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Geophysical Support
Ground-Penetrating Radar (GPR) Theory of Operation
GPR uses high frequency pulsed electromagnetic (EM) waves (typically from 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 EM property contrasts (Annan, 1992, Daniels, 1989). GPR equipment utilized for the measurement of subsurface conditions normally consists of a transmitter and receiver antenna, a radar control unit, and suitable data storage and/or display devices. A circuit within the radar control unit generates trigger pulses that are sent simultaneously to the transmitter and receiver electronics.
The transmitter electronics produce output pulses that are radiated into the ground from the transmitting antenna, and the receiver is turned on to wait for radar energy that is transmitted into the subsurface. When the transmitted radar energy encounters and abrupt change in the EM properties (primarily the relative dielectric permittivity) of the subsurface, a portion of the energy is reflected back to the radar antenna and the remaining energy is transmitted downward to deeper material. The reflected EM energy is detected by the receive antenna, and these signals are sent to the control unit for amplification. As the antenna(s) are moved along a survey line, a series of scans are collected at discrete points along the line. These scans positioned side by side to form a display profile of the subsurface.
Schematic diagram of a ground penetrating radar system.
GPR - Conditions for Use
Attenuation
The principle limiting factor in depth of penetration of the GPR method is attenuation of the electromagnetic (EM) wave in earth materials. Attenuation predominantly results from the conversion of EM energy to thermal energy due to high conductivities of the soil, rock, and fluids. Scattering of the EM energy at sharp boundaries may become a dominant factor in attenuation if a large number of inhomogeneities exist on the scale of a wavelength.
Depth of PenetrationGPR 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, penentration 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.
ResolutionGPR 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 grreater penetration with less resolution. Higher frequencies provide less penetration with higher resolution. Vertical resolution ranges from a few centimeters to more than 0.3 of a meter. Horizontal resolution is determined by the distance between station measurements, or the sample rate, or both, and the towing speed of the antenna.
GPR - Data Measurements
Two modes of data collection are normally used in conducting GPR surveys, both being referred to as the reflection profiling method. In the first mode, data are acquired as the antenna(s) are towed or pulled across the survey line, and in the other mode the GPR data are collected at specific points along the survey line both with fixed tranmitter/receiver separation. The choice of operational mode depends upon the characteristics of the target, the field conditions, and the purpose of the survey. In addition to surveys on land and ice, surveys can also be made in lakes and rivers with low conductivity water. When GPR data are collected on closely spaced (less than 1 meter), these data can be used to generate three-dimensional views of radar data.
All-Terrain Vehicle towing GPR antenna.
GPR data being collected at specific points along survey line.
GPR data being collected on ice.
GPR data being collected on water.
A third mode of data collection is transillumination, which is 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.
GPR - Applications
GPR measurements are used in geologic, engineering, hydrologic, and environmental applications. 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 contaminants (Cosgrave and others, 1987; Brewster and Annan, 1994; Daniels and others, 1995), and conducting archaeological investigations.
Integration of GPR data with other surface geophysical methods, such as seismic, resistivity, or electromagnetic methods, reduces uncertainty in site characterization.
GPR record showing two buried tanks. (Courtesy of Geophysical Survey Systems Inc.)
GPR record showing glacial outwash overlying granite bedrock. (Courtesy of Geophysical Survey Systems Inc.)
GPR record showing a trench with buried drums. (Courtesy of Geophysical Survey Systems Inc.)
GPR record of Native American burials and was provided courtesy of the Ho Chunk Nation. (Courtesy of Geophysical Survey Systems Inc.)
GPR - Three-Dimensional GPR
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. Obtaining a good three-dimensional display is a critical part of interpreting GPR data. Targets of interest are generally easier to identify and isolate on three-dimensional data sets than on conventionl two-dimensional profile lines. Finite-thickness time slices and cross sections have many advantages over infinitesimal thin slices that are routinely used to interpreting GPR data.
Various views of 3-D GPR data sets.
3-D GPR time-slice image of an asphalt paved inactive service station. Image shows location of pipes as well as pump island.
