-to-background-noise ratio required at the sonar receiver to enable an operator torecognize the presence of a signal 50 percent of the time. RD is determined for bothauditory and visual displays.
10.3.3 Source Level (SL or LS). For an active sonar, the source level of a projector isthe intensity of the radiated sound in decibels, relative to a reference intensity of 1µPa,referred to at a point 1 yard from the acoustic center of the projector in the direction of
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the target. LS includes whatever increase due to the projector directivity is appropriateto the particular operating mode, such as RDT or PDT.
10.3.4 Target Strength (TS). The target strength of a reflecting object is the amount bywhich the apparent intensity of sound scattered by the target exceeds the intensity ofthe incident sound. The reference distance is 1 yard from the acoustic center of thetarget.
The value of target strength depends on the size, shape, construction, type ofmaterial, roughness, and aspect of the target, as well as the angle, frequency, andwaveform of the incident sound energy. A typical butterfly pattern associated withsubmarine target strength is shown in figure 10-1. Seldom are all of the characteristicsof this typical pattern observed at one time. This pattern is caused by specular andnonspecular reflection of the signal by the target (Urick, 1967; COMCRUDESGRUTWO/DESDEVGRU, 1974).
10.3.5 Noise Level (NL or LN). LE values calculated for the passive sonar equation areat spectrum level. To convert to the noise appropriate for the active sonar equation,10 log BW (BW = receiver bandwidth) must be added to spectrum level LE. Forexample, if an active sonar has a receiver bandwidth of 300 Hz, 25 dB must be addedto the spectrum level LE to get the total noise against which the echo must berecognized.
Figure 10-1. Aspect Variation of Submarine Target Strength (Urick, 1967).
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10.3.6 Propagation Loss (PL). Propagation loss (transmission loss), as a sonarparameter, is the reduction in signal intensity (in dB) between a point 1 yard from thesound source and the receiving sensor. PL may be obtained from curves provided forspecific configurations, frequencies, and environmental conditions. Two-waypropagation loss is used in the active sonar equations, since sound energy musttraverse the propagation path twice.
10.3.7 Receiver Directivity Index (DI or NDI). The receiver directivity index is a measureof the amount by which an array, through its beam pattern, discriminates against noisein favor of a signal. This property of array directionality is highly desirable, for it enablesthe direction of a signal to be determined and enables adjacent signals to be resolved.At the same time, directivity reduces noise, relative to the signal, arriving from otherdirections. DI is defined as the signal-to-noise ratio (in dB) at the terminals of ahydrophone array (or directional hydrophone), relative to the signal-to-noise ratio of anondirectional hydrophone. Thus defined, DI is always a positive quantity, although itmay be determined by measuring the reduction in noise intensity observed in anisotropic noise field.
The directivity of an array is a function of the dimensions of the array, the number andspacing of elements, and the frequency of the received acoustic energy.
10.3.8 Reverberation Level (RL). When an active sonar is reverberation-limited, theterm (LE) that appears in the noise-limited equation is replaced by RL, the reverberationlevel observed at the receiver beamformer output terminals. The reverberation levelcan be calculated in much the same way as the received signal level, to which it isanalogous. RL is, therefore, a function of source level and range, as well as thedominant reverberation scatterers (volume, sea surface, or bottom).
10.4 Active-Sonar Performance Prediction
The active-sonar equations may be used to predict active-sonar performance.Performance may be predicted by direct application of the equations for signal excess ina manner analogous to that described in chapter 9, paragraph 9.3. The Figure-of-Meritconcept, however, is not useful for the reverberation-limited case. This is because asthe source level increases, the reverberation level will increase at the same rate as thereturn from the target.
For a discussion of procedures for predicting active-sonar ranges for currentoperational sonars, see current sonar manual.
Signal excess is related to probability of detection in a manner conceptuallyidentical to the passive-sonar case. A graph typifying the relationship between signalexcess and probability of detection for active sonars is given in figure 10-2.
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Figure 10-2. Probability of Detection for Various Values of Signal Excess.
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Appendix A
Glossary of Terms, Acronyms, and Abbreviations
ABSORPTION. The reduction of sound intensity caused by the conversion of soundenergy into heat as it passes through water.
ACOUSTIC SIGNATURE. The noise output of a particular class of submarine/ship/aircraft expressed as Spectrum Level.
ACTIVE SONAR. See SONAR.
AFTERNOON EFFECT. The solar heating of the surface water, which causes shallownegative temperature gradients. This results in downward refraction of sound rays andreduced surface duct ranges.
AMBIENT LIMITED SPEED (ALS). For a ship or submarine platform, the slowestrecommended search speed. At this speed or slower, acoustic detection ranges arelimited by the ambient noise in the environment, and NOT by the platform’s self noise.(This occurs at the speed where self noise = ambient noise – 6 dB.) Also, see BREAKPOINT SPEED (BPS).
AMBIENT NOISE (AN). Noise in the sea due to biologics, shipping, ice motion,precipitation, and sea surface agitation caused by winds and terrestrial movements.Self noise and reverberation are not considered ambient noise.
AOS. Atlantic Oceanographic Synopsis. A message synopsis of oceanographicconditions in the Atlantic promulgated by NAVLANTMETOCCEN.
ARRAY. A group of two or more hydrophones arranged to provide a variation ofreception with direction when beamformed.
ATTENUATION. The reduction in sound intensity (dB/kyd) caused by the absorptionand scattering of sound in water.
AXBT. Aircraft Expendable Bathythermograph. Bathythermograph launched from anaircraft which can record water temperature versus depth down to 2,500 feet.
BACKGROUND NOISE. All unwanted sounds received by a hydrophone; includesambient and self-noise.
BACKSCATTERING. That part of the reflected sound energy that returns to thetransducer; equivalent to reverberation.
BAND LEVEL. The level of noise or signal in a specified frequency band.
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BATHYTHERMOGRAPH. An instrument used to obtain a permanent, graphical recordof water temperature (°F or °C) with depth (feet or meters) as it is lowered into the sea.The temperature with depth report is often referred to as a BATHY or BT. See XBT,AXBT, SXBT, and SSXBT.
BEAUFORT SCALE. A system for estimating and reporting wind speeds that uses ascale ranging from 0 to 12.
BISTATIC. Refers to the case in active sonar where the active source and the receivinghydrophone are separated. Also, see MONOSTATIC and MULTISTATIC.
BOTTOM BOUNCE (BB). Sound transmission in which sound rays strike the bottom;one reflection may attain ranges up to 20 kiloyards. Bottom-reflected ray paths arethose ray paths whose angles when leaving the source exceed the departure angle ofthe ray which is tangent to the bottom (limiting ray).
BOTTOM INTERACTION. Interaction of underwater sound with the ocean bottom,whether the sound is reflected from the sediment, or refracted through it, or both. At lowfrequencies, refraction may produce a focusing, somewhat similar to a convergencezone.
BOTTOM LIMITED. The ocean bottom occurs at a depth less than the critical depth.CZ propagation is prevented from occurring. DSC propagation is restricted to a deepsource.
BOTTOM LOSS UPGRADE (BLUG). Improved prediction system which models low-frequency sound refraction through the sediments.
BREAK POINT SPEED (BPS). For a ship or submarine platform, the fastestrecommended search speed. At this speed or faster, acoustic detection ranges arelimited by the platform’s self noise, and NOT by the ambient noise in the environment.(This occurs at the speed where self noise = ambient noise.) Also, see AMBIENTLIMITED SPEED (ALS).
CASS (COMMAND ACTIVATED SONOBUOY SYSTEM). Active sonobuoy thattransmits pulses on command.
CAUSTIC. In a 2-dimensional ray diagram, a caustic is a curve formed by theintersections of adjacent rays in the diagram. A focus occurs when a causticdegenerates to a point or a small region of space.
CAVITATION. The formation of local cavities (bubbles) in a liquid as a result of thereduction of total pressure. This pressure reduction may result from a negativepressure produced by rarefaction or from the reduction of pressure by hydrodynamicflow, such as that produced by high-speed movement of an underwater propeller.
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CONJUGATE DEPTH. For a source below the Sonic Layer Depth (SLD), that depthbelow the deep sound channel axis where the sound speed equals the speed at thesource depth.
CONTINENTAL RISE. A gentle slope with a generally smooth surface found betweenthe continental slope and the abyssal plain.
CONTINENTAL SHELF. A zone adjacent to a continent and extending from the lowwaterline (shoreline) to a depth at which there is a marked increase of bottom slope,known as the continental slope, to a greater depth (usually about the 100-fathom curve).
CONTINENTAL SLOPE. A zone from the outer edge of a continental shelf to thecontinental rise.
CONVECTION CURRENTS. Whenever the surface water undergoes intensive cooling,evaporation, or freezing, the density of the surface water increases beyond that of theunderlying water. As this denser water sinks to a level of the same density, currents areproduced by warmer water flowing in to replace the sinking surface water.
CONVERGENCE ZONE (CZ). That region in the deep ocean where sound rays,refracted from the depths, are focused at or near the surface in successive intervals. [Aconvergence zone is a sound-transmission channel in the deep ocean (2,500-15,000feet [750-4500 meters]) produced by the combination of pressure and temperaturechanges. Convergence zones exist in shallow water but have different characteristics.]
CORRELATION. Correlation is the process of comparing two signals and producing anoutput that is a function of some relation between the two signals. The signals may becompared in frequency, amplitude, or phase. A device that accomplishes this processis called a correlator. The output voltage of a correlator is proportional to the similarityof the two signals.
CRITICAL ANGLE. The grazing angle of a sound wave with the sea bottom at whichtotal reflection occurs.
CRITICAL DEPTH. The depth below the Deep Sound Channel (DSC) axis at which thesound speed is the same as it is at the sonic layer depth. The critical depth is thebottom of the DSC.
CUTOFF FREQUENCY. The lowest frequency (or the largest wavelength) that can betrapped in a surface duct or sound channel. The cutoff frequency is determined by thethickness, as well as by the strength (∆C=Cmax –Cmin) of the duct or channel. It is not asharp cutoff, but frequencies much lower than the cutoff will be strongly attenuated,while frequencies much higher than the cutoff will be trapped. Frequencies near thecutoff may or may not be trapped, depending on such parameters as the sound-speedgradients within and below the duct or channel.
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DECIBEL (dB). A value that expresses the comparison of sounds of two differentintensities. The value is defined as 10 times the logarithm to the base 10 of the ratio ofthe two sound intensities.
DEEP LAYER. The layer of water between the lower edge of the main thermocline andthe ocean bottom. It is characterized by a nearly constant temperature and a positivesound-speed gradient caused by pressure.
DEEP SCATTERING LAYER (DSL). The stratified population(s) of organisms inoceanic waters that scatter sound. The scattered sound is recorded on echo-sounderrecords as a uniform horizontal band or stripe. These layers are generally found duringthe day at depths from 100 to 400 fathoms. A layer is rarely less than 25 fathoms thickand may be as much as 100 fathoms thick. Several layers are often recorded at thesame time and may be continuous for many miles. Most layers typically undergodiurnal vertical movements. Also called false bottom or phantom bottom.
DEEP SOUND CHANNEL (DSC). The main sound channel of the ocean, caused bythe negative sound-speed gradient of the thermocline and the positive gradient of thedeep layer.
DENSITY. The density of sea water is the mass per unit volume. It increases withincreasing salinity and pressure and decreases with increasing temperature.
DEPRESSION/ELEVATION (D/E). The feature of a sonar set that enables its beam tobe trained in the vertical direction.
DEPTH EXCESS. The difference between the bottom depth and the critical depth.
DEPTH REQUIRED. Minimum depth required for a reliable convergence zone to exist.It is 200-300 fathoms below the critical depth.
DICASS. Directional Command Activated Sonobuoy System. Directional activesonobuoy.
DIFAR. Directional Frequency Analysis and Recording. Directional passive sonobuoy.
DIRECTIVITY INDEX (DI). The amount by which a hydrophone array, through its beampattern, discriminates against isotropic noise in favor of the signal. It refersconventionally to a plane-wave signal in isotropic noise. DI is the signal-to-noise ratio(SNR) in dB of an array or directional hydrophone relative to the SNR of anondirectional hydrophone, and is always positive.
DIURNAL CYCLE. A regular daily sequence of events or conditions occurring withineach 24-hour day.
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DOWNSLOPE ENHANCEMENT. Also known as the megaphone effect. Acousticenergy from a source in shallow water changes from a bottom bounce path to aconvergence zone or sound channel path as it travels to deeper water, and isconcentrated down the slope to a receiver in deep water. Also, see UPSLOPEENHANCEMENT.
DSL. See Deep Scattering Layer.
DYNE. A unit of force in the centimeter-gram-second system of measurement that isdefined as the force that gives a 1-gram mass an acceleration of 1 cm/sec2.
ECHO. In active sonar, the sound waves generated by the projector to the target andreflected from the target to the hydrophone or source.
ECHO RANGING. Determination of distance by measuring the time interval betweenemission of a sonic signal and the return of its echo from a reflector.
EDDY. A circular body of water usually formed, where currents pass obstructions,between two adjacent currents flowing counter to each other, or along the edge of apermanent current.
EL NINO. Warm current which generally develops from December through March eachyear and flows south along the coasts of Ecuador and Peru. A concurrent shift in thetropical rain belt also takes place. It is part of the Southern Oscillation.
ENSONIFY. See INSONIFY.
EOTS. Expanded Ocean Thermal Structure.
EXTENDED ECHO RANGING (EER). Multistatic active acoustic system, utilizing theSSQ-110 or SSQ-110A sonobuoy as the source and generally using the SSQ-77Bsonobuoy as the receiver.
FIGURE OF MERIT (FOM). A measure of the effectiveness of a sonar set for aparticular situation. It is the maximum allowable propagation loss that a signal cansuffer for a system to meet a desired performance criterion, usually a 50-percentprobability of detection.
FLOW NOISE. The noise produced by water movement past the transducer orhydrophone array housing. The noise produced at the hull of a moving ship. The noisecreated by turbulent flow in the turbulent boundary layer around the hydrophone.
FLENUMMETOCCEN. Fleet Numerical Meteorology and Oceanography Center.Located in Monterey, California.
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FREQUENCY, SOUND. The number of sound waves passing a point in a given time;measured in Hertz: 1 Hz = 1 cycle/second.
GRADIENT. The rate of change in a given distance of an environmental variable. Forexample, in the sea a vertical temperature gradient is the change of temperature withdepth. A positive gradient is a temperature increase with depth; a negative gradient is atemperature decrease with depth.
GRAZING ANGLE. The angle a sound ray makes with an ocean boundary. Measuredin degrees from the boundary surface.
HALF CHANNEL. An upward-refracting condition where the sound-speed gradient ispositive from the surface all the way to the bottom. Behaves like a very thick surfaceduct. Occurs in high latitude waters and in the Mediterranean Sea in winter.
HIGH-FREQUENCY BOTTOM LOSS (HFBL). A data base which supports high-frequency (1,500-4,000 Hz) performance prediction capability for sonar applications.The HFBL data base divides the worldwide ocean bottom into 9 categories, withcategory 1 = low loss and category 9 = high loss. Each category has an associatedbottom loss versus grazing angle curve.
