Ground penetrating radar, which was originally developed as a means of glacier ice sounding, has many similarities with wave propagation methods in subsurface imaging for oil exploration. This analogy has been used to transfer technology from the petroleum industry to the geotechnical arena. This approach does have its limits as the physical processes involved in signal transmission are very different. Nonetheless, GPR has flourished in its own right as witnessed by the many papers on the use of radar presented at the annual SAGEEP meeting and the biennial International Ground Penetrating Radar Conference. GPR has become one of the instruments of choice for many small site investigations where a metallic object that is shallowly buried, such as an underground gasoline storage tank, must be located.
Because GPR data provide an almost continuous record along the measurement path, the data can be used to determine the heterogeneity of the subsurface [ Olhoeft, 1991, 1994]. Using statistical measures of the radar section, Olhoeft tried to estimate the horizontal length scale of various portions of the data set. The resulting image shows boundaries between regions that are statistically homogeneous. If the relationship of these regions to geologic and hydrologic properties can be determined, then these data could serve as a means of developing detailed flow models.
Because the propagation of electromagnetic energy at radar frequencies is controlled by dielectric properties in geologic materials, the method is sensitive to changes in dielectric permittivity of the bulk material. The bulk dielectric permittivity of a rock formation is highly dependent upon the dielectric value of any pore fluid present, the degree of saturation, and the porosity. The presence of water filled pores increases the bulk dielectric permittivity from the value associated with the unsaturated state. This characteristic allows GPR to detect the water table under certain conditions. If pore water is replaced by organic compounds, which typically have a dielectric constant less than water, electromagnetic energy will be reflected. Sander et al. [1992] and Greenhouse et al. [1993] describe the 1991 Borden experiment, in which GPR was used, along with other geophysical techniques, to monitor a controlled spill of percholorethylene (PCE), a dense nonaqueous phase liquid (DNAPL). This study points out the need for time-differential measurements to remove background effects to allow the detection of small dielectric changes. This technique will be most useful for monitoring contaminant movement during remediation efforts.
There has been some work on processing GPR data using techniques developed for seismic reflection data. Fisher et al. [1992a] describe a wide-aperture GPR system that directly uses seismic processing algorithms. A subsequent article [ Fisher et al., 1992b] uses a reverse-time migration procedure to improve data interpretation. Goodman [1994] has developed a GPR modeling procedure based upon exact ray tracing and used it to simulate archeological targets. Powers et al. [1992] have developed a one-dimensional, full-waveform GPR forward modeling method that is useful in estimating layered model parameters.