SEISMIC-REFLECTION AND GROUND-PENETRATING RADAR IMAGING OF A SHALLOW AQUIFER

In 1995 and 1996, researchers associated with the US Air Force's Phill ips and Armstrong Laboratories took part in an extensive geophysical s ite characterization of the Groundwater Remediation Field Laboratory l ocated at Dover Air Force Base, Dover, Delaware. This field experiment offered an opportunity to compare shallow-reflection profiling using seismic compressional sources and low-frequency ground-penetrating rad ar to image a shallow, unconfined aquifer. The main target within the aquifer was the sand-clay interface defining the top of the underlying aquitard at 10 to 14 m depth. Although the water table in a well near the site was 8 m deep, cone penetration geotechnical data taken acros s the field do not reveal a distinct water table. Instead, cone penetr ation tests show a gradual change in electrical properties that we int erpret as a thick zone of partial saturation. Comparing the seismic an d radar data and using the geotechnical data as ground truth, we have associated the deepest coherent event in both reflection data sets wit h the sand-clay aquitard boundary. Cone penetrometer data show the pre sence of a thin lens of clays and silts at about 4 m depth in the nort h part of the field. This shallow clay is not imaged clearly in the lo w-frequency radar profiles. However, the seismic data do image the cla y lens. Cone penetrometer data detail a clear change in the soil class ification related to the underlying clay aquitard at the same position where the nonintrusive geophysical measurements show a change in imag e character. Corresponding features in the seismic and radar images ar e similar along profiles from common survey lines, and results of join t interpretation are consistent with information from geotechnical dat a across the site.