HYDRODYNAMIC NOISE. See FLOW NOISE.
HYDROPHONE. An acoustic device that receives and converts underwater soundenergy into electric waves.
ICAPS. Integrated Carrier ASW Prediction System.
INSONIFY. To project sound energy into any part of the sea.
INTENSITY, SOUND. The amount of sound energy per second crossing a unit area.
INTERNAL WAVE. A wave that occurs in the ocean medium either at a surface ofdensity discontinuity (as in fronts) or at the boundary between the mixed layer and thethermocline.
ISOSPEED. Values of sound speed are the same in all parts of a given water column;no change in sound speed with depth.
ISOTHERMAL. Of equal or constant temperature with respect to space or time; noincrease or decrease in temperature with depth.
ISOTROPIC. Having the same physical properties in all directions.
IVDS. Independent Variable Depth Sonar.
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LAYER DEPTH. The depth of the lower edge of the surface layer, that is, the top of thethermocline. Also may be the depth of maximum sound speed near the surface.
LAYER EFFECT. When sound passes through a layer in which little or no bending ofthe ray path occurs and then passes into a layer with a strong negative gradient(causing sharp downward bending of the ray), increased spreading occurs with aconsequent loss of sound intensity.
LIMITING RAY. The sound ray that becomes tangent at the depth where the soundspeed is at maximum; it delimits the outer boundary of direct (before reflection) soundrays.
LINE COMPONENT. A discrete, narrow band tonal (line) produced by a noise source.
LITTORAL. The region which horizontally encompasses the land/watermass interfacefrom 50 statute miles ashore to 200 nautical miles at sea. This region extends verticallyfrom the bottom of the ocean to the top of the atmosphere at sea and from the landsurface to the top of the atmosphere over land.
LOFAR. Low-Frequency Analysis and Recording. Search technique usingomnidirectional sonobuoys.
LOW-FREQUENCY BOTTOM LOSS (LFBL). A data base which supports low-frequency performance prediction capability for sonar application (50-1,000 Hz). TheLFBL implementation uses geoacoustic parameters, including sediment sound speed,attenuation, density, and sediment thickness to derive bottom loss for input intoperformance prediction models. The LFBL data base is comprised of 803 LFBLprovinces, each of which has 15 geoacoustic parameters. These parameters describethe reflective and refractive characteristics of the ocean bottom.
MAD. Magnetic Anomaly Detection.
MAIN ACOUSTIC RESPONSE AXIS (MRA). The axis of the major lobe of the receivingor transmitting array beam pattern.
MAIN THERMOCLINE. The layer of water between the surface layer and the deeplayer; it is characterized by a negative sound-speed gradient. Also known as thepermanent thermocline.
MARGINAL ICE ZONE (MIZ). The transition region between the solid ice pack and theopen seas in polar regions. Region of high ambient noise across a wide frequencyspectrum.
MDR (Mean Detection Range). The range at which there is a 50-percent chance ofdetecting a particular target, with a particular figure-of-merit (FOM) and propagation lossprofile. It is the range where the FOM line first intersects the propagation loss curve.
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MGS. Marine Geophysical Survey.
MICROBAR. A unit used in sonar work to measure sound pressure. One microbar isequal to one dyne per square centimeter, which is about one millionth of an atmosphere.The symbol is µbar.
MICROPASCAL (µPa). Reference pressure level equivalent to one millionth of oneNewton/meter2; used in underwater acoustics and equal to 10-5 µbar. A signal of 1µPais 100 dB less intense than a signal of 1 µbar. Older publications referenced soundpressure levels to 1 µbar or .0002 µbar.
MIXED LAYER DEPTH (MLD). The point of maximum near-surface temperature.
MKS. Meters Kilograms Seconds.
MONOSTATIC. Refers to the case in active sonar where the active source and thereceiving hydrophone are collocated. Also, see BISTATIC and MULTISTATIC.
MULTISTATIC. Refers to the case in active sonar where there is an active source andmultiple receivers, some of which are separated from the source. Also, see BISTATICand MONOSTATIC.
NAVLANTMETOCCEN. Naval Atlantic Meteorology and Oceanography Center.Located in Norfolk, Virginia.
NAVOCEANO. Naval Oceanographic Office. Located at Stennis Space Center,Mississippi.
NAVPACMETOCCEN. Naval Pacific Meteorology and Oceanography Center. Locatedat Pearl Harbor, Hawaii.
NOISE LEVEL (NL or LN). The Noise Level is the acoustic intensity of the total noisebackground (ambient and self noise) at the location of the receiving sensor.
NUWC. Naval Undersea Warfare Center.
OCEANIC FRONT. The interface between two water masses having differenttemperature and/or salinity characteristics. A tactically significant front will have a largeeffect on sound transmission and propagation loss.
OAML. Oceanographic and Atmospheric Master Library.
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OCTAVE. The interval between two frequencies having a ratio of 2:1. Thus, going oneoctave higher means doubling the frequency, and going one octave lower meanschanging to one-half the original frequency. For example, 440 to 880 Hz is one octave,880 to 1,760 Hz is the next higher octave, and 440 to 220 Hz is the next lower octave.
ONR. Office of Naval Research. Located in Washington, D.C.
PASSIVE SONAR. See SONAR.
PDT. Processed Directional Transmission. An active-sonar mode.
PLANKTON. All passively drifting or weakly swimming plant and animal life in marineand fresh waters. Plankton range in size from microscopic to jellyfishes measuring sixfeet across the umbrella or bell.
POS. Pacific Oceanographic Synopsis. Weekly oceanographic message summarypromulgated by NAVPACMETOCCEN (Eastern Pacific) and NAVPACMETOCCENWEST Guam (Western Pacific).
PROBABILITY OF DETECTION (POD). The probability of detecting a given target,based on figure of merit and propagation loss as a function of range.
PROPAGATION LOSS (PL). Loss of sound intensity due to spreading and attenuationduring travel through a medium on a transmission path. The reduction in signalintensity (in dB) between a point 1 yard from the sound source and the receiving sensor.Also called transmission loss (TL).
RADIATED NOISE. The spectrum level of the sound energy radiated by a platform.Machinery and propeller noise dominate, but hydrodynamic noise is also a factor. It isnormally expressed as a sound level in dB//1µPa referenced to a distance of 1 yardfrom the source in a 1 Hz bandwidth.
RAREFACTION. The condition in a sound wave where the pressure is lower than theaverage pressure exerted by the medium in which the wave propagates.
RAY PATH. A path perpendicular to the acoustic wavefront as the wave travels throughthe water.
RBR. Refracted Bottom Reflected ray path.
RDT. Rotational Directional Transmission. An active sonar mode.
RECOGNITION DIFFERENTIAL (RD or NRD). The special value of the signal-to-noiseratio required at the sonar receiver that permits a 50-percent probability of detecting atarget signal. The symbol is RD (measured in dB).
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REFERENCE LEVEL. In underwater sound, the standard level to which other soundlevels can be related. Three reference levels commonly used are 1 dyne/cm2 (=1 µbar),0.0002 dyne/cm2, and 10-5 dyne/cm2 (=1 µPa).
REFLECTION LOSS. The component of propagation loss resulting from imperfectreflections at the ocean boundaries.
REFRACTION. The bending or curving of a sound ray that results when the ray passesfrom a region of one sound speed to a region of a different speed. The amount of raybending is dependent upon the amount of difference between sound speeds, that is, thevariation in temperature, salinity, and pressure of the water. Controlled by Snell’s Law.
RELIABLE ACOUSTIC PATH (RAP). A Direct Path transmission mode with ashallow/deep or deep/shallow geometry for the source and receiver. RAPs are notrelated to the DSC, half channel, or BB transmission modes.
REVERBERATION. The combined sound of many small echoes returned to thehydrophone due to scattering at the ocean surface (surface reverberation) and at thebottom (bottom reverberation), and/or scattering in the water mass (volumereverberation). Examples of sources of reverberation are air bubbles and suspendedsolid matter.
REVERBERATION LEVEL (RL). Reverberation level is a ratio of the acoustic intensity,expressed in dB units, produced by pertinent scatters (volume, sea surface, or bottom)as a function of source level and range. RL is used in the active sonar equation.
REVERBERATION LIMITED. Refers to the condition in active sonar when thereverberation interference level is higher than the background noise level. In this case,the term RL replaces LE in the active sonar equation.
RMS. Root Mean Square.
RSR. Refracted Surface Reflected ray path.
SALINITY. The amount (in grams) of total dissolved salts present in one kilogram ofwater. This is equivalent to parts per thousand (ppt or ‰). Salinity (S) is determined bymeasuring the electrical conductivity of a seawater sample: the higher the conductivity,the greater the salinity.
SCATTERING STRENGTH. The ratio (in dB) of scattered sound from a surface orvolume, referred to a distance of 1 yard, to the incident plane-wave intensity (energy perunit area or volume).
SEA STATE. A numerical or written representation of the roughness of the sea surface;the symbol is SS.
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SEA STATE LIMITED. Refers to the condition when sea surface noise is thepredominant source of background noise.
SEA SURFACE NOISE. Noise caused by the action of surface waves. Sea surfacenoise is the predominant source of ambient noise in the open ocean.
SELF NOISE. The component of background noise generated by the listening ship orsubmarine; the symbol is Le.
SELF NOISE LIMITED. Refers to the condition when self noise is the predominantsource of background noise. This occurs when a ship or submarine is travelling fasterthan its breakpoint speed (BPS).
SENSITIVITY. The measure of how well a device converts sound level to voltage level.Measured in dB/ µPa/volt.
SHADOW ZONE. A region in which very little sound energy penetrates, dependingupon the strength of the lower boundary of the surface duct. It is usually bounded bythe lower boundary of the surface duct and the limiting ray. There are two shadowzones: the sea surface, beneath which a shadow is cast by the surface in the soundfield of a shallow source, and the deep-sea bottom, which produces a shadow zone inthe upward-refracting water above it.
SHALLOW WATER. Normally considered as being less than 100 fathoms. Usuallyconsidered to be water of such depth that bottom topography affects surface waves.Only refers to the vertical extent from the ocean/atmosphere interface to the bottom ofthe ocean. Acoustically defined as water depth which will not support convergencezone (CZ) or deep sound channel sound propagation paths.
SIGNAL EXCESS (SE). The difference in dB between received signal-to-noise ratioand recognition differential. This is equivalent to the received signal level in dB inexcess of that required for a 50 percent probability of detection.
SIGNAL-TO-NOISE RATIO. The difference in dB between the received signal andthe received noise; the symbol is SNR.
SIGNIFICANT WAVE HEIGHT (H 1/3). The significant wave height is defined as theaverage height of the highest one-third of the selected waves, and is often thought of asthe most typical height reported by an observer. The average is determined by dividingthe time of record by the significant period.
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SIMAS. Sonar In-Situ Mode Assessment System. On-board acoustic prediction systeminstalled on destroyers and frigates equipped with the SQQ-89 Surface AntisubmarineWarfare Combat System. SIMAS provides active and passive range predictions,equipment settings, command summaries, and environmental updates based on XBTdata, equipment selections and target parameters.
SIMAS II is the latest version of this on-board prediction system and will automaticallyprocess an XBT and update the active sonar setting recommendations. SIMAS II isalso connected to the 53B/C/D sonar and SQR-19 towed array and will monitor/displayreverberation and towed array ambient noise in near-real time.
SINGLE-PING, 50-PERCENT PROBABILITY-OF-DETECTION RANGE. That range atwhich the signal excess becomes zero. The single-ping, 50-percent-probability-of-detection criterion has long been a fleet standard. The median detection range and theinner- and outer-range rings of the bottom-bounce annulus and convergence-zoneannulus are each determined by this probability-of-detection criterion.
SNELL'S LAW. When a wave (light or sound) travels obliquely from one medium toanother, the ratio of the sine of the angle of incidence to the sine of the angle ofrefraction is the same as the ratio of the respective wave speeds in the mediums and isa constant for two particular media. (This is true for all angles measured with respect tothe perpendicular to the interface between the two media. If grazing angles are usedinstead, replace “sine” with “cosine.”)
SOFAR. Sound Fixing and Ranging. A position-fixing system by which hyperbolic linesof position are determined by measuring, at listening stations, the difference in time ofreception of sound signals produced in the sound channel.
SOFAR CHANNEL. The deep sound channel. So called from the WWII Sound Fixingand Ranging (SOFAR) system designed for locating aviators downed at sea.
SONAR. Sound Navigation and Ranging. The method or equipment for determining byunderwater sound techniques the presence, location, or nature of objects at sea. Asystem for determining the location and distance of an underwater object by measuringthe time interval between transmission of a sound signal and its reflection back to theprojector (active sonar). Evaluation of a signal received by a hydrophone from a target(passive sonar).
SONIC LAYER DEPTH (SLD). The depth of maximum near-surface sound speedabove the deep sound channel.
SONOBUOY. A free-floating or anchored device that includes a buoy with radiotelemetering equipment and hydrophone suspended beneath. Sound signals receivedat the hydrophone are transmitted by radio to a nearby receiver for analysis. Designedfor delivery from aircraft.
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SOUND CHANNEL. That region in the water column where the sound speed firstdecreases to a minimum value with depth and then increases in value, due to pressure.Above the depth of minimum value, sound rays are bent downward; below the depth ofminimum value, rays are bent upward, resulting in the rays being trapped in thischannel, and permitting their detection at great ranges from the sound source.
SOUND CHANNEL AXIS. The depth of minimum sound speed within a sound channel.Abbreviated as DSCA for the Deep Sound Channel Axis or SSCA for the SecondarySound Channel Axis.
SOUND SPEED. The rate of travel at which sound energy moves through a medium,usually expressed in feet per second or meters per second.
SOUND SPEED EXCESS. The difference between the sound speed at the oceanbottom and at the bottom of the surface layer.
SOUND SPEED GRADIENT. The rate of change of sound speed with depth in theocean.
SOUND SPEED PROFILE (SSP). A graph of the variation of sound speed with waterdepth.
SOURCE LEVEL (ACTIVE) (SL or LS). The total power output of an active transducerin dB/µPa at 1 yard from the transducer; the symbol is SL.
SOURCE LEVEL (PASSIVE) (SL or LS). Amount of acoustic energy in dB radiatedomnidirectionally by the target at a particular frequency; the symbol is SL. SL isgenerally expressed as the average plane wave-radiated acoustic intensity in a 1-Hzband at a reference distance of 1 yard from the source and relative to a referenceintensity of 1 µPa.
SOUTHERN OSCILLATION. Multiyear variation in the surface temperature of theequatorial Pacific, which appears to have far-reaching effects on worldwide rainfall andtemperature patterns.
SPECTRUM LEVEL. The level of noise or a broadband signal in a frequency band 1Hz (1 cps) wide.
SPECULAR REFLECTION. A mirrorlike reflection of sound rays from the oceansurface, bottom, or a target, having small irregularities compared with the wavelength ofthe incident sound.
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SPREADING LOSS. The phenomenon whereby transmitted sound intensity decreasesin a constant relation to distance from the sound source. The spreading laws relatesound intensity to a ratio of distance from the sound source. These spreading laws are:
IR= Io/R = Cylindrical Spreading,
IR= Io/R2 = Spherical Spreading, and
IR= Io/R4 = Dipolar Spreading;
where
Io = target output intensity, IR = sound intensity at range R, and R = distancefrom target.
