SHALLOW GEOPHYSICAL STUDIES OF NATURAL HAZARDS AND GROUNDWATER SYSTEMS IN THE SOUTHEASTERN U.S

We use geophysical surveys to investigate geologic hazards and groundwater systems in three different geologic settings in the southeastern U.S. The electrical resistivity method was used to study the internal structure of a landslide in the Blue Ridge Mountains of southwestern North Carolina. Evidence of movement includes large boulders, rock debris, and hummocky topography. We used a dipole-dipole array with an electrode spacing of 5 m and a maximum current-potential electrode separation of 40 m. Three profiles at different elevations provide a total coverage of 900 m across the top, middle, and toe of the landslide. We invert the data for 2D models of resistivity structure. The goal of the project is to identify the lateral extent and thickness of the mobile body, to image potential internal sliding surfaces, and to better understand the stability of the slope. A similar electrode configuration was used to investigate a small watershed in amphibolite-grade gneisses in the Inner Piedmont of northeast Georgia. The focus of this study is the soil-saprolite-bedrock transition and its relationship to groundwater flow. Three parallel resistivity profiles (total profile length: 460 m) and surface waves recorded with two intersecting multichannel seismic profiles (total length: 570 m) image the 3D geometry of the bedrock surface and variations within the soil-saprolite cover. One resistivity profile was occupied twice over a 7-month period (April to November). The interpretations are constrained by outcrops and well information. A more extensive set of coincident resistivity/seismic profiles was recorded over the covered karst terrain of the Dougherty Plain in southwest Georgia. Here, Quaternary sands and clays overlie limestone of the Eocene Ocala Formation. A 2D grid of 9 resistivity profiles (total length: 1050 m) is used along with existing seismic profiles, borehole data, and standard penetration tests to investigate active sinkhole development and variations in fracture density within bedrock.