session 9 ground penetrating radar

8/6/2019 Session 9 Ground Penetrating Radar

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GROUND PENETRATING RADAR


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Outline

This lecture has the following structure:

TheoryInstrument characteristicsData interpretationApplications

Source: GSSI (www.geophysical.com)


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Introduction

Ground penetrating radar (GPR) is one of the most commonlyused geophysical techniques in high resolution studies of theshallow subsurface.


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THEORY


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Theory

Ground penetrating radar operates by transmitting a shortelectromagnetic pulse into the subsurface and then recording thereflected energy.

transmitter receiver


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Theory

Ground penetrating RADAR comes from Ra dio Detection a ndRanging:radar waves are transmitted in the radio range of theelectromagnetic spectrum


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Theory

GPR can operate with several antennas, which transmit signalsof frequencies ranging from 10 MHz to ~ 5GHz.

The higher the frequency of the antenna, the shorter thewavelength

Increasing frequency


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Theory

Wavelength and frequency are related by: V = fwhere V = velocity (m/s), = wavelength (m), and f = frequency(Hz)

The higher the frequency, the higher the resolution of the radaroutput (or the more details are visible).

Increasing frequency


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Theory

This can be understood by thinking of waves in the ocean. Athin stick (small object compared to wavelength) will not affectthe wave; however a large pillar (large object compared towavelength) will break/reflect the wave.

Source: University of Cambridge; http://www-diva.eng.cam.ac.uk/fluids/hydrodynamics.html


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Theory

Not only does a higher frequency result in a higher resolution , italso results in a shallower penetration of the subsurface.

At higher frequencies, the wavelengths are shorter in thesubsurface they encounter many small reflectors they arereflected before they can penetrate any deeper.


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Theory

To summarize:

ResolutionFrequency

+-

+-

Penetration depth


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Theory

Three properties of the subsurface control the propagation andreflection of the radar waves:

1. Dielectric constant2. Magnetic permeability3. Conductivity


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Theory

Dielectric constant / Relative static permittivity is a measure forthe amount of electric and magnetic energy a material can store,relative to the energy in a vacuum.

Or in other words:

Dielectric constant / Relative static permittivity ( r): the ratio

between electric permittivity of the medium ( ) to the electricpermittivity of a vacuum ( 0) r = / 0


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The relative permittivity ( r) is related to the velocity of radarwaves (V) in earth materials by:

V = C/ (rr)

Where C (= 3*10 8 m/s) is the velocity of electromagnetic waves in freespace;r = / 0 the relative magnetic permeability of the medium;and

r= /

0the dielectric constant.

This is a theoretic relationship which is only valid in earthmaterials with zero conductivity!

Theory


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Theory

V = C/ (rr)

As the relative magnetic permeability ( r) is close to unity (1) inearth materials, the velocity of radar waves is mainly controlledby the dielectric constant.

That means that contrasts in the reflected signal are caused bydifferences in radar wave velocity!


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Theory

Material r

Air 1

Dry sand/gravel 4-10Wet sand/gravel 10-20

Dry clay/silt 3-6

Wet clay/silt 7-40

Granite 4-9

Limestone 4-8Dry salt 5-6

Permafrost 4-5

Glacier ice 3.5

Fresh water 81

Some common dielectric constants


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Theory

Signal reflection on materials with different dielectricconstants ( r ):

transmitter receiver

= 7

= 3

= 20

= 5

Transition indielectric constant

No changes indielectric constant

Received signal


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Theory

The relationship V = C/ (rr) is valid in an environment withoutconductivity.

However, earth materials are always conductive.

In conductive environments, electromagnetic waves areattenuated (= gradually loosing intensity)


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Theory

To determine at which depth the radar waves have lost allenergy, or in other words, to determine the maximumpenetration depth , the C in the formula must be replaced by(2/ ):

= (2/)/ (rr)

Where = penetration depth, and = conductivity (mS/m)


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Theory

In words: the depth of penetration is controlled by theconductivity, the dielectric constant and the magneticpermeability.

