Kansas Geological Survey, Open File Report 96-49
Ground-penetrating radar (GPR) is a high-resolution near-surface geophysical technique that uses antennas to send electromagnetic pulses into the ground in order to image the subsurface via returned reflection energy. Similar to seismic reflection methods, where reflections are caused by boundaries associated with acoustical impedance contrasts, GPR reflections are caused by the electromagnetic waves encountering media of different electrical properties - namely boundaries consisting of a contrast in the dielectric constant of the material above and below the boundary. Values for dielectric constants (K) range from 1 for air, 4-8 for limestone, 5-13 for shale, 5-40 for clay, and 81 for water (Davis and Annan, 1989). Dielectric constant values affect the velocity of electromagnetic waves through a material, and are related to velocity in non-magnetic materials by:
where c = 3 x 108 m/s (speed of light in a vacuum). Using the relationship given above, one-way velocities for materials range from 0.3 m/ns (meters/nanosecond) for air, 0.11-0.15 m/ns for limestone, 0.077-0.134 m/ns for shale, 0.047-0.134 m/ns for clay, and 0.033 m/ns for water. Antenna frequencies typically range from 10 MHz to 1000 MHz (a 500 MHz antenna was used in this study), and imaging resolution is proportional to antenna frequency, whereas penetration depth is inversely proportional to antenna frequency (the greater the antenna frequency, the less penetration but greater detail). Vertical resolution varies from 1.0-1.5 m (3-5 ft) for low-frequency antennas (10-100 MHz), to 0.02-0.3 m (1-12 in) for high-frequency antennas (500-1000 MHz) for most materials (Davis and Annan, 1989).
GPR profiles have a similar appearance to seismic profiles, and usually are represented as common depth point (cdp) data, with amplitude variations representing differences in reflection energy. As with seismic data, vertical scales are in time (or depth if the data have been depth migrated), whereas lateral scales are in distance. However, the scales differ by several orders of magnitude; GPR records have lengths measured in nanoseconds (1x10-9 s), compared to milliseconds (1x10exp-3 s) in seismic records. Also, the distances between cdps in GPR profiles are usually much smaller than seismic profiles (an average GPR cdp spacing of 3 cm for this study, versus standard, near-surface seismic reflection cdp spacing of 0.3-1.5 m for high-resolution profiles).