Characterization of an aquitard and direct detection of LNAPL at Hill Air Force Base using GPR AVO and migration velocity analyses

Large quantities of non-aqueous phase liquids (NAPL), contaminate the near surface sediments at Operable Unit 1 (OU1), Hill Air Force Base (HAFB), Utah. In October 2000, a 3D, multi-offset GPR survey was acquired at OU1 with two objectives: (1) to image the aquifer/aquitard boundary at a depth of about 30 ft, and (2) to evaluate quantitative processing and interpretation methodologies for direct detection of NAPL. Using pre-stack depth migration, we map the aquitard boundary to about {+-} 1 ft throughout the survey area. An unusual reflection is identified within the vadose zone that does not correlate with known geology. The region below this reflection has anomalously high velocity, implying low electric permittivity, and the amplitude of the anomalous reflection deviates significantly from the background AVO trend. Fitting the Fresnel equation to the AVO data, we estimate the velocity contrast at the anomaly boundary and find that it is in good agreement with the migration velocity model. We interpret the anomaly as a previously unidentified NAPL rich zone. Subsequent coring and chemical analyses verify our interpretation. This exciting result implies that these methodologies may be useful for direct detection of NAPL at other HAFB locations and at sites with similar hydrogeology.