INTEGRATED GEOPHYSICAL IMAGING TECHNIQUES FOR DETECTING NEOTECTONIC DEFORMATION IN THE FLUORSPAR AREA FAULT COMPLEX OF WESTERN KENTUCKY

The Fluorspar Area Fault Complex of southern Illinois extends southwest beneath the northern Mississippi embayment sediment in western Kentucky. Accurate identification and characterization of neotectonic structure within this area has proved problematic due to approximately 100 m of unlithified Cretaceous, Tertiary and Quaternary sediments that conceal bedrock. In addition, the long recurrence interval for large earthquakes allows erosion and/or deposition mechanisms to mask possible geomorphic signatures. An on-going non-invasive geophysical study is being conducted in order to determine the extent of neotectonic structure in the sediment cover. The integrated geophysical methods being used include shear-wave (SH mode) seismic reflection and electrical resistivity (ER) imaging profiles. The non-homogeneous saturated characteristics of the sediments make both techniques ideal for evaluating the existence of near-surface structure. SH-waves travel in the sediment matrix rather than the ground water, and although the frequency of the SH-wave is lower than the P-wave, our experience shows the SH-wave's much lower velocity nonetheless yields an increase in resolution by a factor of two to three, facilitating the imaging of subtle features. ER surveys are being acquired using the dipole-dipole array. This configuration has been shown to optimize horizontal resolution, particularly in the identification of conductive vertical to near vertical features. Preliminary results have found the two methods to be corroborative. S-wave seismic reflection data have successfully imaged the entire 100 m sediment column, and electrical resistivity data have provided acceptable images of sedimentary structure to a depth of approximately 50 m.

Both geophysical methods indicate high-angle faults extending to within 10 m of the surface. Good correlation exists between the methods on fault location and degree of near surface offset. Preliminary time displacement calculations show offsets ranging from 15 to 25 m at the top-of-bedrock horizon, located at depths of approximately 90 to 100 m. Less displacement exits within the near surface, with offsets of 2 to 5 m observed at 20 to 25 m below the surface.