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Seismic Techniques to Delineate Dissolution Features In the Upper 1000 Ft At a Power Plant Site

Summary Shallow seismic techniques (Steeples and Miller, 1990; Park et al., 1999; Xia et al., in press) were used to enhance the effectiveness of a drilling program designed to locate dissolution features large enough to put the integrity of equipment or environment at Alabama Electric Cooperative’s proposed Damascus site at risk (“A” on Figure 1). Dissolution features will directly impact engineering design specifications and future plant safety. This applied research program identified acoustic characteristic unique to voids, subsurface subsidence, and/or karst features; evaluated the potential of acoustic methods to enhance drill-assisted mapping of major stratigraphic units and structural features; identified the maximum and minimum depths of seismic investigation; estimated resolution potential (vertical and horizontal features detectable and resolvable); defined optimum geometries and equipment; and incorporated production 2Ω-D reflection and shear wave profiles with exploratory drilling. The shear wave velocity field provided valuable information about areas that might be at risk of subsiding. From that feasibility survey, areas studied with “young” sinkholes, directly tied to karst features, produced pronounced velocity inversions in close proximity to large velocity gradients (generally forming a closure on contoured cross-sections). These anomalous areas were interpreted to indicate increased stress associated with roof rock loading over rubble zones or void areas. Data from both the reflection and shear wave profiles from the feasibility and 2Ω-D surveys possessed several unique features that are probably related to dissolution and subsidence. Geology Information about the local geologic setting, accompanied by physical examination of surface expressions and exposures of geologic features and anomalies similar to those targeted by this seismic imaging program, was the necessary backgr ound for evaluating the feasibility and optimizing these techniques. Dissolution caverns in the Glendon and Marianna limestones are postulated to be the origin of voids and collapse features within the Bucatunna Clay and river terrace deposits throughout this part of southern Alabama and nearby northern Florida. Incorporating seismic reflection and surface wave profiling allowed geologic features interpreted in boreholes to be extended. Coincident interpretations of these acoustic methods provided insight into the lateral coherency of geologic anomalies undetected by drilling alone. Present at depth (about 100 ft) is the Ocala Limestone, the geologic unit proposed to be responsible for many of the karst-related sinkholes in Florida. The Glendon, Marianna, and Ocala limestones, all present within the upper 150 ft at this site, have a history of dissolution. Critical to safe construction and operation of the proposed power station is the presence of an intact geologic section between the top of the river terrace deposits (ground surface) down to the base of the Ocala Limestone. A swarm of sinkholes in Auburn University’s Solon Dixon Forestry Research Center, less than 5 miles from the proposed site, provided an ideal setting to study the acoustic characteristics of pre-subsidence earth (“B” on Figure 1). Tests designed to identify acoustic signatures associated with developing sinkholes targeted areas near these sinkholes with no apparent surface expression. Since drilling in the national forest was not possible, survey lines were located in close proximity to sinkholes, with receiver stations adjacent to the sinkholes classified as subsidence prone areas. Results of seismic testing permitted estimation of the detectability of cavities, minimum resolvable dimensions of subsurface features, and depth ranges affected by dissolution.