Ground Penetrateing Radar System

[백향목]

Description

Ground penetrating radar (commonly called GPR) is a geophysical method that has been developed over the past thirty years for shallow, high-resolution, subsurface investigations of the earth. GPR uses high frequency pulsed electromagnetic waves (generally 10 MHz to 1,000 MHz) to acquire subsurface information. Energy is propagated downward into the ground and is reflected back to the surface from boundaries at which there are electrical property contrasts (click to see a schematic diagram of the process). GPR is a method that is commonly used for environmental, engineering, archeological, and other shallow investigations.

Typical Uses

GPR is used to map geologic conditions that include depth to bedrock, depth to the water table (Wright and others, 1984; Knoll and others, 1997), depth and thickness of soil and sediment strata on land and under fresh water bodies (Beres and Haeni, 1991; Smith and Jol, 1997), and the location of subsurface cavities and fractures in bedrock (Imse and Levine, 1985). Other applications include the location of objects such as pipes, drums, tanks, cables, and boulders, mapping landfill and trench boundaries (Benson, and others, 1983) mapping contaminants (Cosgrave and others, 1987; Brewster and Annan, 1994; Daniels and others, 1995; Guy and others, 2000), and conducting archeological investigations (Conyers and Goodman, 1997).

Integration of GPR data with other surface geophysical methods, such as seismic, resistivity, or electromagnetic methods, reduces uncertainty in site characterization.

Ground penetrating radar is now a widely accepted field screening technology for characterizing and imaging subsurface conditions. The American Society for Testing and Materials (ASTM) has an approved Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation.

Useful EPA and USACE resources for geophysical methods include:

EPA/625/R-92/007 Use of Airborne, Surface, and Borehole Geophysical Techniques at Contaminated Sites
EPA Geophysics Advisor Expert System Version 2.0

USACE, EM1110-1-1802, Geophysical Exploration for Engineering and Environmental Investigation
Theory of Operation

GPR uses high frequency pulsed electromagnetic waves (typically from 10 MHz to 1,000 MHz) to acquire subsurface information. The electromagnetic wave is radiated from a transmitting antenna, travels through the material at a velocity which is determined primarily by the electrical properties of the material. As the wave spreads out and travels downward, if it hits a buried object or boundary with different electrical properties, then part of the wave energy is reflected or scattered back to the surface, while part of its energy continues to travel downward. The wave that is reflected back to the surface is captured by a receiving antenna, and recorded on a digital storage device for later interpretation. The most common display of GPR data is one showing signal versus amplitude, and is referred to as a trace. A single GPR trace consists of the transmitted energy pulse followed by pulses that are received from reflecting objects or layers. A scan is a trace where a color or gray scale has been applied to the amplitude values. As the antenna(s) are moved along a survey line, a series of traces or scans are collected at discrete points along the line. These scans are positioned side by side to form a display profile of the subsurface.

Electromagnetic waves travel at a specific velocity that is determined primarily by the electrical permittivity of the material. The velocity is different between materials with different electrical properties, and a signal passed through two material with different permittivities 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 the receive antenna is simply called the transit time. The basic unit of electromagnetic wave travel time is the nanosecond (ns), where 1 ns=10-9 s. Since the velocity of an electromagnetic wave in air is 0.3 m/ns, then the travel time for an electromagnetic wave in air is approximately 3.3333 ns per m traveled. The velocity is proportional to the inverse square root of the permittivity of the material, and since the permittivity of earth materials is always greater than the permittivity of the air, the travel time of a wave in a material other than air is always greater than 3.3333 ns/m. Click to see a table that shows permittivities and velocities for various earth materials.

System Components

GPR equipment utilized for the measurement of subsurface conditions normally consists of a radar control unit, transmit and receive antennas, and suitable data storage and/or display devices. The radar control unit generates synchronized trigger pulses to the transmitter and receiver electronics in the antennas. These pulses control the transmitter and receiver electronics in order to generate a sampled waveform of the reflected radar pulses.

Antennas are transducers that convert electrical currents on the metallic antenna elements (usually simple bowtie dipole antennas) to transmit electromagnetic waves that propagate into a material. Antennas radiate electromagnetic energy when there is a change in the acceleration of the current on the antenna. Radiation occurs along a curved path, and radiation occurs anytime that the current changes direction (e.g. at the end of the antenna element). Controlling and directing the electromagnetic energy from an antenna is the purpose of antenna design. Antennas also convert electromagnetic waves to currents on an antenna element, acting as a receiver of the electromagnetic energy by capturing part of the electromagntic wave.