SST. Sea Surface Temperature.
SSXBT. Submarine Expendable Bathythermograph. Bathythermograph, launched froma submarine, which can record water temperature versus depth down to 2,500 feet.
STATIC PRESSURE. The portion of the total pressure in the ocean that increases withdepth and does not vary with time. (The pressure that would exist in the ocean if nosound waves were present.)
SUBBOTTOM. Term used to describe the variation in density and structure of theocean floor. With the penetration of the ocean floor by lower frequencies, density andstructure of the layers of materials making up the near-surface bottom region must beconsidered in acoustic range propagation.
SURFACE DUCT. A zone below the sea surface where sound rays are refractedtoward the surface and then reflected. The rays alternately are refracted and reflectedalong the duct out to relatively long distances from the sound source.
SURTASS. Surveillance Towed Array Sensor System. Passive USW towed arraystreamed by specially configured T-AGOS non-combatant survey ships.
SVP. Sound Velocity Profile is the older, less accurate, term for SSP, Sound SpeedProfile.
SXBT. Surface Expendable Bathythermograph. Bathythermograph launched from asurface ship which can record water temperature versus depth down to 2,500 feet.
TACTAS. Tactical Towed Array Sonar. Passive USW towed array designed to betowed at tactical ship speeds by USN surface combatants. The current version is theAN/SQR-19.
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TARGET STRENGTH (TS). A measure of the reflecting power of the target stated indB. The ratio of the target echo is measured 1 yard from the target to the soundincident on the target.
THERMOCLINE. A temperature gradient in a layer of sea water where the temperaturedecreases continuously with depth. Usually the gradient is greater than 2.7 oF per 165feet (1.5 oC per 50 meters) of depth.
TOPOGRAPHIC NOISE STRIPPING (TNS). Ambient noise from distant shipping(>1 CZ range away) is attenuated, or stripped, by interaction with the ocean bottom,while the acoustic signal of interest is received via the submerged CZ propagation path.TNS may be exploited in areas where the distant shipping noise is bottom limited, but abelow-layer source is not bottom limited, resulting in an increased signal-to-noise ratio.
TOPOGRAPHIC SHADING. The disruption of convergence zone (CZ) or deep soundchannel propagation by ocean bottom features such as seamounts, guyots, ridges, orislands. This disruption causes large shadow zones. Depth excess is destroyed for CZpropagation when a source is one-half the CZ range from such a bottom feature.
TRANSDUCER. A device for converting electrical energy to underwater sound energyor vice versa. When sound energy received through the water is converted to electricalenergy, the device is termed a hydrophone; when electrical energy is converted tosound energy and transmitted into the water, the device is termed a sonar projector oran echo sounder.
TRANSMISSION LOSS (TL). The reduction in signal intensity (in dB) between a point1 yard from the sound source and the receiving sensor. Graphically depicted as afunction of range on a computer-generated propagation loss (PL) curve.
TRANSPONDER. An automated acoustic device, capable of transmitting and receiving,similar to a sonobuoy, that can be activated upon receipt of a sound or radio signal.
TURBIDITY CURRENT. A highly turbid, relatively dense current carrying largequantities of clay, silt, and sand in suspension which flows down a submarine slopethrough less dense water.
TURBULENCE. Fluid flow in which the instantaneous velocities show irregular andapparently random fluctuations. These are often caused by obstructions (such as roughbottoms or eddies) to the fluid flow.
UPSLOPE ENHANCEMENT. Also known as the inverse megaphone effect. Acousticenergy from a source in deep water changes from a convergence zone or soundchannel path to a bottom bounce path as the bottom shoals, and is concentrated up theslope to a receiver in shallow water. Also, see DOWNSLOPE ENHANCEMENT.
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VDS. Variable Depth Sonar. A shipborne sonar system in which the transducer can belowered below the thermocline.
VERTEX DEPTH. The depth in the water at which a refracted sound ray becomeshorizontal.
VERTEX SOUND SPEED. The speed at which a refracted sound ray becomeshorizontal.
VLAD. Vertical Line Array DIFAR. Advanced DIFAR buoy using a vertical line array ofhydrophones to discriminate against ambient noise.
WAVELENGTH, SOUND. The distance between corresponding points of adjacentsound waves; measurement is determined by the ratio of speed to frequency.
WMO. World Meteorological Organization.
XBT. Expendable BathyThermograph. Bathythermograph launched from a ship(SXBT), submarine (SSXBT), or aircraft (AXBT). Fleet XBTs can record watertemperature versus depth down to 2,500 feet.
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Appendix B
Sound Levels
B.1 Sound Intensity and Pressure
Sound intensity is a measure of the sound power (energy per second) crossing aunit area normal to the direction of wave propagation. In a plane wave theinstantaneous acoustic intensity is related to the instantaneous acoustic pressure by:
I = P2
ρc
where (in the MKS system)
I = intensity of sound (in joules/m2s or watts/m2),
P = rms sound pressure (in newton/m2 = pascals),
ρ = density (in kg/m3), and
c = sound speed (in m/s).
Underwater sound pressure and intensities are measured with pressure-sensitivehydrophones with voltage outputs proportional to sound pressure.
B.2 Sound Intensity in Decibels
The decibel (dB), one tenth of a bel, is used by the scientific and engineeringcommunities to express the wide range of sound pressure fluctuations, performanceparameters, and power ratios encountered in transmitting and sensing equipment. Thedecibel is defined as 10 times the logarithm of the ratio of the two powers. In acoustics:
I1 (Intensity Units) I1 (dB//Intensity Unit) = 10 Log ________________ 1 (Intensity Unit)
Since intensity is proportional to the square of sound pressure, sound pressure levelsare expressed in decibels as follows:
P1 (Pressure Units)2
P1 (dB//Pressure Unit) = 10 Log _________________ 1 (Pressure Unit)2
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P1 (Pressure Units) P1 (dB//Pressure Unit) = 20 Log _________________ 1 (Pressure Unit)
In underwater sound, the basic unit of intensity historically has been the intensityof a plane wave having a root mean square (rms) pressure equal to 1 dyne/cm2.Recently the micropascal (µPa), equal to 10-5 dyne per square centimeter, has beenaccepted as the reference-standard pressure for underwater sound measurements. Forexample, if a pressure level of 5,000 µPa were measured, it would be expressed as 20log 5,000 or 74 dB//µPa. Reference pressure levels other than the µPa have been usedin acoustics. Some of these sound-pressure levels and the corresponding conversionfactors to convert to dB//µPa are indicated in table B-1.
Table B-1. Sound-Pressure Level Conversion Factors.
Sound PressureReference Level
To Convert todB//µ Pa Add
µbar 100
dyne/cm2 100
.0002 dyne/cm2 26
Example: 22 dB// µbar = 122 dB//µPa
96 dB//.0002 dyne/cm2 = 122 dB//µPa
When a ratio of pure numbers such as array gain (G), propagation loss (PL)between two points in the ocean, signal excess (SE), or recognition differential (RD) isexpressed in decibels, the appropriate level (L) is
L (dB) = 10 Log (L1 /L2 )
Decibels are based on a logarithmic scale; thus, ten times the logarithm of theproduct of two terms (each of which has been expressed in dB) is the sum of their dBlevels. In a similar manner, when a term is to be divided by another term, subtraction oftheir dB levels is used. (See section B.2.2, Laws of Logarithms.)
I1 (µPa) x g (Dimensionless) = l2 (µPa)
log l1 + log g = log l2
20 log l1 + 20 log g = 20 log l2
Let L1 = 20 log l1, G = 20 log g, L2 = 20 log l2
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then L1 + G = L2
L1 (dB//µPa) + G(dB) = L2 (dB//µPa)
L1 (µPa) = L2 (µPa) PL (Dimensionless)
L1 (dB//µPa) - PL(dB) = L2 (dB//µPa)
Note the following explanation for the two previous equations B8:l = lowercase L1 = numeral one
Some of the properties of the decibel are illustrated by converting several ratiosinto their dB equivalents, as in table B-2; for example, increasing a power of 10 watts bya factor of two is equivalent to adding 3 dB to its initial dB level. Notice particularly that0 dB represents a factor of unity, that is, the ratio of observed sound to the referenceunit is one to one. It does not indicate the absence of sound. Decimal numbersbetween unity and zero have negative dB numbers.
Table B-2. Common Decibel Equivalents.
Numerical Ratio (R) dB (10 Log R)1000.0 30.0
100.0 20.0
10.0 10.0
5.0 7.0
3.0 4.8
2.0 3.0
1.0 0.0
0.7 -1.6
0.5 -3.0
0.01 -20.0
0.001 -30.0
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B.2.1 Derivation of the Passive Sonar Equation
Definition of detectability: D = SO
SD
D = detectability, SO = observed signal-to-noise ratio (SNR), SD = designed SNR
From the definition of SO : SO = IS INIS = signal intensity, IN = noise intensity
By substitution: D = IS . 1 IN SD
The relationship between the signal intensity and the output intensity of the target is
IS = IO L
IO = target output intensity, L = propagation loss factor
By substitution: D = IO . 1 . 1 L IN SD
Introducing a reference intensity ( IR ) and rearranging terms
D = IO . IR . 1 . 1 IR IN L SD
This is the passive sonar equation. Taking 10 log of both sides of this equation yieldsthe more familiar equation (in dB units):
SE = SL – NL – PL – RD where
10 log (D) = SE = Signal Excess 10 log (IO/IR) = SL = Source Level
10 log (IN/IR) = NL = Noise Level 10 log (L) = PL = Propagation Loss
10 log (SD) = RD = Recognition Differential
B.2.2 Laws of Logarithms
1. Product Rule: log (ab) = log (a) + log (b) [Log of product = sum of logs.]
2. Quotient Rule: log (a/b) = log (a) – log (b) [Log of quotient = difference oflogs.]
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3. Power Rule: log (an ) = n log (a)
B.2.3 Power Summing
When two noise sources (expressed in dB) are to be combined, use the followingsteps:
1. convert the values to their original units (intensity units, like µPa2/ρc),
2. add the two noise values in their original units, then
3. compute 10 times the log of the sum; the answer will be in dBs.
For example, suppose we have two noise sources with intensities IA and IB, where
a = noise intensity 1 = IA/IR, b = noise intensity 2 = IB/IR, IR = reference intensity
c = total noise intensity = (IA + IB)/IR
A = noise level 1 = 10 log (a), B = noise level 2 = 10 log (b)
C = total noise level (what we’re trying to find)
Now, since a = 10A/10, and b = 10B/10, c = a + b = 10A/10 + 10B/10 = 10A/10 (1 + 10-(A-B)/10)
So, C = 10 log(c) = A + 10 log (1 + 10-(A-B)/10).
Now, assume A is greater than or equal to B (A ≥ B), and therefore x = A – B ≥ 0, and
C = A + 10 log (1 + 10-x/10)
This last equation shows that the total noise level is equal to the greater of the two noiselevels plus a correction which is a function of the difference between the two levels.
(Note: In this derivation, a, b, and c are dimensionless, while A, B, and C are in dBs.)
For example, suppose a noise level of 22 dB//µPa is to be added to another noise levelof 25 dB//µPa. The combined level is:
10 Log 1022/10 + 1025/10 = 26.8 dB
Symbolically, 22 dB + 25 dB = 26.8 dB,
where, + = Power Sum.
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Figure B-1. Nomogram for Combining Spectrum Levels.
If desired, this arithmetic operation can be avoided by using the graph in figureB-1. Repeating the previous example, apply the difference of 3 dB to the curve and addthe resultant 1.8 dB to the higher level to obtain 26.8 dB.
B.2.4 System Performance. The performance of a sonar system is frequentlyevaluated by comparing it to a sonar equation, discussed in greater detail in chapters 8and 9. One form of the sonar equation is stated as:
SE = SL - NL + DI - RD - PL
The first term, signal excess (SE), is a measure of the ability of a sonar to detecta target. As shown in this equation, SE is equally sensitive to a change in any of thesonar parameters indicated on the right-hand side of the equation. That is, doubling thetarget-radiated noise SL (which is equivalent to raising its levels by 3 dB) has the sameeffect on detection capability as halving noise, NL, and decreasing it by 3 dB. Anunderstanding of this equation assists the sonar designer, sonar operator, and ASWtactician in obtaining optimum performance inasmuch as some of these parameters areeasier to control than others.
B.2.5 Sonar Sound-Pressure Levels. A high-frequency sonar can require a sourcelevel increase of 27 dB (a 500-fold power increase) to double its range. Raising asonar's source level from 100 dB to 127 dB would double its range; hence, 500 timesmore power would be required to produce that level of sound energy.
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Decibels are also applicable to receiver sensitivity in sonar, radar, and radio. Areceiver with a sensitivity of -117 dB is 3 dB better (or can detect a 50-percent weakersignal) than a receiver with a sensitivity of -114 dB. The larger the minus number ofdecibels, the better the receiver sensitivity. It is to be noted, additionally, that a 3-dBloss in receiver sensitivity is as bad as a 3-dB loss in transmitted signal level.Maintaining a sonar receiver's sensitivity is just as important as maintaining theprescribed transmit-power level. The decibels gained through "noise" reduction providethe same increase in performance as an equal increase in source level.
B.2.6 Sound-Pressure Levels of Common Noises. The decibel was originally used asan arbitrary unit based on the faintest sound a person could hear. The dB scale islogarithmic, so that an increase of 10 dB means a tenfold increase of sound intensity: a20-dB rise indicates a hundredfold increase; and a 30-dB increase indicates athousandfold increase in sound intensity. Sound-pressure levels of some commonnoises, expressed in micropascal (µPa), and in decibels relative to a micropascal (dB//µPa), are tabulated in table B-3.
Table B-3. Sound-Pressure Levels of Common Noises.
Sound Pressure LevelNoise µPa dB//µPa
Jet plane at 100 ft 200,000,000 166
Pneumatic riveter 63,000,000 156
Rock music with amplifiers at 4 to 6 ft 20,000,000 146
Loud automobile horn at 23 ft 2,000,000 126
Very heavy traffic (New York City) 200,000 106
Loud peal of thunder 63,000 96
Conversational voice at 12 ft 6,300 76
Quiet suburban street 630 56
Rustling leaves 63 36
Faintest audible sound 20 26
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B.3 Spectrum Levels and Band Levels
Most measurements of broadband noise in sonar are made in frequency bandsthat are hundreds or even thousands of Hertz (Hz) wide. For many applications, it isnecessary to reduce these broadband measurements to an equivalent level in a band of1 Hz within the measurement band. These 1-Hz values, spectrum levels ofL(SPECTRUM), refer to the average level of that part of a signal contained within a1-Hz bandwidth, centered at the particular frequency. A conversion factor can beapplied to the band level, L(BAND LEVEL), measurements to obtain the spectrum levelwithin the band. This conversion factor is a number, in decibels, that is equal to 10 logof the measured bandwidth. Thus,
L(SPECTRUM) = L(BAND LEVEL) – 10 LOG BW
This reduction process is valid for continuous “white” noise having a flatspectrum. It is also valid for a noise having a continuous spectrum falling off at the rateof –6 dB per octave if the center frequency of the band is taken to be its geometricmean frequency (GMF).
