MULTIMETHOD GEOPHYSICAL CHARACTERIZATION OF FAULT SYSTEMS FOR ENVIRONMENTAL PLANNING

摘要

The accurate mapping of bedrock fault and fracture systems remains one of the most challenging components of geologic assessment for environmental investigations. Many times, these systems can have a dramatic impact on the conceptual site model used for engineering or environmental design, and can ultimately alter the approach used in the evaluation of various civil projects such as dams, tunnels and bridges. This paper presents a case study of a petroleum hydrocarbon-impacted site in Leitchfield, Grayson County, Kentucky in which a major fault system runs directly through the area of a former gasoline release and controls its ultimate movement and distribution in the subsurface. Based on limited soil/bedrock boring information and geologic cross-sections developed during the initial site characterization efforts, the shallow bedrock varied significantly in, relative position and composition, with a notable stratigraphic offset present from one side of the fault to the other. Because of the complex nature of the subsurface conditions, geophysics was utilized to enhance the understanding of the fault trace on a localized scale in order to direct subsequent investigation and remedial efforts. Four geophysical survey methods were utilized to image the lateral and vertical extent of the existing fault system: frequency domain electromagnetic (FDEM) terrain conductivity mapping, two-dimensional electrical resistivity imaging (2-D ERI), refraction microtremor (ReMi) shear wave profiling, and very low frequency electromagnetics (VLF-EM). The combination of these four techniques offered a cost-effective and highly-sensitive approach for providing the information used to identify potential preferential groundwater flow pathways and zones where subsurface petroleum hydrocarbon impacts could accumulate.