side scan sonar introduction.pdf
TRANSCRIPT
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Side Scan Sonar Introduction
Presenter’s Name
Date
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SIDESCAN
APPLICATIONS:
•Pipeline Inspection
•Site Surveys
•Wreck Hunting
•Environmental & Sediment Classification
•ROV Operations
•Rig Move Operations
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Sidescan Systems
SeaMARC (I, II)
MR1 (SEAMAP)
TOBI
EDO
EG&G / EDGETECH
Klein
GLORIA
Seascan (MSTL)
GeoAcoustics
SSI, TAMU, Etc
AMS -120 (DSL)
Datasonics
Sonar Systems
Multi- Beam Systems
SIMRAD: EM300, 1000, 3000
HYDROSWEEP:Fansweep
SEABAT: 9001/8101/8102/8125
SEABEAM: Elac/XSE
Odom Echoscan
Interferometric Systems
AMS120 (Woods Hole DSL)
SeaMARC
Ultra Electronics Deepscan
*GeoDAS can be customized to support
any underwater sensor system.
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•SONAR
•Sidescan data
•Beam-width
•Ping
•Towfish
•Waterfall
OVERVIEW OF TERMS
•Bottom-tracking
•Altitude
•Oscilloscope
•Slant Range
•Ground Range
•Imaging Artifacts
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Overall view of sonar survey
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Overall view of sonar survey cont.
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Introduction to Sonar
SONAR Sound Navigation And Ranging
~What Direction is the target and how far is it.
Early sonars returned single values for range to target,
Sidescan Sonar extends the beam of sound laterally, in a fan,
mapping a swath across the seafloor.
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Recall: SONAR == SOund Navigation And Ranging
i.e., What direction is the target and how far is it
• What direction is the target?
Straight down, out to the side, or somewhere in between.
Without angle measurement, can only know range
• How far is the target? Distance = Rate * Time
Speed of Sound in Salt Water = VOS =~ 1500 meters / second
Function of pressure, temperature and salinity
Pulse Length = time (in milliseconds) in which the sonar is actively
transmitting energy
Ping Period = time (in seconds or milliseconds) in which the sonar
is listening for echoes before it pings again.
General Terms and Definitions
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Defining a Ping
Each firing of the sonar illuminates an
area on the seafloor - the echoes from
which we record, and call a “ping”.
The pulse- length and beam-width
define the sonar’s ability to resolve
items on the ground.
Shorter pulse lengths and narrower
beam patterns give higher resolution.
100% coverage requires that we go
slow, or ping fast, so that no gaps
occur between pings. This implies a
trade- off between survey speed, sonar
range and survey coverage.
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Where is the sonar?
Hull mounted or in a “Towfish”.
Vessel position calculated from a DGPS (hopefully);
and sonar towfish position calculated relative to
vessel, via estimates of offsets, depth, cable-out
and relative bearing to ship (or USBL).
Sidescan “towfish”
Sonar Placement
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Sonar Placement cont.
Here’s another perspective…
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Sidescan & Data Transfer
Lets begin with the sensor…
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Next…
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Then…
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Finally…
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In a nutshell…
Each ping illuminates an area of the seafloor beneath and perpendicular to the
sensor - the backscatter from which we derive an image of a thin slice of the
seabed. This image is sent to a waterfall display, such as seen in GeoDAS, and
provides a continuous display of pings, forming a scrolling image of the
seafloor.
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The waterfall display is what
appears on the computer
screen via a data acquisition
Software program such as
GeoDAS.
A waterfall display is used
to portray the recorded
sonar pings as a vertically
scrolling image of sequential
scan lines abutted to portray
a continuous image of the
bottom, optionally corrected
for vessel speed.
What is a “waterfall”?
Purpose of a Waterfall
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Bottom Tracking
The process of detecting the arrival time of the first echo from the bottom is known as
“bottom-tracking” and is mandatory for successful operations.
With an estimate of the sensor’s height above bottom, or altitude, and the assumption
that the bottom is nominally flat across-track within the coverage of the ping, we can
convert the raw time-domain (slant range) imagery of any target at point “P” to ground
range images, and represent the actual ground range to the target on the bottom, via
the Pythagorean’s Theorem.
What is “bottom-tracking”?
22GRSRGR
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Detecting the first return, as shown by the black lines superimposed
on the raw data, gives us the sonar’s height off seafloor bottom also
know as “altitude”. GeoDAS uses its own built in oscilloscope to
detect this first return and begin bottom tracking which is important
to properly apply some of the image enhancement routines.
Oscilloscope
Sonar
echoes
(port and
starboard)
from one
“ping”
Detection of 1st
return gives alt.
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Bottom Tracking
Given the speed of
sound in water, the
arrival time of each
acoustic backscatter
sample produces a
direct measure of the
range to the reflecting
target. This allows
GeoDAS to track the
seafloor bottom (this
will be discussed
further in the Playback
presentation).
Bottom-tracking is
indicated in GeoDAS
by the red lines in
the bottom-tracking
waterfall and in the
sidescan waterfall.
Bottom-tracking is most helpful in avoiding dredging…
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Slant - Range Imagery:
; A “raw” display of sonar data, object shapes may be distorted
Image without Bottom-Tracking Image with Bottom - Tracking applied
Ground - Range Imagery:
Requires knowledge of terrain, slope or the "flat - bottom assumption“
Slant-Range vs. Ground-Range
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Uniformity of sidescan imagery is dependent on uniformity of illumination.