The center frequency of commercially available antennas range from 10 to 1000 MHz. These antennas generate pulses which typically have 2 to 3 octaves of bandwidth. In general, lower-frequency antennas provide an increase in depth of penetration but have less resolution than higher-frequency antennas.

GPR systems are digitally controlled, and data are usually recorded digitally for post-survey processing and display. The digital control and display part of a GPR system generally consists of a microprocessor, memory, and a mass storage medium to store the field measurements. A small micro-computer and standard operating system is often utilized to control the measurement process, store the data, and serve as a user interface. Data may be filtered in the field to remove noise, or the raw data may be recorded and the data processed for noise remove at a later time. Field filtering for noise removal may consist of electronic filtering and/or digital filtering prior to recording the data on the mass data storage medium. Field filtering should be normally minimized except in those cases where the data are to be interpreted immediately after recording.

Mode of Operation

The most common mode of GPR data acquisition is referred to as the reflection profiling method. In the reflection mode of operation, a radar wave is transmitted, received and recorded each time the antenna has been moved a fixed distance across the surface of the ground, in a borehole, or across any other material that is being investigated. In addition to surveys on land and ice, surveys can also be made in lakes and rivers with low conductivity water.

Three-dimensional ground-penetrating radar (GPR) consists of collecting GPR data on closely spaced (less than 1 meter) lines. Powerful computers are then used to composite these lines into a three-dimensional data volume that can be observed from any angle using any subset of the data.

Transillumination measurements can be used in locations such as mines and boreholes where the transmitter and receiver can be put on opposite sides of a medium so as to look through it. Tomographic reconstruction techniques can be used to image the volume between the measurement points.

Data Display and Interpretation

The objective of GPR data presentation is to provide a display of the processed data that closely approximates an image of the subsurface, with the anomalies that are associated with the objects of interest located in their proper spatial positions. Data display is central to data interpretation, and is an integral part of interpretation.

There are three of displays of surface GPR data, including: (1) a one-dimensional trace, (2) a two dimensional cross-section, and (3) a three-dimensional display. Borehole data can be displayed as a two-dimensional cross section, or processed to be displayed as a velocity or attenuation tomogram. A one-dimensional trace is not very much value until several traces are placed side-by-side to produce a two dimensional cross section, or placed in a three dimensional block view.

The wiggle trace (or scan) is the building block of all displays. A single trace can be used to detect objects (and determine their depth) below a spot on the surface. By moving the antenna over the surface and recording traces at a fixed spacing, a record section of traces is obtained. The horizontal axis of the record section is surface position, and the vertical axis is the round-trip marvel time of the electromagnetic wave. A GPR record section is very similar to the display for an acoustic sonogram, or a fish finder. Wiggle trace displays are a natural connection to other common displays used in engineering (e.g. an oscilloscope display), but it is often impractical to display the numerous traces that are measured along a GPR transect in wiggle-trace form. Therefore, scan displays have become the normal mode of two dimensional data presentation for GPR data. A scan display is obtained by simply assigning a color (or a variation of color intensity) to amplitude ranges on the trace.

Three dimensional displays are fundamentally block views of GPR traces that are recorded at different positions on the surface. Data are usually recorded along profile lines, in the case of a continuous recording system, or at discrete points on the surface in fixed-mode recording. In either case, the accurate location of each trace is critical to producing accurate 3D displays. Normally, 3D block views are constructed, then they may be viewed in a variety of ways, including as a solid block or as block slices.

Obtaining good three-dimensional images are very useful for interpreting specific targets. Targets of interest are generally easier to identify and isolate on three dimensional data sets than on conventional two dimensional profile lines. Simplifying the image, by eliminating the noise and clutter is the most important factor for optimizing the interpretation. Image simplification may be achieved by: 1) carefully assigning the amplitude-color ranges; 2) displaying only one polarity of the GPR signal; 3) using a limited number of colors; 4) decreasing the size of the data set that is displayed as the complexity of the target increases; 5) displaying a limited range (finite-thickness time slice); and 6) carefully selecting the viewing angle. Further image simplification in cases of very complex (or multiple) targets may also be achieved by displaying only the peak values (maximum and minimum values) for each trace. Finite-thickness (pillow) time slices and cross sections have many advantages over infinitesimal thin slices that are routinely used for interpreting GPR data.

Performance Specs

The GPR method is site specific in its performance depending upon the surface and subsurface conditions. Performance specifications include requirements for or information about reflections, depth of investigation, resolution, interferences, calibration, quality control, and precision and bias.