As the magnetic permeability is almost unity and the dielectricconstant doesnt vary more than a factor 10, the conductivity isthe controlling parameter for the penetration depth! Theconductivity can vary several orders of magnitude!!!


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Theory

In summary:The propagation of radar waves is controlled by conductivity,dielectric constant and magnetic permeability

Penetration depth is mainly determined by conductivity

Changes in received signal are mostly caused by changes indielectric constant

Magnetic permeability has almost no influence (is close tounity).


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INSTRUMENT


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Instrument

The GPR consists of thefollowing essential parts:

1. Signal transmitter2. Receiver

3. Recording unit (laptop)

1 & 2

3


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Instrument

The transmitter and receiver can be integrated in one antenna orseparate in two antennasCommon frequencies of antennas :

Frequency(MHz)

Penetration depth (m)(depending on soil type)

100 0 25200 0 9400 0 - 4900 0 - 11600 0 0.52600 0 0.4



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Instrument

The antenna is connected via a fiber optic cable to therecording unit.

Fiber optic cable

Recording unit

(laptop)



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Instrument

The received signal is instantly visible on the screen of thelaptopMeasurement settingscan also be adjustedinstantly, so that theresult of theadjustment isimmediately visible.

Example of screen output(source: RADAN manual, GSSI (www.geophysical.com))


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Instrument

The following parameters should be known before measurementsare made:

What are the characteristics of the earth material?

How deep should the radar waves penetrate?Which antenna is going to be used?Once the output is visible on the recording unit: should thesignal be enhanced or filtered?


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Instrument

What are the characteristics of the earth material?It is important to know at least the range of dielectric constant(D.C.) to avoid measuring in material that is less appropriate for

GPR (for example, peat)In most GPR systems, an estimate of the D.C. can be enteredin the settings. The true D.C. can be adjusted later duringanalysis.


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Instrument

With deeper penetration, the signal is attenuated. If the object ofinterest is deeper than the maximum penetration depth of acertain antenna, an antenna with lower frequency should be

chosen.(Remember: higher frequency antenna results in shallowermeasurements with higher resolution)


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Instrument

Sometimes the object of interest is at a depth at which the signalhas weakened significantly. To better distinguish the object fromthe background, the signal can be enhanced using gain .

Usually a positive gain is applied to the deeper part of the signal(signal strength is enhanced), while a negative gain is applied tothe shallow part of the signal (signal strength is reduced).


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Instrument

Normal signal Enhanced signalGain Gain

- + - +

Gain = 0 Negative gain in upperpart, positive gain inlower part of signal


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Instrument

Background noise can be removed from the signal using filtersMost commonly used filters are:

Low pass filters (filter out high frequencies)High pass filters (filter out low frequencies)Background removal filters (filter out low frequencies)Stacking (filter out high frequencies)


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Instrument

GPR can make point measurements and continuousmeasurements.For continuous measurements, the distance that the GPR has

covered should be known.The distance can be measured in two ways:

By using an additional measuring wheel,By using GPS.

GPS should be used when there are no clear reference points inthe landscape or on the map


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INTERPRETATION

I i


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Interpretation

Time range(nanoseconds)

Distance (meters)

Time slice ofsubsurface

Objects

The time slice (display of a 2D image of the subsurface) is called aradargram.

I i


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Interpretation

Information aboutthe properties of the signal and thenumber of measured signalsper second / meter

Dielectric constant.D.C. can still bechanged duringanalysis.

Gain. In this casethere are 5breakpoints, wheregain is ranging from-8.0 to +35.0

These numbers areused for time-depthconversion (will beexplained in the nextslides)

Time range,number of nanosecondsthat thesignal isreceived.

Example of properties of radargram

I i


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Interpretation

What we see is a time slice of the subsurface. How can this be translatedto depth?

The easiest way is to derive the depth from the velocity of the signal.

