Ground Penetrating Radar Fundamentals
Ground Penetrating Radar FundamentalsbyJeffrey J. Daniels, Department of Geological Sciences, The Ohio State UniversityPrepared as an appendix to a report to the U.S.EPA, Region VNov. 25, 2000IntroductionGround penetrating radar (commonly called GPR) is a high resolution electromagnetictechnique that is designed primarily to investigate the shallow subsurface of the earth,building materials, and roads and bridges. GPR has been developed over the past thirtyyears for shallow, high resolution investigations of the subsurface. GPR is a time-depen dent geophysical technique that can provide a 3-D pseudo image of the subsurface, includ ing the fourth dimension of color, and can also provide accurate depth estimates for manycommon subsurface objects. Under favorable conditions, GPR can provide precise infor mation concerning the nature of buried objects. It has also proven to be a tool that can beoperated in boreholes to extend the range of investigations away from the boundary of thehole.GPR uses the principle of scattering of electromagnetic waves to locate buried objects.The basic principles and theory of operation for GPR have evolved through the disciplinesof electrical engineering and seismic exploration, and practitioners of GPR tend to havebackgrounds either in geophysical exploration or electrical engineering. The fundamentalprinciple of operation is the same as that used to detect aircraft overhead, but with GPRthat antennas are moved over the surface rather than rotating about a fixed point. This hasled to the application of field operational principles that are analogous to the seismicreflection method.GPR is a method that is commonly used for environmental, engineering, archeological,and other shallow investigations. The fundamental principles that are described in the fol lowing text applies to all of these applications.November 25, 20001
Basic PrinciplesThe practical result of the radiation of electromagnetic waves into the subsurface for GPRmeasurements is shown by the basic operating principle that is illustrated in Figure A1.The electromagnetic wave is radiated from a transmitting antenna, travels through thematerial at a velocity which is determined primarily by the permittivity of the material.The wave spreads out and travels downward until it hits an object that has different electri cal properties from the surrounding medium, is scattered from the object, and is detectedby a receiving antenna. The surface surrounding the advancing wave is called a wavefront. A straight line drawn from the transmitter to the edge of the wavefront is called aray. Rays are used to show the direction of travel of the wavefront in any direction awayfrom the transmitting antenna. If the wave hits a buried object, then part of the wavesenergy is “reflected” back to the surface, while part of its energy continues to travel down ward. The wave that is reflected back to the surface is captured by a receive antenna, andrecorded on a digital storage device for later interpretation.Antennas can be considered to be transducers that convert electric currents on the metallicantenna elements to transmit electromagnetic waves that propagate into a material. Anten nas radiate electromagnetic energy when there is a change in the acceleration of the cur rent on the antenna. The acceleration that causes radiation may be either linear,(e.g., atime-varying electromagnetic wave traveling on the antenna), or angular acceleration.Radiation occurs along a curved path, and radiation occurs anytime that the currentchanges direction (e.g. at the end of the antenna element). Controlling and directing theradiation from an antenna is the purpose of antenna design.Antennas also convert electromagnetic waves to currents on an antenna element, acting asa receiver of the electromagnetic radiation by capturing part of the electromagnetic wave.The principle of reciprocity says that the transmit and receive antennas are interchange able, and this theory is valid for antennas that are transmitting and receiving signals in theair, well above the surface of the ground. In practice, transmit and receive antennas are notstrictly interchangeable when placed on the ground, or a lossy material surface, because ofattenuation effects of the ground in the vicinity of the transmit antenna.Electromagnetic waves travel at a specific velocity that is determined primarily by the per mittivity of the material. The relationship between the velocity of the wave and materialproperties is the fundamental basis for using GPR to investigate the subsurface. To statethis fundamental physical principle in a different way: the velocity is different betweenmaterials with different electrical properties, and a signal passed through two materialswith different electrical properties over the same distance will arrive at different times.The interval of time that it takes for the wave to travel from the transmit antenna to thereceive antenna is simply called the travel time. The basic unit of electromagnetic wavetravel time is the nanosecond (ns), where 1 ns 10 -9 s. Since the velocity of an electro magnetic wave in air is 3x108 m/s (0.3 m/ns), then the travel time for an electromagneticwave in air is approximately 3.3333 ns per m traveled. The velocity is proportional to theinverse square root of the permittivity of the material, and since the permittivity of earthmaterials is always greater than the permittivity of the air, the travel time of a wave in aNovember 25, 20002
