Ground-penetrating radar investigation of the shallow subsurface at the Groundwater Remediation Field Laboratory, Dover Air Force Base, Delaware.

摘要

To promote the development of technologies to remediate DNAPL's, and other environmental contaminants, the Groundwater Remediation Field Laboratory (GRFL) was established at Dover Air Force Base, Delaware (Clement at al., 1997). A major characterization, including the use of ground penetrating radar (GPR), seismic reflection, electro-magnetic and cone penetrometer (CPT) techniques to develop an image of the subsurface to depths approaching 15 m, of the laboratory site was conducted prior to the beginning of controlled contaminant releases. The Department of Geology at the University of Delaware participated by collecting GPR data using their pulseEKKO IV system with 200, 100, 50, and 25 MHz antennas. This dissertation project focused on the analysis and interpretation of the 50 and 200 MHz GPR data, and the integration of this data with the CPT borehole information with the 25 and 100 MHz results from earlier work by Hills (1998) and Schafstall (1998).; Nine major reflection events were observed in the subsurface to depths of approximately 10--13 meters in the 50 MHz common offset profiles. Due to its higher frequency and therefore greater attenuation, reflection events were observed to only 4--5 meters in the 200 MHz data. The velocities of these reflections varied between 0.100 m/ns to 0.169 m/ns, with average velocities slightly higher in the 200 MHz. These velocity values are consistent with the sediment types (e.g., mostly dry sands, silts, and clays) observed in the region. Five reflection events were observed to be fairly continuous. The general structural trend of the reflection events indicated a deepening of the events towards north and west. Correlations of the GPR reflection events with the stress ratios and resistivity data were fairly consistent. The prominent basal silt-clay unit was observed to be consistent in the CPT logs and the 25 MHz data. This silt-clay layer acts as the major aquitard in the region. To first order, reflection events 3 and 4 appear to coincide with lenses of silt-clay within an overall sand unit. Data from sediment cores in the study area in the future may provide conclusive evidence for correlation with the results obtained in this study.