Typical velocities are given in the table on the next slide (from Davis &Annan, 1989)

I t t ti


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Interpretation

Davis, J. L. and A. P. Annan (1989). "Ground-penetrating radar for high-resolutionmapping of soil and rock stratigraphy." Geophysical Prospecting 37: 531-551.

I t t ti


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Interpretation

Beware that the time display in the time slice is the two-way traveltime, or in other words the time for the signal to travel to the objectand back to the receiver.

Example: calculate the depth to the first signal in the figureto the right. The material is clayey.

Answer: the two-way travel time is 8 ns the time for thesignal to reach the object is 4 ns.Assuming a velocity of the signal in clay of 0.06 m/ns thedistance to the first signal is (4 * 0.06) = 0.24 m

I t t ti


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Interpretation

As an additional verification of the time-depth conversion, anobject of known depth should be included in the survey.

The real velocity can be determined when the depth of an

object is knownFrom the velocity, the dielectric constant of the material can bedetermined ( V = C/ (rr))Adjusting the dielectric constant in the analysis improves thedepth estimation from objects at unknown depth.

Interpretation


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Interpretation

A typical feature of a radargram is that objects look like parabolas.In the image below three parabolas are visible:

Interpretation


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Interpretation

Parabolas in the image are caused by the fact that the signal isalready reflected by the object before the antenna is directlyabove it.

At locations 1&3 the travel time to the object is longer than atlocation 2 (directly above the object), resulting in a deeperreflection. The real depth of an object is therefore at the top of theparabola.

1 2 3

Object

1 2 3

Interpretation


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Interpretation

Deriving the location of an object from a parabola is done by themigration module in analysis software.

Before After migration


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Interpretation


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Interpretation

Filtering can be done when the features of interest are not well visible:

Enhancement of image features

Background noisefiltering

Interpretation


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Interpretation

When measurements were made in a raster, a 3D image of thesubsurface can be created:

2D 3D

Source: www.copijn.nl Source: www.copijn.nl


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APPLICATIONS

Applications


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Applications

GPR studies include the following fields:GeologicalGlaciological

EnvironmentalEngineering and constructionArchaeologyForensic science

Applications


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Applications

Examples of geological applications:Detection of natural cavities and fissuresSubsidence mapping

Mapping of superficial depositsSoil stratigraphy mappingGeological structure mappingMapping of faults, dykes, coal seams

Lake and riverbed sediment mappingMineral exploration and resource evaluationDepth to water table

Applications


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Applications

Structural mapping

Applications


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Applications

Subsidence of a road at the surface can sometimes be explainedby the structures in the subsurface:

Subsidence

Applications


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Applications

Glacial ice thickness measurementsOne of the first applications of GPR was measuring glacial ice thickness.The story goes that an airplane using GPR on board was trying to land on

an ice sheet. Unfortunately, the pilot interpreted the transition betweenthe ice and the subsurface as being the surface on which he could land,not knowing that GPR signal went right through the ice sheet. Theairplane crashed on the ice sheet, but this event marked the beginning ofglacial ice thickness measurements.

Applications


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Applications

Glacial ice thickness measurements(making use of large contrasts in dielectric constants between ice (low)and wet sediments (high))

Applications


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Applications

Environmental applications:Pollution plume detectionLandfill cover thickness measurements

http://landfill.files.wordpress.com/2008 Source: Geofox-Lexmond b.v. (www.geofox-lexmond.nl)

Applications


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Applications

Environmental applications:Seepage (of water, hydrocarbons) detectionSalt intrusion

Applications


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Applications

Engineering and constructionRoad pavement analysis

Void detection

Location of reinforcement (rebars) in concrete

Location of public utilities (pipes, cables, etc)

Testing integrity of building materials

Concrete testing


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Applications


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pp

ArchaeologyLocating burial moundsLocating ancient settlements

Foundation research ofhistoric churches

Forensic researchLocating buried corpses

Locating (mass) graves

Foundation of historic churchSource: University of Arkansas, www.cast.uark.edu

session 9 ground penetrating radar

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