material other than air is always greater than 3.3333 ns/m. The travel time of an electro magnetic wave through two different materials is shown in Figure A2(a).FIGURE A1. Transmitted electromagnetic wavefront scattered from a buried object with acontrasting permittivity. Permittivity of the host media is e1, and the permittivity of the buriedobject is e2.Considering the wave scattered from the object in Figure A1, if a receive antenna isswitched-on at precisely the instant that the pulse is transmitted, then two pulses will berecorded by the receive antenna. The first pulse will be the wave that travels directlythrough the air (since the velocity of air is greater than any other material), and the secondpulse that is recorded will be the pulse that travels through the material and is scatteredback to the surface, traveling at a velocity that is determined by the permittivity (e) of thematerial. The resulting record that is measured at the receive antenna is similar to one ofthe time-amplitude plots in Figure A2(b), with the “input” wave consisting of the directwave that travels through air, and the “output” pulse consisting of the wave reflected fromthe buried scattering body. The recording of both pulses over a period of time with receiveantenna system is called a “trace”, which can be thought of as a time-history of the travelof a single pulse from the transmit antenna to the receive antenna, and includes all of itsdifferent travel paths. The trace is the basic measurement for all time-domain GPR sur veys. A scan is a trace where a color scale has been applied to the amplitude values. Theround-trip (or two-way) travel time is greater for deep objects than for shallow objects.Therefore, the time of arrival for the reflected wave recorded on each trace can be used todetermine the depth of the buried object, if the velocity of the wave in the subsurface isNovember 25, 20003
known. The principles of constructing a scan from a sequence of traces is shown in FigureA3.FIGURE A2. Relationship between travel effect of permittivity on travel time through a sample:(a) input and output pulse through samples with different permittivities and velocities, and (b)time versus amplitude plots showing the time differential between the input and output signals foreach sample.The trace is the time-history record (measured in nanoseconds for radar waves) of a tinypiece (in the spatial sense) of a pulse of electromagnetic energy that travels from the trans mit antenna and ends up at the receive antenna. If a portion of the wavefront encounters anobject with a permittivity different from the surrounding material (host media), then thatportion changes direction by a process that is called scattering. Scattering at the interfacebetween an object and the host material is of four main types: 1) specular reflection scat tering, 2) diffraction scattering, 3) resonant scattering, and 4) refraction scattering, asshown in Figure A4.Specular reflection scattering is the common model for the seismic reflection technique. Ifthe transmit and receive GPR antennas separate entities, then the system is called a bistaticantenna arrangement. If bistatic GPR antennas are deployed with the transmit and receiveantennas located closely together, then the energy that is recorded is often called backscattered energy. If the same antenna is used for transmitting and receiving the signal,then the antenna system is called a monostatic system. Specular scattering is based on theLaw of Reflection, where the angle of reflection is equal to the angle of incidence, orf 1 f 2 in Figure A4(a).When a wave impinges on interface, it scatters the energy according to the shape androughness of the interface and the contrast of electrical properties between the host mate rial and the object. Part of the energy is scattered back into the host material, while theother portion of the energy may travel into the object. The portion of the wave that propa gates into the object is said to be refracted. The angle that the wave enters into the object isdetermined by Snell’s law, which can be stated as follows:November 25, 20004