Reflections

Reflections are created by an abrupt change in the electrical and magnetic properties of the material the electromagnetic waves are traveling through. In most situations, magnetic effects are small. Most GPR reflections are due to changes in the relative permittivity of material. The greater the change in properties the more signal is reflected. In addition to having a sufficient electromagnetic property contrast, the boundary between the two materials needs to be sharp.

Depth of Penetration

The principle limiting factor in depth of penetration of the GPR method is attenuation of the electromagnetic wave in the earth materials. The attenuation predominantly results from the conversion of electromagnetic energy to thermal energy due to high conductivities of the soil, rock, and fluids. Scattering of electromagnetic energy may become a dominant factor in attenuation if a large number of inhomogeneties exist on a scale equal to the wavelength of the radar wave.

GPR depth of penetration can be more than 30 meters in materials having a conductivity of a few milliSiemens/meter. In certain conditions such as thick polar ice or salt deposits, penetration depth can be as great as 5000 meters. However, penetration is commonly less than 10 meters in most soil and rock. Penetration in mineralogic clays and in materials having conductive pore fluids may be limited to less than 1 meter.

Interferences

The GPR method is sensitive to unwanted signals (noise) caused by various geologic and cultural factors. Geologic (natural) sources of noise can be caused by boulders, animal burrows, tree roots, and other inhomogeneties can cause unwanted reflections or scattering. Cultural sources of noise can include reflections from nearby vehicles, buildings, fences, power lines, and trees. Shielded antennas can be used to limit these types of reflections. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may cause noise on GPR records.

Resolution

GPR provides the highest lateral and vertical resolution of any surface geophysical method. Various frequency antennas (10 to 1000 MHz) can be selected so that the resulting data can be optimized to the projects needs. Lower frequency provides greater penetration with less resolution. Higher frequencies provide less penetration with higher resolution. Resolution of a few centimeters can be obtained with high frequency antennas (1 GHz) at shallow depths, while lower frequency antennas (10 MHz) may have a resolution of approximately one meter at greater depths. Horizontal resolution is determined by the distance between station measurements, and/or the sample rate, the towing speed of the antenna, and the frequency of the antenna.

Calibration

The manufacturer's recommendations should be followed for the calibration and standardization of GPR equipment. An operational check should be conducted before each project and before starting fieldwork each day. A routine check of equipment should be made on a periodic basis and after each problem.

Quality Control

Quality control can be appropriately applied to GPR measurements, and are applicable to the procedures, processing, and interpretation phases of the survey. Good quality control requires that standard procedures (such as given in ASTM Standard Guide D6432-99) are followed and appropriate documentation made.

Precision and Bias

Precision is a measure of the repeatability between measurements. Precision can be affected by the location of the antennas, the tow speed, the coupling of the antennas to the ground surface, the variations in soil conditions, and the ability and care involved in picking reflections. Assuming that soil conditions remain the same (that is, soil moisture), repeatability of radar measurements can be 100%.

Bias is defined as a measure of closeness to the truth. The accuracy of a GPR survey is dependent upon picking travel times, processing and interpretation, and site-specific limitations, such as unknown changes in radar velocities (lateral and vertical) or the presence of steeply dipping layers.

Advantages

GPR measurements are relatively easy to make and are not intrusive. Antennas may be pulled by hand or with a vehicle from 0.8 to 8 kph, or more, that can produce considerable data/unit time. GPR data can often be interpreted right in the field without data processing. Graphic displays of GPR data often resemble geologic cross sections. When GPR data are collected on closely spaced (less than 1 meter) lines, these data can be used to generate dimensional views of radar data greatly improving the ability to interpret subsurface conditions.

Limitations

The major limitation of GPR is its site specific performance. Often, the depth of penetration is limited by the presence of mineralogic clays or high conductivity pore fluid.

Cost Data

The cost of GPR systems vary widely depending on the complexity of the systems. Most systems fall in the $15,000 to $50,000 range. GPR systems can be rented for about $1,000 per week and a $300 mobilization charge. GPR surveys can be conducted by contractors with costs ranging from $1,000 to $2,000 per day depending on the amount of interpretation needed and if a report is required.

Vendor/Instrument Information
Vendor Instrument Trade Name
Geophysical Survey Systems Inc. SIR Systems, Pathfinder
MALÅ GeoScience RAMAC/GPR System
Sensors & Software Inc. pulseEKKO Systems, Noggin Systems


Additional Resources

References

Annan, A.P., 1992. Ground penetrating radar workshop notes. Sensors and Software Inc., Mississauga, Ontario, 128 p.

Benson, R.C., R.A. Glaccum, and M.R. Noel, 1983. Geophysical techniques for sensing buried wastes and waste migration. Environmental Monitoring Systems Laboratory, U. S. Environmental Protection Agency, Contract #68-03-3050, Las Vegas, NV, 1983, 236 p.

