Ground Penetrating Radar GPR Methods

Environmental engineers often use the GPR methods to locate buried objects, map the depth to shallow water tables, and delineate soil horizons. The principles involved in GPR technology are similar to those in seismic refraction, except that in GPR, electromagnetic energy is used instead of acoustic energy, and the resulting image is relatively easy to interpret.

In a GPR survey, a transmitting and a receiving antenna are dragged along the ground surface as shown in Figure 9.14.4. The small transmitting antenna radiates short pulses of high-frequency radio waves into the ground, and the receiving antenna records variations in the reflected return signal. The attenuation loss of the signal in the ground increases with ground conductivity and with frequency for a given material. Changes in ground electric conductivity are associated with natural hydrogeological conditions such as bedding cementation, moisture, clay content, voids, and fractures. Therefore, an interface between two soil or rock layers with sufficient contrast in electric conductivity shows up in the radar profile (Benson and Glaccum 1979).

The advantages of the GPR methods include rapid areal coverage, where site conditions are favorable, and great

FIG. 9.14.4 Ground-penetrating radar survey. (Reprinted from U.S. Environmental Protection Agency, 1993, Subsurface characterization and monitoring techniques, a desk reference guide, U.S. EPA/625/R-93/003a [May] U.S. EPA.)

resolution and penetration in dry, sandy, or rocky areas. The use of GPR, however, is limited in moist and clayey soils and soils with high electrical conductivity.

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