v1sin f----- -------------1v2sin f 2(1)where v1 and v 2 are the velocities of the wave through the upper and lower materials,respectively, and f 1 and f 2 are the angles of the raypath for the incident and refractedwaves, respectively.FIGURE A3. The process of constructing a scan from a series of traces measured along thesurface. Sequence of producing a GPR profile: 1. transmit and receive electromagnetic energy, 2.the received energy is recorded as a trace at a point on the surface, 3. traces are arranged side-by side to produce a cross section of the earth recorded as the antennas are pulled along the surface.Traces are displayed as either wiggle trace, or scan plots (gray scale or color assigned to specificamplitudes).November 25, 20005
FIGURE A4. Scattering mechanisms: (a) specular reflection scattering, (b) refraction scattering,(c) diffraction scattering, and (d) resonant scattering.If the interface is smooth and continuous (e.g., a layer boundary), and velocity of the wavein the lower boundary (e.g., the object, or lower layer) is greater than velocity in the hostmaterial, then the wave within the object we’ll travel along the interface with a velocitythat is equal to velocity of wave in the object. The angle where this occurs is called criti cal angle, and can be determined by the following equation:ev---------2 -----1 sin f 1v2e1(2)The distance that the receiver must be from the transmitter to receive a refracted wave iscalled the critical distance. Refracted waves are uncommon as a propagation path forGPR, since the electromagnetic wave velocity tends to decrease with depth. This is a con sequence of the fact that seismic and electromagnetic wave velocities in partially saturatedand unconsolidated materials are affected primarily by the water content.Diffraction (Figure A4-c) is the bending of electromagnetic waves. Diffraction scatteringoccurs when a wave is partially blocked by a sharp boundary. Huygen’s Principle ofspherical spreading applies, but since the wave scatters off of a point, the wave spreads outin different directions, as first noted by Fresnel (1788-1827). The nature of the diffractedenergy depends upon the sharpness of the boundaries and the shape of object relative tothe wavelength of the incident wave. Diffractions commonly can be seen on GPR data assemi-coherent energy patterns that splay out in several directions from a point, or a alongNovember 25, 20006
a line. Geologically, they often are measured in the vicinity of a vertical fault, or a discon tinuity in a geologic layer (abrupt facies change).Resonant scattering occurs when a wave impinges on a closed object (e.g., a cylinder), andthe wave bounces back-and-forth between different points of the boundary of the object.Every time the wave hits a boundary, part of the energy is refracted back into the hostmaterial, and part of the energy is reflected back into the object. This causes the electro magnetic energy to resonate (sometimes called ringing) within the object. The resonantenergy that is trapped inside of the object quickly dissipates as part of it is re-radiated tothe outside of the object. Closed objects are said to have a resonant frequency that is basedon the size of the object, and the electrical properties of the object and the surroundingmaterial. However, the ability of an object to resonate depends on the wavelength (veloc ity of the object, divided by the frequency of the wave) with respect to dimensions of theobject. The length of time that an object resonates is determined by the permittivity con trast between the object and the surrounding material.In practice, GPR measurements can be made by towing the antennas continuously over theground, or at discreet points along the surface. These two modes of operation are illus trated in Figure A5. The fixed-mode antenna arrangement consists of moving antennasindependently to different points and making discreet measurements, while moving-modekeeps the transmit and receive antennas at a fixed distance with the antenna pair movedalong the surface by pulling them by hand, or with a vehicle. The fixed-mode arrangementhas the advantage of flexibility, moving-mode has the advantage of rapid data acquisition.In practice, a combination of fixed-mode and moving-mode provides an optimum mixtureof flexibility and mobility. Measurements made in the fixed mode can be used to deter mine the best spacing and antenna orientation for making moving mode measurements.Some systems enable the operator to make both types of measurements with the sameantennas and electronics.GPR Field SystemsThe transmit and receive antennas are moved independently in the fixed mode of opera tion. This allows more flexibility of field operation than when the transmit and receiveantennas are contained in a single box. For example, different polarization componentscan be recorded easily when the transmit and receive antennas are separate. In the fixedmode of operation, a trace is recorded at each discreet position of the transmit and receiveantennas through the following sequence of events in the GPR system: 1) a wave is trans mitted, 2) the receiver is turned on to receive and record the received signals, and 3) aftera certain period of time the receiver is turned off. The resulting measurements that arerecorded during the period of time that the receiver is turned on is called a trace, asdescribed earlier. Figure A5(a) shows a trace in over a single layer. The idealized trace forthis simple case consists of a direct pulse, and a single reflection from the layer.In the moving mode of operation, a radar wave is transmitted, received and recorded eachtime that the antenna has been moved a fixed distance across the surface of the ground, ormaterial, that is being investigated. Since a single record of a transmitted pulse is called atrace, the spacing between measurement points is called the trace spacing. The trace spacNovember 25, 20007
