Seismic Characterization Of The Karst Bedrock Surface In The Southeast Industrial Area At Anniston Army Depot, Alabama

Abstract

Environmental concerns at the Southeast Industrial Area (SIA) of Anniston Army Depot (ANAD) have been the impetus for geotechnical investigations over the last 20 years. These previous investigations have all failed in one respect, to map the bedrock topography of the underlying Knox Group dolomites. The bedrock surface is an important target: a) lows points provide a greater column of overburden in which to trap contaminants; b) bedrock pinnacles may act as rapid access points into the karst conduit systems; and c) hydraulically transmissive zones are often found within the weathered portion (epikarst) of the bedrock. The approach taken by Argonne was to construct a bedrock surface map integrating existing borehole data with selective seismic-refraction profiling. Initial inspection revealed that depth-to-bedrock measurements based solely on auger refusal were not acceptable in many cases because refusal occurred on top of either large boulders or chert horizons. Seismic refraction profiling within the SIA required overcoming several technical challenges. First, numerous sumps, fans, and blowers produced strong background noise, as did the constant vehicular traffic and sporadic railroad activity. Secondly, construction design for the SIA was not conducive to seismic profiling as compacted gravel-fill, reinforced concrete, and asphalt paving comprised the ground surface. One solution was simple brute force with upwards of 30 stacks using a weight-drop source to overcome the background din. Other solutions included collecting data during off-hour periods, mounting geophones in gravel, grassy, and asphalt areas adjacent to the major roadways, and paying careful attention to where the shot point was located to avoid impacting on the hardened concrete surfaces. In the latter case, the concrete roadbed acts as a wave-guide, primarily transmitting the seismic energy horizontally and obscuring the underlying geology. Seismic profiling and analysis of borehole data indicate that at least 4 major bedrock depressions, averaging 20-to-30 m in local relief, occur along the southeast and northeast boundaries. Drilling records indicate more frequent occurrences of clay-filled cavities and fractured and weathered rock along the southeast boundary, suggesting a causal relationship for the location of the bedrock lows. Eight confirmatory boreholes were used to ground truth the seismic models. Four of these boreholes confirmed the presence of the bedrock lows, with one low located where previous investigations had indicated shallower bedrock. The ability to acquire seismic data in an active industrial area where utility lines and sewer systems preclude using electrical or electromagnetic methods helps ANAD to intelligently guide drilling, regulatory, and remediation strategies.