The GMF of the band is given as:
GMF = (f1 x f2 )1/2
where f1 and f2 are the upper and lower frequency limits of the band.
A –6 dB-per-octave slope is typical of ambient sea noise, ownship backgroundnoise (at low speeds), and target-radiated noise at frequencies above a few hundredHz.
B.3.1 Bandwidth Conversion Nomogram. A bandwidth conversion nomogram (figureB-2) is presented for determining (a) the correction factor needed for a passbandconversion and (b) the bandwidth conversion factor readouts for one octave, one-halfoctave, one-third octave, and one-tenth octave bandwidths, respectively.
In figure B-2, the upper line is used to determine the quantity (10 log bandwidth)for bands up to 10 kHz in width. For example, if a conversion factor is required for apassbandwidth of 50 Hz, a correction of 17 dB is derived from the top line labeled“Bandwidth.” This conversion can then be applied to the specified 50-Hz band levels.
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Figure B-2. Bandwidth Conversion Curves.
The four lower lines are used to determine the bandwidth correction factor of aproportional band. After locating the GMF of the particular band on the frequency scaleand determining the point where the frequency line intercepts the appropriate bandcurve, the correction factor can be read directly from the dB scale to the left. Forexample, to compute the spectrum level at 1,000 Hz, having been given the half-octaveband level of 50.4 dB, we find from figure B-2 that the conversion for a half-octave bandat 1,000 Hz is 25.4 dB. Thus, the computed spectrum level at this frequency is 50.4 –25.4 = 25.0 dB.
These corrective factors apply to noises typical of ambient sea noise, own shipbackground noise (at low speeds), and lower frequency (for example, several hundredHertz) target-radiated noise. These factors do not apply to noise that has a slopegreater than –6 dB per octave.
B.3.2 Bandwidth. The width of a proportional band can be described as a percentageof the center frequency; these percentages are shown for three kinds of bands in tableB-4, together with the spectrum-level conversions for convenient center frequenciesfound in commonly used filters.
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This table means, for example, that the width of a half-octave band centered at 1,000Hz is 348 Hz (that is, 34.8% x 1,000 Hz and that conversion is 25.4 dB).
Table B-4. Bandwidths as Percentages and Selected Conversions.
Width Conversion in dBOctave % 100 Hz 106 Hz 125 Hz 1,000 Hz 1,700 Hz
1 70.7 18.5 18.7 19.5 28.5 30.81/2 34.8 15.4 15.7 16.4 25.4 27.71/3 23.1 13.6 13.9 14.6 23.6 25.9
B.3.3 Discrete Frequencies. The conversion process is valid only if (a) the band levelcontains no strong discrete frequencies and (b) the noise is basically continuous, asshown in figure B-3. Large energy peaks in discrete frequency regions, as shown infigure B-4, will yield spectrum levels lower than the level of the line component of thespectrum.
Figure B-3. Ideal Continuous Noise.
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Figure B-4. Noise Containing Discrete Frequencies.
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Appendix C
Optical Oceanography
C.1 Introduction
Even in the clearest ocean water, light transmission is attenuated many timesmore than sound transmission. Underwater light travels only a few hundred meters,while sound can travel around the world. In severe cases, water turbidity may evenprevent a diver from seeing his hand against his face plate. In air the transmission oflight is considerably better than the transmission of light in water because the air is 800times less dense than water. However, the physics of light and sound transmission inair and underwater are very similar.
Light is a form of electromagnetic energy that is selectively absorbed in wateraccording to its color or wavelength. Clear oceanic water has the greatest transmissionand least attenuation in the blue-green region of the spectrum. Even at this color, lightintensity is reduced 4 percent for every meter traveled. When moving from clear oceanicwater to the more turbid near-shore water, contaminants from offshore runoff absorbmore in the longer blue-green wavelengths and shift the region of maximum lighttransmission toward the yellow-green wavelengths. This absorption of blue light iscaused by multiple particle scattering in turbid waters nearshore. Blue light travels greatdistances during the scattering process and is absorbed.
Light is attenuated in water by two means: absorption, which converts lightenergy to heat, and scattering, which merely deflects the light to a different direction. Inboth cases, light formed from an image will be attenuated as it travels through thewater. In fact, scattered background light may enter the images path and adverselyaffect the image contrast with its background. Scattered light can blur the fine detail ofan image and can even obliterate an image altogether. Scattering is one of the mostformidable problems in underwater visibility, regardless of viewing media such asphotography, video camera, satellite, or human eye.
NAVOCEANO has a mathematical model called "Visibility Evaluation ofUnderwater Systems" (VEUWS). VEUWS, given the optical parameters of the lightsource, target, and water, can predict the visibility for large targets.
The optical properties of the ocean vary greatly in space and time. Images fromsatellites show this variability on a large scale. However, on a small scale, nearshorewater clarity can vary greatly. For example, clarity can vary spatially on either side of agyre, front, or river plume and can vary in time on tidal cycles, hours, or days. Waterclarity is also weather related. Severe weather with high winds causes waves andcurrents to mix the ocean and thereby reduce water clarity. The clearest waters arefound after the ocean has been calm for at least a few days. Plankton blooms andschools of fish can also reduce water clarity.
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C.2 Sources of Light
Sources of light in the ocean can be divided into categories, natural and artificial.Natural sources of light, including the sun, moon, and stars, are incident on the surfaceand propagate down to depth with diminished intensity. Also, light can be generated byorganisms within the sea, and the light they produce is called bioluminescence.Bioluminescence varies greatly with location. Its intensity ranges from near zero to asbright as a full moon on a clear night.
Table C-1 summarizes several light sources at sea level. The effects of lightattenuation in the atmosphere have been excluded. Figure C-1 summarizes thedistribution of solar radiation as a function of wavelength.
Table C-1. Ground-Level Illumination from Several Common Sources.
SOURCEGROUND-LEVEL
ILLUMINATION (1m/m 2)Sun-clear sky 1 x 105
Sun-cloudy bright 2 x 104
Sun-heavy overcast 2 x 103
Full moon-clear sky 3 x 10-1
Twilight-sunset 1 x 10-1
Full moon-overcast 3 x 10-2
Quarter moon-clear sky 3 x 10-2
Clear sky-no moon 1 x 10-3
Starlight 2 x 10-4
Figure C-1. Standard Relative Luminosity, or Visibility, Curve and Luminous Efficiency.
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C.3 Air-Water Interface
Light incident on the water surface will obey Snell's Law. In a similar way soundis refracted and reflected at an interface of density discontinuities in the ocean. Light,however, pronouncedly polarizes during the process. A detailed discussion ofpolarization will not be given, but the fact that it happens is important. The reflectedlight can be considered as a vector with two components: one componentperpendicular and the other parallel to its path. The degree of polarization depends onincident angle. At an angle of 53 degrees, the reflected light will be completelypolarized. The effect of polarization is the reason why polarized sunglasses increasevisibility through the water interface so well. The glasses block the one vectorcomponent of reflected light and thus cut the glare considerably. Figure C-2 is agraphic representation of Snell's Law. Lower case "n" is the index of refraction. Theangle phi is the angle of refraction and reflection. The angular dependence of refractionand reflection is shown in figures C-3 and C-4, respectively.
n1 sin φ1 = n2 sin φ2 r =
(Snell's Law)
An important phenomenon shown by Snell's Law occurs at the critical angle of48.6 degrees. At this angle and greater, all incident light is reflected and no light isrefracted into the water. The converse is true for light underwater incident on thesurface from below. This phenomenon is the reason why it is possible to see throughthe surface into the water in only a small circle around an observer sitting in a boat andalso is the reason why standing up increases the viewing area so greatly.
C.4 Attenuation of Light
Light is attenuated by two independent physical processes, absorption andscattering. Absorption is the process by which light is absorbed into the water and itsenergy is transformed into heat. Scattering is the process of light changing directionafter it hits a molecule of water (Rayleigh scattering) or a particle suspended in thewater (Mie scattering). Artificial light is attenuated in water by the following formula:
E(r)=E(o)e- αr
= E(o)e-1
/ L , where L = -1/ α
Alpha (α) is the volume attenuation coefficient, (units of 1/m)E(r) is the light intensity left at distance rE(o) is the light intensity at the source r = 0e is the base of the natural logarithm 2.718L is the attenuation length (in meters)
r⊥ + r " 2
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Light is reduced by approximately 37 percent of its original intensity for each attenuationlength traveled.
Alpha (α), the volume attenuation coefficient for artificial light, assumes there is single orno scattering. The formula is
α = a + b
Figure C-2. Reflection and Refraction of a Linearly Polarized Light Wave with itsElectric Vector Parallel to the Plane of Incidence.
where "a" is the volume absorption coefficient and "b" is the volume scatteringcoefficient.
Both of these coefficients depend on wavelength. Scattering depends ongeometry and scatters in all directions. Forward backscattering reduces visibilitygreatly. Scattering also polarizes light as well as reflection but will not be discussedhere. Figures C-5 and C-6 summarize light attenuation in the sea.
Both "a" and "b" are functions of wavelength. Note that blue light scatters themost but is absorbed the least, and red conversely scatters the least but is absorbed themost. This phenomenon is the reason why a clear sky or ocean appears blue.
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C.5 Sunlight in Water
Sunlight is the major source of light in the ocean. It consists of direct sunlightand indirect sunlight scattered through the atmosphere. This light enters the water andis multiply scattered as it penetrates to depth. It becomes so diffuse with depth that itsintensity is dependent only on the zenith angle; increasing depth merely provides aconstant light from all directions.
Figure C-3. Reflectance as a Function of Angle of Incidence.
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Figure C-4. Angle of Incidence and Fraction of Light Refracted into Wateras a Function of φ2.
Figure C-5. Volume-Attenuation Coefficient of Typical Estuary, Coastal, and ClearOceanic Water Compared with that of Distilled Water.
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Much more light will exist at depth than that predicted by using the volumeattenuation coefficient alpha (α). The diffuse extinction coefficient "k" is used instead of"a" to predict ambient light intensity. Ambient light is attenuated in the sea using thisformula:
Ed(z) = E(o) e -k(z)
Ed(z) = light intensity at depth zE(o) = light intensity at the surface, z = o
k = a + B
where "k" is the diffuse attenuation coefficient. "a" is the absorption coefficient and isthe same for artificial light. However, "B" is the scattering coefficient for multiplyscattered light where "b" for artificial light assumed little or single scattering. "k" issmaller than alpha (α) by 1/2 to 1/3. Figure C-7 shows the value of "k" for differentnatural light sources.
C.6 Instrumentation
In situ measurements of light in the sea can be made with special opticalinstruments, satellites, and the common Secchi Disc. Accurate measurements of theattenuation coefficient alpha are made using a transmissometer. The diffuseattenuation coefficient "k" is more commonly measured using an illuminometer,satellites, or Secchi Disc. The transmissometer and illuminometer are precision opticalinstruments. The satellite and Secchi Disc are ballpark estimates of "k" but are spatiallyand seasonally published in atlases.
Figure C-6. Volume-Attenuation Coefficient α and Attenuation Length Lin the Visible Spectrum for Distilled Water.
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Figure C-7. Approximate Illumination as a Function of Depth forSeveral Natural Light Sources.
(Clear oceanic water is assumed with a diffuse attenuation constant k of about 0.05 m-1
for the solid curves. Coastal water with k=0.15 m–1 is assumed for the dashed curve,and maximum-clarity water with k=0.034 m-1 is assumed for the dot-dashed curve.)
Transmissometers measure transmittance, the light intensity attenuated in a1-meter path expressed as a ratio of the light transmitted E(o) to the light received E(r)in percent:
T% = = e -r
, and = -ln 1/T = lnT
T% = transmittanceE(r) = light intensity received, r = 1mE(o) = light intensity transmitted r = 0α = volume attenuation coefficient, alpha (1/m)
For example, T = 75%, then α = - ln 1/T = ln T = ln (0.75) = 0.288(-m)
Illuminometers measure the attenuation of ambient light as a function of depth byusing a ratio of the light intensity incident on the surface E(z1) to the light intensity atdepth E(z2):
E (r)E (o)
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E(z2) E(z)
= e-k(z
1 - z
2) if z1 = 0, = e
-kz
E(z1) E(o)
or E(z) E(o) k = -ln = ln E(o) E(z)
E(z1) = the light intensity at depth z1 (in meters).
E(z2) = the light intensity at depth z2 (in meters).
Note the following explanation for the above equations:
l = lowercase L 1 = numeral one
C.7 Underwater Visibility
Even the underwater visibility of large targets, where fine detail is not consideredimportant, is still not an easy prediction. Natural light tends to be so diffuse it does notcast strong shadows. Multiply scattered light from particles in the image transmissionpath is superimposed on the image, and the image itself is degraded by refractivediscontinuities in the transmission path. Underwater visibility can be a complex subjectthat depends on wavelength, geometry, reflection and refraction, optical properties ofthe water, light source, detector, and contrast. The following discussion is greatlysimplified and intended to provide ballpark estimates on visibility.
Once image-forming light reaches a detector, there is no guarantee the target willbe visible. A white target against a white background is not necessarily visible. Colorplays an important role in the visibility of a target. For example, an olive-green mine onan olive-green mud bottom will be hard to see. Conversely, an olive-green mine on awhite sand bottom will be easy to see. The longest range of visibility is for a whitetarget against a black background or vice versa. The contrast of the target to itsbackground is used to determine if a target is visible. Contrast is defined as
Bt(r) - Bb(r) C =
Bb(r)
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r = path distanceC = contrastBt = target radiance
Bb = background radiance
Bp = path radiance
Bt(r) = e-αr(Bt(o)+Bp(r))
Bb(r) = e-k(Z-B)
Bp(r) = path radiance at the target
Note: If Bb(r) is greater than Bt(r), the contrast is negative.
Figure C-8 shows the geometry and terms used in computing contrast. Inherentparameters are independent of the light intensity in the water. Apparent parametersdepend on light intensity in the water.
Rough estimates of alpha (α) can be made by estimating the distance two dark-suited divers mutually disappear horizontally. This range is approximately 4/α -m = 4Lor α = 4/r and is independent of "k" in this case.
The image radiance Bt(o) and path radiance Bp(r) are both attenuated using thevolume attenuation coefficient, alpha, between the target and the detector at a distance(r) through the water. However, the background radiance at the target Bb(o) is
attenuated by the diffuse attenuation coefficient "k." Bt(r) is a function of the distance (r)
and angle θ. Bb(r) is a function of the depths of the target and detector, and theirgeometry. Figures C-9 and C-10 give examples of contrast or visibility for black andwhite targets. For the human eye, the threshold of visibility has a contrast ofapproximately 0.02, depending on the individual. Other detectors, such as films andvideo cameras, have to be considered separately. Each has its own contrast threshold.Low-light-level video cameras have excellent light sensitivity even in the low-light-levelequivalent to starlight.
The greatest range of visibility for a white target is looking down into the darkdepths (+ contrast), and the least range of visibility for a white target is lookinghorizontally against the ambient light field. When looking up at a white target fromdepth, the target looks black from a distance and is indistinguishable from a blacktarget. However, the target will be visible because of its negative contrast, even thoughno image-forming light is reaching the detector.