Major limits on uniformity are:
Spreading Losses:
Intensity (2R) = Intensity (R) / 4
(1 over R squared rule);
approximate, but reasonably close;
often estimated
Fix these with TVG
Beam -Pattern Variations
(side - lobes):
Illumination by sonar is not perfectly
even; can cause severe image
banding
Fix these with AVG
Imaging Artifacts
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REVIEW
•SONAR: acronym for SOund Navigation And Ranging
•Sidescan data: arrive as time series (port & stbd) of echo amplitude
•Beam-width: footprint of the sonar ping on the ground, usually in
degrees.
•Ping: all the samples recorded from 1 active transmission
•Towfish: hydrodynamic towbody containing sonar electronics
•Waterfall: display of sequential pings as grayscale images
•Bottom-tracking: the process of detecting the first return (gives the
altitude)
•Altitude: the height of the sonar off the seabed
•Oscilloscope: backscatter sampled by sonar in 1/SAMPLE_RATE
•Slant Range: cross-track resolution is constant in time
•Ground Range: cross-track resolution is constant in space
•Image Artifacts: distortions in sonar data
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LIMITATIONS & ADVANTAGES
Disadvantages:
Cannot give geo-referenced bathymetry data
Difficult to position due to long cable
Semi-Skilled interpretation necessary
Advantages:
Low cost
large area coverage reduces survey time
Natural visual interpretation
Proven Technology
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TOW FISH WITH DECK UNIT
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Basic Sonar Principles
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SONAR SYSTEM ELEMENTS
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TWO WAY TRAVEL TIME
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SWATHE FROM SIDESCAN
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Side Scan Swath
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Spherical Spreading of Sound
Spreading loss
When it hits the sea bottom or surface,
spreading becomes cylindrical
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Absorption of sound
Vibrating molecules
Viscosity of medium
Chemical
Depends on Frequency of
Sound
High Frequency Sound
Low Frequency Sound
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Refraction of Sound
Bending of a sound wave
towards a region of slower
sound speed
Effects on imaging the
bottom
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Reflection of Sound
Deflection of the path of a
sound wave by an object or
by the boundary between
two media
Acoustic properties of the
media boundaries…
Similar = less
reflection
Dissimilar = more
reflection
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Scattering of Sound
Scattering affects the
distance sound can travel
Amount of scattering
depends on:
•Size of scatter
•Wave length of sound
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Traveling through the sea, an underwater sound signal becomes delayed, distorted and weakened, reflecting on boundaries of underside surface of waves, bottom and shores, bubbles, suspended particles and marine life.
Tide, current, temperature variances and wind also play on a sound's final quality.
Man made noise can also affect the results
Noise
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Time Varying Gain
Time Varying Gain (TVG) is
accurately controlled
amplification (gain) relative
to time after transmission.
Used to correct for
transmission loss.
Time
Gain
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Sonar Equation
EL = SL + TS - 2TL
Where EL (echo level) is the
level of the reflectedsound,
SL (source level) is the level
of the incident sound, TS is
target strength, and 2 TL is
two-way transmission loss
due to spreading and
absorption.
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SOURCE LEVEL
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PULSE LENGTH
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ACROSS TRACK RANGE
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ALONG TRACK RESOLUTION
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HORIZONTAL BEAM PATTERN
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VERTICAL BEAM PATTERN
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BEAM FORMATION
VP/frequency
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NEAR FIELD BEAM FORMATION
VP/frequency Length of array
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SPREADING, ABSORPTION AND NOISE
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TOWFISH IMAGE
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SIDESCAN RECORD BUILD UP
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CODA SYSTEM
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CODA PIPELINE
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SIDESCAN GEOMETRY
A
B
C
D
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A B
C
D
A= Surface echo
B = Bottom Echo
C = Target Echo
D = Shadow
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PADDLE STEAMER WRECK
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ICEBERG SCOURS
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MOSAIC
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FREQUENCY ALLOCATION
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100kHZ WRECK
SCHOONER
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100kHZ AIRCRAFT
WELLINGTON BOMBER
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500kHZ WRECK
LIGHTSHIP
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PIPELINE CROSSING
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MINE-HUNTING
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ENGINEERING
Steel rod protruding from
mound evident by its acoustic
shadow
Fill mound on previous sink
hole
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BACKGROUND THRESHOLD
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SEA SURFACE RETURN
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SURFACE INTERFERENCE
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LOWERING TOW-FISH
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WATCH OUT FOR THE BOTTOM!
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WATCH THE ECHO-SOUNDER
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NO SURFACE RETURN
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SEA CLUTTER
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PROJECTIONS & DEPRESSIONS
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TARGET IN WATER COLUMN
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TARGET IN WATER COLUMN
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TARGET ON BOTTOM
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MULTIPLE ECHO’S
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MULTIPLE PIPELINE ECHO’S
Pipeline
multiple Pipeline
multiple
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MULTIPLE SEA-BOTTOM
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100kHZ WRECK
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POWER BOAT WAKE
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SAIL BOAT WAKE
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AIR BUBBLES
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SHADOW FROM BUOY
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PIPELINE SUSPENSION
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FISH
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FISH SHOAL
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TARGET AT LONG AND SHORT RANGE
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FIRST WRECK CONTACT
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SECOND WRECK CONTACT
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TYRES ON SEA BED
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WOODEN LADDER
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MISTAKEN WRECK
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SIDE LOBE RETURNS
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CROSSTALK AND DIRECT IMAGE
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CROSSTALK
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SECOND SWEEP
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SECOND SWEEP WRECK
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REFRACTION PROBLEMS
SIDE SCAN SONAR
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REFRACTION IMAGE
TOTAL LOSS OF SIGNAL AT
THERMOCLINE
Focussing of beams
due to refraction
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REFRACTION
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TOWING PITCH
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TOWING PITCH
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TOWFISH YAW
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TOWFISH YAW
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SWEEPING TURN
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DEVIATION FROM COURSE
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DON’T DO THIS !
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YOU CAN DO THIS
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LOST TAIL FIN
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