ing that is chosen should be a function of the target size and the objectives of the survey. Asingle trace over two layers is shown in Figure A5(a). Traces that are displayed side-by side form a GPR time-distance record, or GPR cross section, which shows how the reflec tions vary in the subsurface. If the contrasts in electrical properties (e.g. changes in permit tivity) are relatively simple, then the GPR time-distance record can be viewed as a twodimensional pseudo-image of the earth, with the horizontal axis the distance along the sur face, and the vertical axis being the two-way travel time of the radar wave. The two-waytravel time on the vertical axis can be converted to depth, if the permittivity (which can beconverted to velocity) is known. The GPR time-distance record is the simplest display ofGPR data that can be interpreted in terms of subsurface features. A GPR time-distancerecord can also be produced by making a series of fixed-mode measurements at a constantinterval between traces on the surface.FIGURE A5. Fixed and continuous mode GPR operation on the surface. The primary electronicsmay be mounted on the antennas, in the antenna box, or in a box that is separate from theantennas. The transmit pulser is usually located in close proximity to the transmit antennaelement. (a) fixed mode produce a trace, (b) moving mode produces a GPR time-distance record.GPR equipment consists of antennas, electronics and a recording device, as shown in Fig ure A5(b). The transmitter and receiver electronics are always separate, but in a fixedmode configuration, they are often contained in different boxes, while in some systemsthat are designed for moving-mode operation, all of the electronics are contained in onebox. In some cases, the electronics may be mounted on top of the antennas, which makesfor a compact system, but it also decreases the operational flexibility of the system.November 25, 20008
GPR systems are digitally controlled, and data are usually recorded digitally for post-sur vey processing and display. The digital control and display part of a GPR system generallyconsists of a micro-processor, memory, and a mass storage medium to store the field mea surements. A small micro-computer and standard operating system is often utilized to con trol the measurement process, store the data, and serve as a user interface. Data may befiltered in the field to remove noise, or the raw data may be recorded and the data pro cessed for noise removal at a later time. Field filtering for noise removal may consist ofelectronic filtering and/or digital filtering prior to recording the data on the mass data stor age medium. Field filtering should normally be minimized except in those cases where thedata are to be interpreted immediately after recording.Data Display and InterpretationThe objective of GPR data presentation is to provide a display of the processed data that isclosely approximates an image of the subsurface, with the anomalies that are associatedwith the objects of interest located in their proper spatial positions. Data display is centralto data interpretation. In fact, producing a good display is an integral part of interpretation.There are three types of displays of surface data, including: 1) a one-dimensional trace, 2)a two dimensional cross section, and 3) a three dimensional display. Borehole data can bedisplayed as a two-dimensional cross section, or processed to be displayed as a velocitytomogram. A one-dimensional trace is not of very much value until several traces areplaced side-by-side to produce a two dimensional cross section, or placed in a threedimensional block view.The wiggle trace (or scan) is the building block of all displays. A single trace can be usedto detect objects (and determine their depth) below a spot on the surface. By towing theantenna over the surface and recording traces at a fixed spacing, a record section of tracesis obtained. The horizontal axis of the record section is surface position, and the verticalaxis is round-trip travel time of the electromagnetic wave. A GPR record section is verysimilar to the display for an acoustic sonogram, or the display for a fish finder. Two typesof wiggle-trace cross-sections of GPR traces are shown in Figure A6. Wiggle trace dis plays are a natural connection to other common displays used in engineering (e.
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