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The greatest range of visibility for a black target is viewed looking up from depth.Again, no image-forming light is reaching the detector. The target is seen by virtue of itscontrast only. The poorest visibility is viewed looking down against the dark depths.Black targets are black because they absorb all light incident on them, regardless ofcolor. A colored target absorbs all the incident light except the colors reflected.
C.8 References for Appendix C
1. Mertins, Lawrence E., In-Water Photography (Theory and Practice), John Wiley andSons Publishers, New York, NY, 1970.
Figure C-8. Geometry and Terms Used in Computing Apparent Target Contrast.
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Figure C-9. Contrast as a Function of Viewing Distance for Black-and-White ObjectsWhen Viewed Downward, Upward, and Horizontally against
Ambient Background Radiance.(Background radiance is assumed to have reached its asymptotic distribution, and
α =0.39 and k=0.18 m-1
. Contrast is positive for white object (θ = π/2 and θ = π). Allother contrasts are negative.)
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Figure C-10. Apparent Contrast of Black Marks on Diffuse White Target When Viewedfrom Different Directions.
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Appendix D
Bioluminescence
D.1 Bioluminescent Marine Organisms
Bioluminescence is the emission of light by living organisms. The color of the light isusually blue-green and corresponds to those wavelengths which are transmitted farthestthrough seawater. The intensity of the light is a function of organism type and abundance. This community of organisms includes both plants (phytoplankton) and animals (zooplankton);their limited mobility allows them to be passively transported by currents.
The following organisms can occur in concentrations to be of concern to Navaloperations: (1) microscopic phytoplankton called dinoflagellates which generate sheet-typedisplays. This is a uniform glow which may cover large areas, small patches, or wide ribbons. (2) Crustaceans, such as copepods and euphausiids. These organisms create spark-typedisplays consisting of distinct points of light which are particularly conspicuous in the wake ofa ship, along the hull line, or in agitated waters. (3) Larger plankton, such as jellyfish,ctenophores, and salps. These organisms produce globular displays of light of variousdiameters which may be very bright. These displays may glow for extended periods of timeand are most common in warmer waters.
Bioluminescent displays usually occur as a combination of two types, or occasionally,all three types may be seen at the same time. Frequently the boundaries betweenluminescent and dark water are sharp, and displays may be concentrated in streaks or bandsparallel to wind or current flow. In some regions, spectacular displays such as"phosphorescent wheels," "erupting balls," and "milky seas" have been reported. These"wheels" consist of alternating light and dark bands which rotate around a central hub. "Erupting balls" are described as small balls of luminescence which appear below the surfaceand then rise to the surface, where they spread into large patches. "Milky seas" are describedas large areas of white, blue, or green luminescence which appear to glow continuously,without agitation. These displays are rare but have been reported most frequently in theArabian Sea region.
D.2 Variability
Bioluminescence can be found in all regions of the oceans and at all depths. It is mostprevalent in coastal waters, in frontal zones, and near river outflows. Changes inbioluminescent intensity result from vertical, diurnal, seasonal, and regional variations inplankton abundance. This variability is a consequence of the interaction of light, temperature,stability and mixing rate of the watermass, plant nutrients, and predator abundance. Over thecourse of 24 hours, the intensity normally changes several orders of magnitude.
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Through each season, bioluminescence is expected to be higher in coastal watersrelative to open ocean. Marshlands are breeding grounds for crustaceans, and the potentialfor bioluminescent activity may increase dramatically in adjacent coastal areas. River runoff,agricultural runoff, warm-water effluents, and sewage outfall near coastal cities provideconditions which may be favorable for the rapid development of large concentrations ofluminous dinoflagellates. These blooms may follow rain or strong winds within a week or two. Large concentrations of dinoflagellates may discolor the water red, orange, yellow, green, orbrown during the day. Discolored water often indicates increased nighttime bioluminescenceactivity.
D.3 Displays
Displays can originate at or below the sea surface and are the result of mechanicalstimulation of bioluminescent organisms. Bioluminescence-detected Naval assets includesurface vessels, submarines, SEAL delivery vehicles, and swimmers. Moored mines couldalso create a luminous signature when plankton are stimulated by swell or current-inducedturbulence around a stationary object.
D.4 Detection
Detection by bioluminescent signatures is a nighttime threat and should be of concernfrom about one hour after sunset until sunrise. Most bioluminescent organisms which occur inhigh enough concentrations for detection of Naval platforms will be in the upper 200 feet (60meters) of the water column. Any object moving through the water at night can create aluminous trail that can potentially be detected with devices ranging from the naked eye to off-the-shelf low-light level cameras. Detection depends on the intensity of the bioluminescenceand the optical clarity of the water through which the light passes to the surface.
Potential for detection by bioluminescence is expected to be greater in the coastalareas. Water clarity will decrease and partially attenuate the increased bioluminescencesignal; however, operations are limited to near-surface waters, and detection is probable. Operations during twilight hours would mask luminous trails. If nighttime operations arerequired, avoidance of areas near river mouths, lagoon/marsh systems, and developed areasis recommended. If a swimmer observes bioluminescence as he moves through the water,detection is possible.
D.5 Intensity
The intensity of bioluminescence is less in the open ocean than nearshore; however,open-ocean water is more clear, and the light will propagate to greater distances. Bioluminescence intensity may change dramatically near frontal regions and tends to behigher on the colder side of frontal zones. In some areas, maximum values ofbioluminescence are often at or below the seasonal thermocline.
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Appendix E
Tactical Oceanography Reference Packet
E.1 Introduction
The following packet of diagrams and listings has been organized to provide aconvenient location for information which may be used in developing and presentingtactical oceanography topics. The data may be applied to training scenarios and canserve as a hard-copy backup for periods of computer unavailability. Contents includegraphs, tables, and charts illustrating environmental values related to frequencies andlocations.
Worksheets, summary forms, acoustic model guidance, and environmentalmonitoring recommendations all provide forms that may be used on a daily basis.
161
Tactical Oceanography Reference Packet
Contents
Ambient Noise Value Guide – Worldwide Shipping Densities .......................... 162Ambient Noise Value Guide – AN Level vs. Frequency.................................... 163Rain Level Spectra in Spectrum Level.............................................................. 164Sea State Spectra in Spectrum Level (10 Hz - 1,000 Hz)................................. 165Sea State Spectra in Spectrum Level (1,250 Hz - 100,000 Hz)........................ 166Shipping Level Spectra in Spectrum Level (10 Hz - 700 Hz)............................ 167Ocean Turbulence Spectra/Molecular Agitation Spectra .................................. 168Bandwidth Conversion Curves ......................................................................... 169Surface Duct Cutoff Frequency Graph ............................................................. 170Sound Channel Low-Frequency Cutoff Graph.................................................. 171Probability of Detection (Signal Excess)........................................................... 172Standard Deviation of FOM (Sigma)................................................................. 173Probability of Convergence Zone (CZ) Occurrence (%) ................................... 174Ambient Limited Speed (ALS)/Breakpoint Speed (BPS) .................................. 175Ambient Noise, Self Noise, and Total Background Noise as a Functionof Own Ship’s Speed ........................................................................................ 176Omnidirectional FOM Worksheet...................................................................... 177Beam-forming FOM Worksheet ....................................................................... 178Tactical Oceanography Summary (Page 1 of 4)............................................... 179Tactical Oceanography Summary (Page 2 of 4)............................................... 180Tactical Oceanography Summary (Page 3 of 4)............................................... 181Tactical Oceanography Summary (Page 4 of 4)............................................... 182Representative Prediction Frequencies - World Ocean/Sea Salinity Values .... 183Passive Acoustic Model Guidance ................................................................... 184Environmental Awareness................................................................................ 185Useful Formulas and Definitions....................................................................... 186
162
Ambient Noise Value Guide
Worldwide Shipping Density
163
Ambient Noise Value GuideAmbient Noise Level Versus Frequency
164
Frequency (Hz)
Intermittent Moderate Heavy
(dB//1 Pa/Hz) µRain Level Spectra in Spectrum Level
600630700800900100012501500160020002500300031504000500060006300700080009000100001250015000
8080797978787675757472717069676565646362615957
8181818181818080808079787877767574747372716968
8282828282828282828282818181818080807979787775
165
Frequency (Hz)
Sea State Spectra in Spectrum Level (dB//1 Pa/Hz)µ
0 1 2 3 4 5 6
1012.5151620253031.5405060637080901001251501602002503003154005006006307008009001,000
51515151515151515151515151515151515151515050504948484747464545
58585858585858585858585858585858585757575757575655555554545353
62626262626262626262626262626262626262626262626161616160605959
65656565656565656565656565656565656565656565656564646464636362
67676767676767676767676767676767676767676767676766666666656565
68686868686868686868686868686868686868686868686868686868676767
71717171717171717171717171717171717171717171717171717070707069
166
Frequency (Hz)
Sea State Spectra in Spectrum Level(dB//1 Pa/Hz) µ
0 1 2 3 4 5 6
12501500160020002500300031504000500060006300700080009000100001250015000160002000025000300003150040000500006000063000700008000090000100000
43424240383736343331313029282725242322201918161413131211109
525150494746464442414039383736353333312928272624222221201918
585756555452525048474646454443414039373634343230292827262625
626160595756565452515049484747454343413938373634333231302928
646363616058585654535352515049474645444240403836353434333231
666565636260605856555554535251494847454442424038373736353433
696867666463636159585756555453525050584645444341
403939383736
167
Shipping Level Spectra in Spectrum LevelFrequency (Hz)
Basins Chokepoints
0 I II III IV V VI VII VIII IX
101315162025303240506063708090100125150160200250300315400500600630700
N/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/AN/A
57596060616262626261606059575655535151N/AN/AN/AN/AN/AN/AN/AN/AN/A
616263646466666667676766656362605755545250N/AN/AN/AN/AN/AN/AN/A
66686969707171727272727271696866626060575553535149N/AN/AN/A
71737474757677777777777776747371686564625957565453N/AN/AN/A
75777778798081818282828181797876737069666361615957N/AN/AN/A
79808181838485858686868685848281777574716865656260N/AN/AN/A
82838585868788888990909090898786827979757270696664N/AN/AN/A
84858787888990909192909089878685828079777574747270696868
88899191929394949596949393919089868383817978787674737272
168
Ocean TurbulenceSpectra
Molecular AgitationSpectra
Frequency(Hz)
Frequency(Hz)
Spectrum Level(dB//1 µPa/Hz)
Spectrum Level(dB//1 µPa/Hz)
1.0 109 6,000 1
1.25 105 6,300 1
1.5 103 7,000 2
1.6 102 8,000 3
2.0 99 9,000 4
2.5 96 10,000 5
3.0 93 12,500 7
3.15 92 15,000 8
4.0 89 16,000 9
5.0 86 20,000 11
6.0 83 25,000 13
6.3 82 30,000 14
7.0 81 31,500 15
8.0 79 40,000 17
9.0 78 50,000 19
10.0 76 60,000 21
12.5 73 63,000 21
15.0 70 70,000 22
16.0 69 80,000 23
20.0 66 90,000 24
25.0 63 100,000 25
30.0 61
31.5 60
40.0 56
50.0 53
169
Bandwidth Conversion Curves
Frequency (Hz)
Note: Use upper curve when width of pass band is known; use lower curves if an octave band is specified.
170
171
Sound ChannelLow-Frequency Cutoff Graph
172
Probability of DetectionSignal Excess
(+) Plus Minus (-)
To select the appropriate Sigma Value, use the following guidelines:
a. A Sigma of 6, if Ambient Noise measurements
have been made and Submarine Speed and Type
are known.
b. A Sigma of 8, if Ambient Noise is estimated from
forecasts, Submarine Speed is known to within 3
knots, and Type is known.
c. A Sigma of 10, if Ambient Noise is estimated
from forecasts and Submarine Speed and Type
are uncertain.
Probability of CZ Detection ( 8 dB Uncertainty)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
50
55
59
64
69
73
77
80
84
86
89
91
93
94
96
96
97
97
98
98
99
Average Signal Excess
in the CZ (dB)
Probability of
CZ Detection (%)
20 18 16 14 12 10 8 6 4 2 0 2 4 6 8 10 12 14 16 18 20
SigmaVal
6
8
9
10
11
12
14
100 100 99 99 98 95 90 84 74 63
99 98 97 96 93 89 84 77 69 59
98 97 96 94 91 86 81 74 67 56
98 96 95 92 89 84 79 73 66 55
97 95 93 90 86 82 76 71 65 54
95 93 91 88 84 79 74 69 63 54
92 90 87 84 80 76 71 67 61 54
50
50
50
50
50
50
50
37 26 16 10 5 2 1 1 0 0
41 31 23 16 11 7 4 3 2 1
44 33 26 19 14 9 6 4 3 2
45 34 27 21 16 11 8 5 4 2
46 35 29 24 18 14 10 7 5 3
46 37 31 26 21 16 12 9 7 5
46 39 33 29 24 20 16 13 10 8
173
0.2 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99
To select the appropriate Sigma Value, use the following guidelines:
a. A Sigma of 6, if Ambient Noise measurements have been made and Submarine Speed and Type are known. b. A Sigma of 8, if Ambient Noise is estimated from forecasts, Submarine Speed is known to within 3 knots, and Type is known. c. A Sigma of 10, if Ambient Noise is estimated from forecasts and Submarine Speed and Type are uncertain.
Probability of Detection (%)
-25
-10
+5
Sig
nal E
xces
s (d
B)
-20
-15
-5
0
+10
+15
+20
10 dB
8 dB
6 dB
Standard Deviation of FOM (Sigma)
174
Probability of Conver gence Zone (CZ)Occurrence (%)
175
Ambient Limited Speed (ALS)/Breakpoint Speed (BPS)
LE = Total Back ground Noise
La = Ambient Noise
Le = Self Noise
LE = La + Le
Ambient Limited Speed (ALS):- that speed at which LE = L a + 1 dB.
This occurs at the speed whereLe = La - 6 dB.
Breakpoint Speed (BPS):- that speed at which LE = L a + 3 dB.
This occurs at the speed whereLe = La.