Beres, M., and F.P. Haeni, 1991. Application of ground penetrating radar methods in hydrogeologic studies. Ground Water, vol. 29, no. 3, p. 375-386.

Conyers, L.B. and D. Goodman, 1997. Ground-penetrating radar, an introduction for archaeologists. Altamira Press, Walnut Creek, CA, 232 p.

Daniels, J.J., 1989. Fundamentals of ground penetrating radar. Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems. Colorado School of Mines, Golden, Colorado, p. 62-142.

Daniels, J.J., J. Bower, and F. Baumgartner, 1998. High resolution GPR at Brookhaven National Lab to delineate complex subsurface structure. Journal Environmental and Engineering Geophysics, vol. 3, Issue 1, p. 1-6.

Daniels, J.J., D.L. Grumman, and M. Vendl, 1997. Coincident antenna three dimensional GPR. Journal of Environmental and Engineering Geophysics. vol. 2, p. 1-9.

Daniels, J.J., R. Roberts, and M. Vendl, 1995. Ground penetrating radar for the detection of liquid contaminants. Journal of Applied Geophysics, vol. 33, p. 195-207.

Davis, J.L., and A.P. Annan, 1989. Ground penetrating radar for soil and rock stratigraphy. Geophysical Prospecting, vol. 37, p. 531-551.

Fisher, E., G.A. McMechan, and A.P. Annan, 1992. Acquisition and processing of wide-aperture ground penetrating radar data. Geophysics, vol. 57, no. 3, p. 495-504.

Guy, E.D., J.J. Daniels, J. Holt, S.J. Radzevicius, and M.A. Vendl, 2000. Electromagnetic induction and GPR measurements for creosote contaminant investigation. Journal Environmental and Engineering Geophysics, vol. 5, Issue 2, p. 11-19.

Guy, E.D., J.J. Daniels, S.J. Radzevicius, and M.A. Vendl, 1999. Demonstration of using crossed dipole GPR antenna for site characterization. Geophysical Research Letters, vol. 26, no. 22. p. 3421-3424.

Haeni, F.P., 1996. Use of ground penetrating radar and continuous seismic-reflection on surface-water bodies in environmental and engineering studies. Journal of Environmental & Engineering Geophysics, vol. 1, no. 1, p. 27-36.

Holt, J., Vendl, M., Baumgartner, F., Radziviscius, S., and Daniels, J.J., 1998, Brownfields site-characterization using geophysics: A case history from East Chicago: Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems: Environmental and Engineering Geophysical Society, March 22-26, 1998, Chicago, IL, p. 389-395.


Imse, J.P. and E.N. Levine, 1985. Conventional and state-of-the-art geophysical techniques for fracture detection. Proceedings Second Annual Eastern Groundwater Conference, July 16-18, 1985, National Water Well Assoc., Portland, Maine, p. 261-278.

Knoll, M.D., F.P. Haeni, and R.J. Knight, 1991. Characterization of a sand and gravel aquifer using ground penetrating radar, Cape Cod, Massachusetts. U.S. Geological Survey Water Resources Investigations Report 91-4035, p. 29-35.

Olhoeft, G.R., 1998. Electrical, magnetic, and geometric properties that determine ground penetrating performance. in Proc. Seventh International Conference on Ground Penetrating Radar, May 27-30, 1998, the University of Kansas, Lawrence, KS, 786 p.

Placzek, G., and F.P. Haeni, 1995. Surface-geophysical techniques used to detect existing and infill scour holes near bridge piers. U.S. Geological Survey Water Resources Investigations Report 95-4009, 44 p.

Smith, D.G., and H. Jol, 1997. Radar structure of a Gilbert-type delta, Peyto Lake, Banff National Park, Canada. Sedimentary Geology, vol. 113, p. 195-209.

Ulriksen, C.P.F., 1982. Application of impulse radar to civil engineering, PhD. Thesis, Department of Engineering Geology, Lund University of Technology, Sweden, 175 p.

Internet Sites:

Jeff Daniels, Department of Geological Sciences, The Ohio State University, GPR Research

GRORADAR by Gary R. Olhoeft

Branch of Geophysical Applications and Support, U.S. Geological Survey

GPR 2002 Ninth International Conference on Ground Penetrating Radar April 29 - May 2, 2002, Santa Barbara, CA, U.S.A.

EPA Contact

Mark Vendl
U.S. Environmental Protection Agency
77 W. Jackson Blvd.
Chicago, IL 60604
312-886-0405
FAX 312-353-9281
vendl.mark@epa.gov


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