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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
Source: GSSI (www.geophysical.com)
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Instrument
The antenna is connected via a fiber optic cable to therecording unit.
Fiber optic cable
Recording unit
(laptop)
Source: GSSI (www.geophysical.com)
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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