176
Ambient Noise, Self Noise, andTotal Background Noise
As a Function of Own-Ship’s Speed
177
178
UNCLASSIFIEDSECRET WHEN FILLED IN
UNCLASSIFIEDSECRET WHEN FILLED IN
179
UNCLASSIFIED
SECRET WHEN FILLED IN
(Page 1 of 4)
DTG (Local Time)____________
FROM: Sonar Watch SupervisorTO: Commanding Officer
Tactical Action Officer
SUBJ: TACTICAL OCEANOGRAPHY SUMMARY
1. SITUATION:Based on INSITU BT at ________________(Local Time)
In Position_______________LAT___________________LON
ASW Prediction Area____________________Location
Sea State__________________ Shipping Density___________________
General/Specific Threat Search__________________________________
Own Ship Speed __________kts Target Speed______________kts
2. OCEANOGRAPHY:
SD SST__________F SLD__________ft COF__________Hz
SC DSCA__________ft SSC (Yes/No)
SSC from__________ft to__________ft Thickness__________ft
SSCA__________ft Delta C__________ft/sec COF__________Hz
CZ Depth Excess__________FA CZ Range__________Kyds
Submerged CZ (Yes/No) Conjugate Depth__________ft
BB Bottom Depth__________FA Topography__________
Bottom Loss Class: High Freq.__________ Low Freq.__________
BB Propagation: HF (Good/Marginal/Poor) LF (Good/Marginal/Poor)
UNCLASSIFIED
SECRET WHEN FILLED IN
180
UNCLASSIFIED
SECRET WHEN FILLED IN
(Page 2 of 4)
3. PASSIVE ACOUSTIC SPREADSHEET:
Freq. (Hz)/Source
SL Sonar/Processor
FOM/FDM
S/RGeometry
Range Predictions(MDR,BB,CZ,PCZD)
Detection:____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Counterdetection:____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
4. ANALYSIS & APPLICATION:
Our Best Sensor_______________ Our Best Search Frequency____________Hz
ALS__________kts BPS__________kts RSS__________kts
Best Towed-Array Depth__________ft Best Sonobuoy Depth__________ft
Target’s Best Sensor____________ Target’s Best Search Frequency__________Hz
Best Listening Depth_________ft Best Depth to Avoid Detection__________ft
Acoustic Advantage (kyds):DP:_________________________________________________________________________
CZ:_________________________________________________________________________
UNCLASSIFIED
SECRET WHEN FILLED IN
181
UNCLASSIFIED
SECRET WHEN FILLED IN
(Page 3 of 4)
5. ENVIRONMENTAL CONSIDERATIONS IN THE ACTIVE SONAR EMPLOYMENTDECISION:
SEARCH MDR (KYDS) COUNTERDETECTION MCDR (KYDS)
Active Sonar:
AN/SQS-53B PD ____________ Passive Acoustic ____________
BD ____________ Ping Intercept ____________
AN/SQS-53C PD ____________ Mean Effective Torp. RNG ____________
BD ____________ ESM ____________
AN/SQS-56 PD ____________ Mean Effective SSM RNG ____________
BD ____________ Imputed Mission ____________
AN/SSQ-62 S/S ____________ Periscope Visual ____________
CL ____________
D/D ____________
SEARCH MDR (KYDS) SEARCH MCDR (KYDS)
Non-Acoustic:
VISUAL-AIRCRAFT ____________ VISUAL-SHIP ____________
FLIR/IRDS ____________ AN/SPS-10/67 ____________
AN/APS-115 ____________ AN/SPS-55 ____________
AN/APS-124 ____________ AN/SPS-64 ____________
AN/APS-137 ____________ AN/SPQ-9 ____________
ESM ____________ MK 92 FCS ____________
UNCLASSIFIED
SECRET WHEN FILLED IN
182
UNCLASSIFIED
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(Page 4 of 4)
5. ENVIRONMENTAL CONSIDERATIONS IN THE ACTIVE SONAR EMPLOYMENTDECISION:
a. Passive Sonar and non-acoustic detection opportunities depend upon the threat submarine,i.e., she must snorkel or expose a mast.
b. Active Sonar detection opportunities are dependent on the environment.
c. Consider ROE, EMCON, PMI, Imputed Threat Submarine Mission, and sensor opportunities.In peacetime or times of rising tensions, the threat submarine’s mission may be to conductundetected transit to insert SOF, lay mines, or position for ASUW patrol. At these times, activeSonar and Radar may not endanger own force and may deter the submarine.
d. In Hot War, within a poor acoustic environment (Zero Layer Depth – ZLD), where MDR iswell within the threat’s Mean Effective Torpedo Range, non-acoustic sensors (primarily airborneand shipboard Radars) provide the best potential for detection outside the threats weaponrange.
IS PREDICTED ACTIVE MDR WITHIN THE THREAT SUBMARINE’S EFFECTIVE TORPEDORANGE?
e. Consider delaying active search until CUED by shipboard Non-Acoustic, Towed-Array, orAirborne Sensors.
UNCLASSIFIED
SECRET WHEN FILLED IN
183
Select Prediction Frequencies to represent the PrimarySearch Tonals (PST) and Passive Broadband PredictionFrequencies (PBBPF) from the following table:
PST/PBBPF Prediction Frequency
10-90 5090-200 150
200-450 300450-750 600
750-1100 9001100-1500 1200
WORLD OCEAN/SEA SALINITY VALUESOcean/Sea Salinity (PTS/1000)
Mediterranean Sea 38North Atlantic Ocean 35Atlantic Ocean 35Indian Ocean 35Pacific Ocean 35North Pacific Ocean & Marginal Ice Zones 32Red Sea 39-42Black Sea 18-22South China Sea 34East China Sea 33Korea Strait 33Yellow Sea 31.5Sea of Japan 34Sea of Okhotsk 32.5Kuril Basin 32.5Bering Sea 33
184
Passive AcousticModel Guidance
185
Environmental Awareness
Any change, of equal or greater value, inthe listed parameters should be followed by a newEnvironmental/Acoustic Range Prediction:
Sea State Any Change
Wind Speed 5 Knots
Sea Surface Temperature 2 ° F
Sonic Layer Depth (SLD) 50 Ft.
Gradient Below Layer 0.5 ° F / 100 Ft.
Ambient Noise 2 dB
Water Depth 100 Fathoms
Bottom Province Any Change
Biologics Low to High
Own Ship’s Speed 3 Knots
186
Useful Formulas and Definitions
1. C = λ F , where C = Speed of Sound, λ = wavelength, and F = Frequency
2. Snell's Law: C1 C2 where C = Speed of Sound
Cos θ1 Cos θ2 θ = Angle of Incidence
3. Spreading Loss: Cylindrical = 10 log r where r = rangeSpherical = 20 log rDipolar = 40 log r
4. Geometric Mean Frequency: GMF = √ f1 x f2
where GMF = Geometric Mean Frequencyf1 = Lowest Frequency in Bandf2 = Highest Frequency in Band
5. Signal Excess Form of the Passive Sonar Equation:
SE = SL - PL - NL + DI - RD
where SE = Signal Excess, SL = Source Level, PL = Propagation Loss,NL = Noise Level (Total Background Noise), DI = Directivity Index, andRD = Recognition Differential.
6. Active Sonar Equation (Noise-Limited, Monostatic Case):
SE = SL + TS - NL + DI - RD - 2PL
where SE = Signal Excess, SL = Source Level, TS = Target Strength,NL = Noise Level (Total Background Noise),DI = Directivity Index, RD = Recognition Differential,and PL = Propagation Loss.
=
187
Useful Formulas and Definitions
7. Cutoff Frequency - the lowest frequency which may be trapped within a SurfaceDuct or within a Sound Channel; limiting frequency is based onrespective wavelength; optimum frequency which will be trapped is1.8 to 2.0 times the cutoff frequency.
Formula for Surface Duct: fc = 0.3978 x Co
1.5
Zld x (∆ C).5
where fc = cutoff frequency in Hz, ∆C = Cld - Co,
Cld = sound speed at sonic layer depth, andCo = surface sound speed , Zld = sonic layer depth,0.3978= constant, independent of measuring system(feet or meters).
Formula for Sound Channel: fc = 0.2652 x Ca 1.5
∆Ζ x (∆C) .5
where fc = cutoff frequency in Hz, ∆Z = sound channel thickness,∆C = Cb - Ca, Ca = sound speed at sound channel axis depth,Cb = sound speed at channel boundaries.0.2652 = constant, independent of measuring system(feet or meters).
8. Sonic Layer Depth (SLD) - depth on a Sound Speed Profile where the maximumnear-surface sound speed is attained; the bottomdepth of the Surface Duct; upper boundary of theDeep Sound Channel.
9. Critical Depth (CD) - deep depth on a Sound Speed Profile where the soundspeed at the Sonic Layer Depth is reacquired; lower boundary ofthe Deep Sound Channel.
188
Useful Formulas and Definitions
10. Depth Excess (DE) - difference in depth from the Critical Depth to the OceanBottom for near-surface sources or from the ConjugateDepth to the Ocean Bottom for submerged (below-layer)sources; usually measured in fathoms; relates to theprobability of convergence zone propagation path occurrence.
11. Sound Channel - any location on the sound speed profile where a negativegradient is followed by a positive gradient, which forms an axisat the sound speed minimum occurring between the gradients;Strength (Magnitude) is the difference in sound speed betweenthe axis and the boundaries; Thickness is the difference indepth between the upper and lower boundary.
12. Deep Sound Channel (DSC) - sound channel on the sound speed profile with itsaxis (DSCA) as the lowest sound speed occurring on the entireprofile. Usually occurs at several thousand feet of depth, butmay migrate to shallow depths during winter season in highlatitudes and in the Mediterranean Sea.
13. Secondary Sound Channel (SSC) - sound channel occurring within either the upper portion of the Deep Sound Channel or the Surface Duct;
may occur for short duration due to ocean front interactions;occur for long duration in a variety of watermasses around theworld; usually shallow enough and of long enough duration towarrant tactical investigation; axis is entitled Secondary SoundChannel Axis (SSCA).
14. Submerged Convergence Zone - propagation path occurring in the upper regionof the Deep Sound Channel; focusing of acoustic energy definespath as convergence zone propagation and is most intense atdepths approximately equal to submarine depth; extent ofpropagation path is determined by sound speed profile and depthof submarine; ranges are shorter than surface convergence zonepaths.
189
Useful Formulas and Definitions
15. Conjugate Depth - relatively deep depth on the sound speed profile at which the sound speed equals the sound speed at the depth of a submarine
below the layer.
16. Wilson's Equation for Speed of Sound:
Metric System:C = 1449.2 + 4.623T - .0546T2 + 1.391(S-35) + .017D
where C = Speed of Sound in meters/secondT = Temperature in CelsiusS = Salinity in parts per thousand (ppt)D = Depth in meters
English System:C = 4427.2 + 11.962T - .0553T2 + 4.562(S-35) + .017D
where C = Speed of Sound in feet/secondT = Temperature in FahrenheitS = Salinity in parts per thousand (ppt)D = Depth in feet
17. Sound Speed Factors - the factors relating to changes in sound speed areTemperature (T), Salinity (S), and Pressure (P). The relationship to thechange in sound speed for the change of each factor is as follows:
Metric System:∆CT = ( 4.62 - .11 TC ) ∆ TC , where ∆ TC = 1o C; for TC =13o C , ∆C = 3.2 m/sec∆CS = 1.4 ∆S , for ∆S = 1 ppt, ∆C = 1.4 m/sec∆CD = .017 ∆D , for ∆D =100 meters, ∆C = 1.7 m/sec
English System:∆CT = ( 11.96 - .11 Tf ) ∆ Tf, where ∆ Tf = 1o F; for Tf =54oF , ∆C = 6.0 ft/sec∆CS = 4.56 ∆S ; for ∆S = 1 ppt, ∆C = 4.6 ft/sec∆CD = .017 ∆D for ∆D =100 feet, ∆C = 1.7 ft/sec
190
Appendix F
FOM Terminology
F.1 Introduction
The Figure of Merit (FOM) terminology used in the U.S. Navy is not universalbetween USW communities or platform types. The following list of FOM formulasshould help to clarify the differences.
Airborne Platform FOM Equation:
FOM = SL - AN - RD
Where: FOM = Figure of MeritSL = Source LevelAN = Ambient NoiseRD = Recognition Differential
Surface Platform FOM Equation:
FOM = SL - LE - RD
Where: FOM = Figure of MeritSL = Source LevelLE = Total Background NoiseRD = Recognition Differential
Submarine Platform FOM Equation:
NFM = LS - LE - NRD
Where: NFM = Figure of MeritLS = Source LevelLE = Total Background NoiseNRD = Recognition Differential
191
Appendix R
References
1. Bell, T.G.; Comparison of Target Detection Results with Expectations based onUSL Range Prediction Methods (U), U.S. Navy Underwater Sound Laboratory ResearchReport Number 576, U.S. Navy Underwater Sound Laboratory, New London, CT, April1963. CONFIDENTIAL
2. Bell, T.G.; Operating the AN/SQS-26 Sonar in the Ocean Environment (U), USLResearch Report No. 726, U.S. Navy Underwater Sound Laboratory, New London, CT,1966. CONFIDENTIAL
3. Blumenthal, B.; Guide to Common Shipboard Expendable Bathythermograph(SXBT) Recording Malfunctions, Reference Publication 21, Naval OceanographicOffice, NSTL, MS, August 1978.
4. Carter, D.J.T.; Echo Sounding Correction Tables, 3rd Edition, NP 139,Hydrographic Department, Minister of Defense, Taunton, Somerset, 1980.
5. Carter, R.G., LCDR USN, ed.; Destroyer Sonar Manual (U), Technical Report 8-74,COMCRUDESGRUTWO/DESDEVGRU, 1974. CONFIDENTIAL
6. Convergence Zone Range Slide Rule, Naval Undersea Warfare Center, NewLondon, CT, Revised 1973.
7. Del Santo, Jr., R.F., and T.G. Bell; A Comparison of Predicted Versus ActualSubmarine Sonar Detection Ranges (U), U.S. Navy Underwater Sound LaboratoryReport 544, U.S. Navy Underwater Sound Laboratory, New London, CT, 1962.CONFIDENTIAL
8. Diachok, O.I., and R. S. Winokur; “Spacial Variability of Underwater Ambient Noiseat the Arctic Ice-Water Boundary,” Journal of the Acoustic Society of America, 55, No. 4[1974]:750.
9. Hanssen, G.L.; Application and Display (U), Volume 6, Special Publication Number106, U.S. Naval Oceanographic Office, Washington, D.C., First Edition, 1966.CONFIDENTIAL
10. Hanssen, G.L.; Operational Display of Oceanographic Charts, Informal Report No.67-86, U.S. Naval Oceanographic Office, Washington, D.C., December 1967.
11. Huff, R.P. Lt., USN; COMPATWINGSPAC ASW Oceanography News,Oceanography – How to Get the Most From It, FASOTRAGRUPAC DET,PATWINGSPAC, NAS Moffett Field, CA, n.d.
192
12. Jitkovskiy, Yu, and L. Volovova; Sound Scattering from the Ocean Bottom,Proceedings of the Fifth International Acoustic Congress, Paper E67, Liege, Belgium,1965.
13. Lehmann, Richard, 1992 and 1998.
14. Levenson, C.; Atlas of Non-Submarine Sonar Targets (Whales and BottomFeatures for the Western North Atlantic), unpublished report, U.S. Naval OceanographicOffice, Washington, D.C., 1969.
15. Lyons, A.M.; Sea Water Sound Speed Expressed in English Units, U.S. NavalOrdnance Laboratory Technical Report 63-168, U.S. Naval Ordnance Laboratory, WhiteOak, MD, 1963.
16. Matthews, D.J.; Tables of the Velocity of Sound in Pure Water and Sea Water forUse in Echo Sounding and Sound Ranging, Hydrographic Department, MinistryDefense (Naval), London, England, 1939.
