ELECTRICAL RESISTIVITY IMAGING (ERI) BENEATH SINKHOLES AND SUSPECT SINKS: IMPLICATIONS FOR KARST RISK ASSESSMENT IN CLARK COUNTY, OHIO

Karst features (sinkholes, subsurface voids, solution-enlarged fractures, and springs) occur in Silurian carbonates (mostly dolostones) of west- and south-central Ohio. The Division of Geological Survey of Ohio published a report identifying known and suspect karst features associated with these carbonates in a 24 km x 16 km area covering most of Clark Co., Ohio. Potential sinkholes were identified on a DEM generated from LiDAR data. Most were then examined in the field. The study also included locations of voids in cliff faces and springs. Results were presented as maps and aerial images showing the locations of features. Karst features were classified as field verified (59 features), suspect - field visited (17), suspect - not visited (4), and spring (32). We examined a site with 3 field verified sinks and a site with 3 suspect - field visited sinks using electrical resistivity imaging (ERI) to evaluate processes associated with their formation. The study areas are underlain by middle Silurian dolostones overlain by glacial till and outwash. Interpretations of ERI profiles were informed by nearby well logs.

The 3 field verified sinks are ~1 m wide and dm's deep. In the 2 ERI surveys conducted (78 m & 50 m long, both 12 m depth), bedrock (resistivities, ρ's, of ~700-3300 Ωm) is readily differentiated from overlying sediment (ρ's of ~20-300 Ωm). The sinks occur in a dry drainage controlled by bedrock topography. No anomalies indicative of bedrock collapse are present beneath the sinks, suggesting that they formed by subsurface erosion and collapse of sediment associated with solution-enlarged fractures, similar to those apparent in outcrops. The 3 suspect sinks are ~10 m wide and ~1 m deep. The 2 ERI surveys conducted (each 81 m long, 17 m depth) imaged discontinuous gravel lenses (ρ's of 200-500 Ωm) and sand, silt, & clay (ρ's of 10-100 Ωm), but did not intercept bedrock, consistent with nearby well logs. The depressions likely are not sinkholes, but rather cultural features, as this site appears disrupted on historical aerial imagery.

This study shows that ERI is an effective tool for evaluating depressions when used with well logs and surface examination. Results suggest that the karst risk associated with collapse sinkholes in the area may be minimal and that other suspect depressions may not be sinks and warrant further evaluation.