Accounting For Near Field Conditions When Interpreting 3-D Gpr Data

Many GPR data sets are collected to characterize the earth's subsurface at depths less than 5.0 m. Depending on the soil's electrical properties and the frequency of the GPR antenna, near field conditions may extend to this depth and deeper. GPR measurements of a half-wavelength calibration sphere buried at different depths in the Ohio State University test pit have permitted near field calibration of the OSU bistatic antennas. Data collected along multiple profile lines at increasing offset distances from the buried sphere have been processed to extract the amplitude and time arrival of the 125 MHz component in the sphere reflections. Contour maps of the amplitude indicate strong antenna orientation sensitivity in the extreme near field, At<br>increasing depths the reflection strength becomes less sensitive to antenna orientation. Time arrivals from profile lines with antennas oriented transverse to the profile line direction are amenable to accurate velocity-depth inversion, even in the extreme near field. Accurate velocity-depth inversion was not successful on profile line data with transmit and receive antennas<br>oriented parallel to the profile line direction.