17. Officer, C.B.; Introduction to the Theory of Sound Transmission, McGraw-Hill, NewYork, NY, 1958.
18. Operating Guidelines for the CG-47 Class Ship with the AN/SQQ-89(V)3 SurfaceAntisubmarine Warfare Combat System, Sonar Supervisor Manual (U), NUSC TD-8063-1, August 1988. SECRET
19. Submarine Tactics (U), Vol. 7, No. 2, pp. 2-6, COMSUBDEVRON 12, September1986. CONFIDENTIAL
20. Surface Ship Acoustic Prediction Systems and Tactics (U), NWP 3-21/34, Chief ofNaval Operations, Department of the Navy, Washington, D.C., October 1998.CONFIDENTIAL
21. Swanson, B.K.; Submarine Sonar Environmental Manual (U), Special PublicationNumber 140, U.S. Naval Oceanographic Office, Washington, D.C., 1974.CONFIDENTIAL
22. Urick, R.J.; Principles of Underwater Sound for Engineers, McGraw-Hill, New York,NY, 1967.
23. Urick, R.J.; Principles of Underwater Sound for Engineers, 2nd ed., McGraw-Hill,New York, NY, 1975.
24. Urick, R.J., Sound Propagation in the Sea, Defense Advanced Research ProjectsAgency (DARPA), Washington, D.C., 1979.
193
25. Vidale, M.L., and M.H. Houston; Estimates of Ambient Noise in the Deep Ocean(U), General Oceanography Report No. 4, LRAPP, Office of Naval Research, December1968. CONFIDENTIAL
26. Wilson, W.D.; “Speed of Sound in Sea Water,” Journal of the Acoustic Society ofAmerica, 1960, 32:641.
194
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
A1J1B PEOASWASM PATUXENT RIVER MD 1
A1J1K PEOUNSEAWAR WASHINGTON DC 1
A1J1M PEOMINEWAR WASHINGTON DC 1
A1J1N PEOSUB WASHINGTON DC 1
A2A CNR ARLINGTON VA 4[32B, 32SO, 322B (2 copies)]
A3 CNO WASHINGTON DC 8[N096 (2 copies), N84, N85, N87, N091(2 copies), N095]
A6 CMC WASHINGTON DC (ASL-44) 1
B2A JWAC DAHLGREN VA 1
B2A JWAC DET WASHINGTON DC 1
B2E DMACSC WASHINGTON DC 1
B2E DMACSC EUR OBERAUERBACH GE 1
B2E DMACSC LANT NORFOLK VA 1
B2E DMACSC PAC HICKAM AFB HI 1
B2E DMACSC EUR DET NAPLES IT 1
B2E DMACSC LATIN AMERICA ALBROOK AFB PM 1
B2E DMACSC PAC DET ATSUGI JA 1
B2E DMACSC PAC DET SAN DIEGO CA 1
B2E NIMA HQ FAIRFAX VA 1
B2E NIMA WASHINGTON DC 1
B2E DMS FT BELVOIR VA 1
195
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
B2G DTIC (OCC) 1
B3 COMDT AFCS NORFOLK VA 1
B3 COMDT NWC WASHINGTON DC 1
B3 IRMC WASHINGTON DC 1
B3 PRES NDU WASHINGTON DC 1
C20C NRL DET STENNIS SPACE CENTER MS 3(7100, 7300, 7400)
C20C NRL DET MONTEREY CA 1(7500)
C20C NRLCHESBAY DET CHESAPEAKE BEACH MD 1
C281 NAVOCEANPROFAC WHIDBEY ISLAND DET 1COOS HEAD OR
C281 NAVOCEANPROFAC WHIDBEY ISLAND DET 1PACIFIC BEACH WA
C281 NAVOCEANPROFAC WHIDBEY ISLAND DET 1PEARL HARBOR HI
C40 FLENUMMETOC DET ASHEVILLE NC 1
C40 NAVPACMETOC DET ATSUGI JA 1
C40 NAVPACMETOCFAC COMP BANGOR WA 2
C40 NAVPACMETOC DET BARBERS POINT HI 1
C40 NAVLANTMETOC DET BRUNSWICK ME 1
C40 NAVLANTMETOC DET CECIL FIELD FL 1
C40 NAVTRAMETOC DET CORPUS CHRISTI TX 1
C40 NAVPACMETOC DET DIEGO GARCIA 1
196
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
C40 NAVPACMETOC DET EL CENTRO CA 1
C40 NAVPACMETOC DET FALLON CA 1
C40 NAVTRAMETOC DET FT WORTH TX 1
C40 NAVLANTMETOC DET GUANTANAMO BAY CU 1
C40 NAVPACMETOC DET KADENA JA 1
C40 NAVLANTMETOC DET KEFLAVIK IC 1
C40 NAVLANTMETOC DET, KEY WEST FL 1
C40 NAVLANTMETOCFAC COMP KINGS BAY GA 2
C40 NAVTRAMETOC DET KINGSVILLE TX 1
C40 NAVPACMETOC DET LEMOORE CA 1
C40 NAVLANTMETOC DET MAYPORT FL 1
C40 NAVTRAMETOC DET MERIDIAN MS 1
C40 NAVPACMETOC DET MIRAMAR CA 1
C40 NAVPACMETOC DET MISAWA JA 1
C40 NAVEURMETOC DET NAPLES IT 3
C40 NAVLANTMETOC FAC COMP NEW LONDON CT 1
C40 NAVTRAMETOC DET NEW ORLEANS LA 1
C40 NAVTRAMETOC DET NEWPORT RI 1
C40 NAVLANTMETOC DET, OCEANA VA 1
C40 NAVLANTMETOC DET PATUXENT RIVER MD 1
C40 NAVPACMETOC DET PT MAGU CA 1
197
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
C40 NAVLANTMETOC DET ROOSEVELT ROADS PR 1
C40 NAVPACMETOC DET SASEBO JA 1
C40 NAVEURMETOC DET SIGONELLA IT 1
C40 NAVEURMETOC DET SOUDA BAY GR 1
C40 FLTNUMMETOC DET TINKER AFB OK 1
C40 NAVPACMETOC DET WHIDBEY ISLAND WA 1
C40 NAVTRAMETOC DET WHITING FIELD FL 1
C40 NAVTRAMETOC DET WILLOW GROVE PA 1
C84D NAVUNSEAWARCEN DET AUTECANDROS ISLAND BAHAMAS 1
C84D NAVUNSEAWARCEN DET ORLANDO FL 1
C84D NAVUNSEAWARCEN DET WAIANAE HI 1
C84D NAVUNSEAWARCEN DET AUTECWEST PALM BEACH FL 1
E3B ONR EUR 1
FA39 NAVOCEANPROFAC DAM NECK VA 7[Attn: SURTASS MIL DET (5)]
FA39 NAVOCEANPROFAC WHIDBEY ISLAND WA 8[Attn: SURTASS MIL DET (6)]
FA43 REDTRAFAC DAM NECK VA 1
FD1 COMNAVMETOCCOM STENNIS SPACE CENTER MS 34[N43 (10 copies), N434 (24 copies)]
FD2 NAVOCEANO STENNIS SPACE CENTER MS 18[N72TS(10), N72MD, N72PD(5), N72JL, N72JP]
198
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
FD3 FLENUMMETOCCEN MONTEREY CA 3
FD4 NAVLANTMETOCCEN NORFOLK VA 5[MET (2 copies)]
FD4 NAVPACMETOCCEN PEARL HARBOR HI 5[MET (2copies)]
FD4 NAVICECEN SUITLAND MD 3
FD5 NAVEURMETOCCEN ROTA SP 5[MET (2copies)]
FD6 NAVLANTMETOCFAC, JACKSONVILLE FL 5[MET (2 copies)]
FD6 NAVPACMETOCFAC, SAN DIEGO CA 5[MET (2 copies)]
FD6 NAVPACMETOCFAC, YOKOSUKA JA 5[MET (2 copies)]
FD6 NAVCENTMETOCFAC, BAHRAIN 5[MET (2 copies)]
FD7 NAVTRAMETOCFAC, PENSACOLA 3
FF6 NAVOBSY WASHINGTON DC 2
FF38 U.S. NAVAL ACADEMY 1(ATTN: OCEANOGRAPHY DEPT.)
FKA12 TRITRAFAC KINGS BAY 2
FKP1E NAVUNSEAWARCENDIV NEWPORT RI 1
FKP1E NAVUNSEAWARCENDIV KEYPORT WA 1
FKP1E COMNAVUNSEAWARCEN NEWPORT RI 5
FS1 ONI WASHINGTON DC (ATTN CODE 26M) 2
199
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
FT13 NATTC 1
FT15 NAVTECTRAU, KEESLER AFB 9[Code OO, OOE, CISO, 50 (4) , & 106(2)]
FT24 FLETRACEN NORFOLK VA 3[N396, N396A (2 copies)
FT38 NAVSUBTRACENPAC PEARL HARBOR HI 3
FT43 SWOSCOLCOM 1
FT46 FLEASWTRACENPAC SAN DIEGO CA 4(N65)
FT78 NETPDTC PENSACOLA FL 3[N311(2), N315]
FT85 TRITRAFAC BANGOR WA 2
FT95 SUBTRAFAC NORFOLK VA 3
V4 MARINE CORPS AIR FACILITY 2(Attn: WXSVCOFF)MCAF KANEOHE BAY HIMCAF QUANTICO VA
V5 MARINE CORPS AIR STATION 9(Attn: WXSVCOFF)MCAS CHERRY POINT NCMCAS NEW RIVER NCMCAS BEAUFORT SCMCAS MIRAMAR CAMCAS CAMP PENDLETON CAMCAS EL TORO CAMCAS YUMA AZMCAS FUTENMA JAMCAS IWAKUNI JA
V12 MARINE CORPS COMBAT DEVELOPMENT 1COMMAND (Attn: DOCTRINE DIVISION)
200
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
V25 MARINE CORPS AIR/GROUND COMBAT CENTER 1(Attn: WXSVCOFF)
N/A CHEMICAL BIOLOGICAL INCIDENT 1RESPONSE FORCE (Attn: S-2/S-3)
21A1 CINLANTFLT NORFOLK VA (Code N37) 1
21A2 CINCPACFLT PEARL HARBOR HI (Code O2M) 1
21A3 CINCUSNAVEUR LONDON UK 1
21A3 CINCUSNAVEUR NAPLES IT 1
21A4 COMUSNAVCENT BAHRAIN 1
21A4 DEPCOMUSNAVCENT MACDILL AFB FL 1
22A1 COMSECONDFLT 1
22A2 COMSEVENTHFLT 1
22A2 COMTHIRDFLT 1
22A3 COMSIXTHFLT 1
22A4 COMFIFTHFLT 1
23A1 COMNAVICE KEFLAVIK IC 1
23A2 COMNAVFORKOREA DET CINC CHINHAE KOR 1
23A2 COMNAVMARIANAS DET CAT GU 1
23A2 COMUSNAVAK JUNEAU AK 1
23A2 COMNAVFORJAPAN YOKOSUKA JA 1
23A2 COMNAVFORKOREA SEOUL KOR 1
23A2 COMNAVFORKOREA NCC DET CHINHAE KOR 1
201
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
23A2 COMNAVMARIANAS GU 1
23B1 USCOMSOLANT 1
23B1 CTF EIGHT FOUR 1
23B1 USCOMEASTLANT LONDON UK 1
23B2 COMASWFORPAC PEARL HARBOR HI 1
23B2 COMCARSTRIKEFORSEVENTHFLT 1
23B2 COMNAVSPECWARCOM CORONADO CA 1
23B2 COMPATRECONFORSEVENTHFLT KAMI SEYA JA 1
23B3 COMAREAASWFORSIXTHFLT 1
23B3 COMBATTLEFORSIXTHFLT 1
23B3 COMARSURVRECFORSIXTHFLT 1
23B3 COMARSURVRECFORSIXTHFLT DET ROTA SP 1
23B3 COMARSURVRECFORSIXTHFLT DET 1SIGONELLA IT
23B4 COMIDEASTFOR 1
23C COMNAVRESFOR NEW ORLEANS LA 1
24A1 COMNAVAIRLANT NORFOLK VA 1
24A2 COMNAVAIRPAC SAN DIEGO CA 1
24D1 COMNAVSURFLANT NORFOLK VA 1
24D2 COMNAVSURFPAC SAN DIEGO CA 1
24G1 COMSUBLANT NORFOLK VA (Code N25) 2
24G2 COMSUBPAC PEARL HARBOR HI (Code N24) 2
202
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
25A COMINEWARCOM CORPUS CHRISTI TX 1
25A1 COMCMRON ONE 1
25A1 COMCMRON TWO 1
25A1 COMCMRON THREE 1
26A1 COMPHIBGRU TWO 1
26A2 COMPHIBGRU ONE 1
26A2 COMPHIBGRU THREE 1
26B3 COMNAVSURFRESFOR NEW ORLEANS LA 2
26B3A NAVSURFRESFOR TSLS NEW ORLEANS LA 1
26D1 SEAL TEAM TWO 1
26D1 SEAL TEAM FOUR 1
26D1 SEAL TEAM SIX 1
26D1 SEAL TEAM EIGHT 1
26D2 SEAL TEAM ONE 1
26D2 SEAL TEAM THREE 1
26D2 SEAL TEAM FIVE 1
26E1 ACU TWO 1
26E1 ACU FOUR 1
26E1 BMU TWO 1
26E1 COMSPECBOATRON TWO 1
26E2 ACU ONE 1
203
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
26E2 ACU FIVE 1
26E2 BMU ONE 1
26E2 COMSPECBOATRON ONE 1
26J1 COMAFLOATRAGRULANT NORFOLK VA 1
26J1 AFLOATRAGRU NORFOLK VA 2
26J1 AFLOATRAGRU MAYPORT FL 2
26J1 COMAFLOATRAGRU INGLESIDE TX 1
26J2 COMAFLOATRAGRUMIDPAC PEARL HARBOR HI 1
26J2 COMAFLOATRAGRUPAC SAN DIEGO CA 1
26J2 COMAFLOATRAGRUWESTPAC YOKOSUKA JA 1
26J2 AFLOATRAGRUPAC PACNORWEST DET 1
26J2 AFLOATRAGRUWESTPACDET SASEBO JA 1
26K COMUNDERSEASURV DAM NECK VA 1
26K COMUNDERSEASURV DET PEARL HARBOR HI 1
26R1 MIUWU TWO ZERO ONE 1
26R1 MIUWU TWO ZERO TWO 1
26R1 MIUWU TWO ZERO THREE 1
26R1 MIUWU TWO ZERO FOUR 1
26R1 MIUWU TWO ZERO FIVE 1
26R1 MIUWU TWO ZERO SIX 1
26R1 MIUWU TWO ZERO SEVEN 1
204
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
26R1 MIUWU TWO ZERO EIGHT 1
