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Urban/Industrial Geophysics: Anniston Army DepotProblem/OpportunityGeophysical investigations in urban and/or industrial areas are limited by significant cultural interference, and thus are rarely attempted. Remediation efforts at the Southeast Industrial Area (SIA) of Anniston Army Depot provided a test site for determining the feasibility of using geophysics in such a complex setting. Previous investigations spanning the last 20 years have all failed in one respect: mapping the underlying bedrock topography. This type of map is critical for guidance of future drilling and remediation efforts, because the low-points on the bedrock surface are likely discharge points for migrating contaminants.
ApproachThe approach taken by Argonne was to construct a map of the bedrock surface using seismic profiling and existing borehole information. Several technical hurdles first had to be overcome. Numerous sumps, fans, and blowers; constant vehicular traffic; and sporadic railroad activity produced strong background noise. Construction design for the SIA was not conducive to seismic profiling because the ground surface was composed of compacted gravel-fill, reinforced concrete, and asphalt paving. Solutions involved careful survey design; collecting data during off-hour periods; mounting seismic-receivers in gravel, grassy, and asphalt areas adjacent the major roadways; and paying careful attention to where the shot point was located to avoid impacting the hardened concrete surfaces.
ResultsAt least four bedrock depressions, averaging 30 meters in local relief, were mapped by the Argonne study (see figure). One of these bedrock lows occurred where previous investigations had suggested that the bedrock surface was shallower. Subsequent drilling confirmed the presence of these bedrock depressions, and indicated an increase in cavity development. The cavity development, concurrent with the bedrock lows, suggests more porous bedrock than previously thought, and thus more potential for contaminant entrapment and migration.
Future PlansThe successful implementation of geophysics at Anniston has led the U.S. Army to fund Argonne studies at other urban/ industrial sites within the Department of Defense Newer processing techniques using ray-path tomography and modeling of the seismic surface-waves are currently planned to help improve the overall state-of-the-art.
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Bedrock Surface Map of the SIA at ANAD
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