INVESTIGATING CRITICAL ZONE PROCESSES IN THE PIEDMONT WITH SEISMIC REFRACTION AND VP/VS RATIOS

Low effective pressures (<1 MPa) and large variations in porosity, fracture density, and saturation complicate geophysical efforts to characterize the critical zone (CZ). We conducted seismic refraction surveys along four 190 m profiles above weathered igneous and metamorphic bedrock in the South Carolina Piedmont. Our seismic data include both p-wave velocity (V_p) data and shear wave velocity data (V_s) from body waves. We used a sledgehammer source and 96 horizontal geophones to acquire shear body wave travel times and inverted those data to determine V_s. Unexpectedly low V_p/V_s values (V_p/V_s < ) were found in a layer approximately 15 m thick in all four profiles, indicating negative Poisson’s ratios. These data were consistent with sonic logs collected in nearby boreholes. While such low V_p/V_s values are rarely identified in Earth materials, we propose that a wider range of V_p/V_s and Poisson’s ratios can be expected in the CZ compared to the deeper crust, where effective pressures are greater. These results raise questions about how critical zone processes can create low V_p/V_s and negative Poisson’s ratios in weathered materials.