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

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