26R1 MIUWU TWO ONE ZERO 1
26R1 MIUWU TWO ONE TWO 1
26R1 MIUWU TWO ONE FOUR 1
26R1 COMNAVIUWGRU TWO 2
26R2 MIUWU ONE ZERO ONE 1
26R2 MIUWU ONE ZERO TWO 1
26R2 MIUWU ONE ZERO THREE 1
26R2 MIUWU ONE ZERO FOUR 1
26R2 MIUWU ONE ZERO FIVE 1
26R2 MIUWU ONE ZERO SIX 1
26R2 MIUWU ONE ZERO EIGHT 1
26R2 MIUWU ONE ZERO NINE 1
26R2 MIUWU ONE ONE ZERO 1
26R2 MIUWU ONE ONE TWO 1
26R2 MIUWU ONE ONE FOUR 1
26R2 COMNAVIUWGRU ONE 2
26S1 COMNCWGRU TWO 1
26S2 NCWGRU ONE 1
26QQ1 COMNAVSPECWARDEVGRU DAM NECK VA 1
26QQ1 NAVSPECWARGRU TWO 1
205
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
26QQ1 NAVSPECWARUNIT EIGHT 1
26QQ1 NAVSPECWARUNIT FOUR 1
26QQ1 NAVSPECWARUNIT TEN 1
26QQ1 NAVSPECWARUNIT TWO 1
26QQ2 COMNAVSPECWARGRU ONE DET KODIAK AK 1
26QQ2 COMNAVSPECWARGRU ONE 1
26QQ2 NAVSPECWARUNIT ONE 1
26QQ3 NAVSPECWARUNIT THREE 1
26QQ4 NAVSPECWARDET TWO 1
26QQ4 SPECBOAT DET FOUR 1
26WW DSU SAN DIEGO CA 1
26YY3 FOSIC EUROPE LONDON UK 1
26KKK1 TACTRAGRULANT DAM NECK VA 1
26KKK2 TACTRAGRUPAC SAN DIEGO CA 1
26OOO NAVSURFPAC MOBTRAEVCOM 1
26WWW NAVTRASUPPU TINKER AFB OK 1
28A1 COMCARGRU TWO 1
28A1 COMCARGRU FOUR 1
28A1 COMCARGRU SIX 1
28A1 COMCARGRU EIGHT 1
28A2 COMCARGRU ONE 1
206
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
28A2 COMCARGRU THREE 1
28A2 COMCARGRU FIVE 1
28A2 COMCARGRU SEVEN 1
28B1 COMCRUDESGRU TWO 1
28B1 COMCRUDESGRU EIGHT 1
28B1 COMCRUDESGRU TWELVE 1
28B1 COMWESTHEMGRU 1
28B1 COMWESTHEMGRU DET PASCAGOULA 1
28B2 COMCRUDESGRU ONE 1
28B2 COMCRUDESGRU THREE 1
28B2 COMCRUDESGRU FIVE 1
28C1 COMSURFWARDEVGRU DET WEST 1CORONADO CA
28C1 COMNAVSURFGRU MED 1
28C1 COMSURFWARDEVGRU LITTLE CREEK VA 1
28C2 COMNAVSURFGRU MIDPAC 1
28C2 COMNAVSURFGRU PACNORWEST 1
28D1 COMDESRON TWO 1
28D1 COMDESRON SIX 1
28D1 COMDESRON FOURTEEN 1
28D1 COMDESRON EIGHTEEN 1
28D1 COMDESRON TWO TWO 1
207
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
28D1 COMDESRON TWO FOUR 1
28D1 COMDESRON TWO SIX 1
28D1 COMDESRON TWO EIGHT 1
28D1 COMDESRON THREE TWO 1
28D2 COMDESRON ONE 1
28D2 COMDESRON SEVEN 1
28D2 COMDESRON NINE 1
28D2 COMDESRON THIRTEEN 1
28D2 COMDESRON FIFTEEN 1
28D2 COMDESRON TWO ONE 1
28D2 COMDESRON TWO THREE 1
28D2 COMDESRON THREE ONE 1
28D2 COMDESRON THREE THREE 1
28D3 COMDESRON FIVE ZERO 1
28K1 COMSUBDEVRON TWELVE (Code N225) 2
28K1 COMSUBGRU TEN (Code 32) 2
28K1 COMSUBGRU TWO (Code N3) 2
28K1 COMSUBGRU EIGHT (Code N3) 2
28K1 COMSUBRON TWO 1
28K1 COMSUBRON FOUR 1
28K1 COMSUBRON SIX 1
208
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
28K1 COMSUBRON EIGHT 1
28K1 COMSUBRON SIXTEEN 1
28K1 COMSUBRON TWO ZERO 1
28K1 COMSUBRON TWO TWO 1
28K2 COMSUBGRU SEVEN (Code N3) 2
28K2 COMSUBGRU NINE (Code N33) 3
28K2 COMSUBDEVRON FIVE SAN DIEGO CA 5
28K2 COMSUBRON ONE 8
28K2 COMSUBRON THREE 8
28K2 COMSUBRON SEVEN 9
28K2 COMSUBRON ELEVEN 7
28K2 COMSUBRON SEVENTEEN 17
28L1 COMPHIBRON TWO 1
28L1 COMPHIBRON FOUR 1
28L1 COMPHIBRON SIX 1
28L1 COMPHIBRON EIGHT 1
28L2 COMPHIBRON ONE 1
28L2 COMPHIBRON THREE 1
28L2 COMPHIBRON FIVE 1
28L2 COMPHIBRON SEVEN 1
28L2 COMPHIBRON ELEVEN 1
209
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
29A1 GUIDED MISSILE CRUISER LANT (CG) 14
29A2 GUIDED MISSILE CRUISER PAC (CG) 13
29B1 AIRCRAFT CARRIER LANT (CV) (CVN) 7
29B2 AIRCRAFT CARRIER PAC (CV) (CVN) 5
29E1 DESTROYER (DD) LANT, 963 CLASS 14
29E2 DESTROYER PAC (DD), 963 CLASS 12
29F1 GUIDED MISSILE DESTROYER LANT (DDG) 15
29F1 PCO OKANE (DDG77) 1
29F1 PCO PORTER (DDG78) 1
29F2 GUIDED MISSILE DESTROYER PAC (DDG) 14
29N1 SUBMARINE LANT (SSN) 35
29N2 SUBMARINE PAC (SSN) 28
29P2 AUXILLARY RESEARCH SUBMARINE PAC (AGSS) 1
29Q1 FLEET BALLISTIC MISSILE SUBMARINE LANT 20(SSBN 734-743, BLUE AND GOLD)
29Q2 FLEET BALLISTIC MISSILE SUBMARINE PAC 20(SSBN 726-733, BLUE AND GOLD)
29S RESEARCH SUBMARINE (NUCLEAR) (NR) 1
29AA1 GUIDED MISSILE FRIGATE LANT (FFG) 23
29AA2 GUIDED MISSILE FRIGATE PAC (FFG) 17
30A USS INCHON (MCS 12) 3
30B MINE HUNTER COASTAL (MHC) ANDFLEINTROTM SAVANNAH GA 12
210
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
30B PCO SHRIKE (MHC) 1
30C MINE COUNTERMEASURES (MCM) ANDFLEINTROTM GREEN BAY WI 15
31A1 USS MOUNT WHITNEY (LCC 20) 2
31A2 USS BLUE RIDGE (LCC 19) 2
31H1 AMPHIBIOUS ASSAULT SHIP LANT (LHA) (LPH) 3
31H2 AMPHIBIOUS ASSAULT SHIP PAC (LHA) (LPH) 3
31N1 MULTI-PURPOSE AMPHIBIOUS ASSAULT SHIP LANT 3
31N2 MULTI-PURPOSE AMPHIBIOUS ASSAULT SHIP PAC 3
32KK MISCELLANEOUS COMMAND SHIP (AGF) 2
42A1 COMFAIR KEFLAVIK IC 2
42A1 COMFAIRCARIB ROOSEVELT ROADS PR 2
42A2 COMFAIRWESTPAC ATSUGI JA 2
42A3 COMFAIRMED NAPLES IT 2
42B1 COMPATWINGSLANT NORFOLK VA 2
42B1 PATWINGSLANT DET AMPO JACKSONVILLE FL 1
42B2 COMPATWINGSPAC BARBERS POINT HI 2
42B2 PATWINSPAC DET TSC NORTH ISLAND 2
42B3 COMRESPATWINGLANT DET JACKSONVILLE FL 2
42B3 COMRESPATWINGPAC DET WHIDBEY ISLAND WA 2
42B3 COMHELWINGRES SAN DIEGO CA 2
211
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
42B3 COMRESPATWINGLANT NORFOLK VA 2
42B3 COMRESPATWINGPAC MOFFETT FIELD CA 2
42D1 FASOTRAGRULANT NORFOLK VA 2
42D1 FASOTRAGRULANT DET BRUNSWICK ME 1
42D1 FASOTRAGRULANT DET CHERRY POINT NC 1
42D1 FASOTRAGRULANT DET JACKSONVILLE FL 1
42D1 FASOTRAGRULANT DET MAYPORT FL 1
42D1 FASOTRAGRULANT DET OCEANA 1
42D2 FASOTRAGRUPAC SAN DIEGO CA 2
42D2 FASOTRAGRUPAC DET ATSUGI JA 1
42D2 FASOTRAGRUPAC DET BARBERS POINT HI 1
42D2 FASOTRAGRUPAC DET SAN DIEGO CA 1
42D2 FASOTRAGRUPAC DET FALLON NV 1
42D2 FASOTRAGRUPAC DET LEMOORE CA 1
42D2 FASOTRAGRUPAC DET WARNER SPRINGS CA 1
42D2 FASOTRAGRUPAC DET WHIDBEY ISLAND WA 1
42E1 COMHSWINGLANT JACKSONVILLE FL 2
42E1 COMHSLWINGLANT MAYPORT FL 2
42E1 COMHSLWINGLANT DET SIGONELLA IT 1
42E1 COMHSLWINGLANT DET WTU 1
42E1 COMHELTACWINGLANT NORFOLK VA 1
212
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
42E2 COMHSLWINGPAC SAN DIEGO CA 2
42E2 COMHSWINGPAC SAN DIEGO CA 2
42E2 COMHELTACWINGPAC SAN DIEGO CA 1
42P1 COMPATWING FIVE 2
42P1 PATRON FIVE 1
42P1 PATRON EIGHT 1
42P1 PATRON TEN 1
42P1 COMPATWING ELEVEN JACKSONVILLE FL 2
42P1 PATRON SIXTEEN 1
42P1 PATRON TWO SIX 1
42P1 PATRON THREE ZERO 1
42P1 PATRON FOUR FIVE 1
42P1 SPEC PROJ PATRON ONE BRUNSWICK ME 1
42P2 COMPATWING ONE KAMI SEYA JA 2
42P2 PATWING ONE DET KADENA JA 1
42P2 PATWING ONE DET MISAWA JA 1
42P2 PATWING ONE DET DIEGO GARCIA 1
42P2 COMPATWING TEN WHIDBEY ISLAND WA 2
42P2 PATRON ONE 1
42P2 PATRON FOUR 1
42P2 PATRON NINE 1
213
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
42P2 PATRON FOUR ZERO 1
42P2 PATRON FOUR SIX 1
42P2 PATRON FOUR SEVEN 1
42P2 PATRON SPEC PROJ UNIT TWOBARBERS POINT HI 1
42P3 PATRON SIX TWO 1
42P3 PATRON SIX FOUR 1
42P3 PATRON SIX FIVE 1
42P3 PATRON SIX SIX 1
42P3 PATRON SIX NINE 1
42P3 PATRON NINE ONE 1
42P3 PATRON NINE TWO 1
42P3 PATRON NINE FOUR 1
42W1 HELMINERON FOURTEEN 1
42W1 HELMINERON FIFTEEN 1
42BB1 HELANTISUBRON THREE 1
42BB1 HELANTISUBRON FIVE 1
42BB1 HELANTISUBRON SEVEN 1
42BB1 HELANTISUBRON ELEVEN 1
42BB1 HELANTISUBRON FIFTEEN 1
42BB2 HELANTISUBRON TWO 1
42BB2 HELANTISUBRON FOUR 1
214
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
42BB2 HELANTISUNRON SIX 1
42BB2 HELANTISUBRON EIGHT 1
42BB2 HELANTISUBRON TEN 1
42BB2 HELANTISUBRON FOURTEEN 1
42BB3 HELANTISUBRON SEVEN FIVE 1
42CC1 HSL FOUR ZERO MAYPORT FL 1
42CC1 HSL FOUR TWO MAYPORT FL 1
42CC1 HSL FOUR FOUR MAYPORT FL 1
42CC1 HSL FOUR SIX MAYPORT FL 1
42CC1 HSL FOUR EIGHT MAYPORT FL 1
42CC2 HSL THREE SEVEN BARBERS POINT HI 1
42CC2 HSL FOUR ONE NORTH ISLAND CA 1
42CC 2 HSL FOUR THREE NORTH ISLAND CA 1
42CC2 HSL FOUR FIVE NORTH ISLAND CA 1
42CC2 HSL FOUR SEVEN NORTH ISLAND CA 1
42CC2 HSL FOUR NINE NORTH ISLAND CA 1
42CC2 HSL FIVE ONE ATSUGI JA 1
42CC2 HSL FIVE ONE DET ELEVEN ATSUGI JA 1
42CC3 HSL EIGHT FOUR NORTH ISLAND CA 1
42CC3 HSL NINE FOUR NAS WILLOW GROVE PA 1
215
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
45A1 FLEET MARINE FORCE COMMANDS 3(Attn: WXSVCOFF)MARFORLANTMARFORPACMARFORRES
45A2 MARINE EXPEDITIONARY FORCE 3(Attn: WXSVCOFF)I MEFII MEFIII MEF
46B MARINE AIRCRAFT WING 4(Attn: WXSVCOFF)FIRST MAWSECOND MAWTHIRD MAWFOURTH MAW
46Q MARINE WING SUPPORT GROUP 4(Attn: WXSVCOFF)MWSG-17MWSG-27MWSG-37MWSG-47
46R MARINE WING SUPPORT SQUADRON 10(Attn: WXSVCOFF)MWSS-171MWSS-172MWSS-271MWSS-272MWSS-273MWSS-274MWSS-371MWSS-372MWSS-373MWSS-374
46U MARINE AVIATION WEAPONS AND 1TACTICS SQUADRON ONE (Attn: WXSVCOFF)
216
DISTRIBUTION LIST
SNDL ACTIVITY # OF COPIES
50A CINCUSACOM NORFOLK VA 1(Attn: J335WX)
50A USCINCEUR ECJ1-AAL VAIHINGEN GE 1(Attn: J33WE)
50A USCINCCENT MACDILL AFB FL 1(Attn: CCJC-OW)
50A USCINCPAC HONOLULU HI 1(Attn: J316)
50A USCINCSO MIAMI FL 1(Attn: SCJ3-SMO)
50A USCINCSPACE PETERSON AFB CO 1(Attn: J33W)
50A USCINCSOC MACDILL AFB FL 1(Attn: J3-OW)
50A USSTRATCOM OFFUTT AFB NB 1(Attn: J315)
50A USTRANSCOM SCOTT AFB IL 1(Attn: PCJ3/J4-ODM)
50D COMNAVSPECWARCOM CORONADO CA 1
50D CINCLANTFLT NORFOLK VA 150D COMUSNAVCENT BAHRAIN 1
50D DEPCOMUSNAVCENT MACDILL AFB FL 1
50D CINCUSNAVEUR LONDON UK 1
50D CINCPACFLT PEARL HARBOR HI 1