GEOPHYSICAL CHARACTERIZATION OF KARST FEATURES ABOVE AN UNDERGROUND MINE NEAR QUINCY, ILLINOIS

Extensive knowledge of subsurface structure is required for safe underground mining. Solution-enhanced fractures may make mining dangerous and/or impossible. In this study two-dimensional (2D) resistivity and electromagnetic (EM) conductivity surveys were made over a known solution-enhanced fracture in in the Mississippian Haight Creek limestone in Quincy, IL. This limestone is covered by approximately 20-30 m of glacial and alluvial deposits. Both methods were used along multiple lines to detect lateral changes in subsurface structure. A surface manifestation of the fracture was observed as a series of soil pipes.

The 2D resistivity surveys used a 5 m spacing dipole-dipole array with a total line length of 114 m to achieve an approximate depth penetration of 24 m. Apparent resistivity values were inverted using RES2DINV software. Unconsolidated sediments (20-580 ohm-m) overlie high-resistivity bedrock (greater than 600 ohm-m). The bedrock fracture location displays an apparent thinning of the sedimentary layers. EM surveys performed with the Geonics EM34 at a 40 m coil spacing achieved a maximum depth penetration of 60 m in the vertical dipole mode. Measurements were taken at 5 m increments. Surveys using 20 m coil spacing were also used to study shallower structures. This method was successful at detecting the fracture as a low conductivity feature.

Future surveys may include seismic and ground penetrating radar (GPR) methods at the same location, as well as additional 2D resistivity and EM surveys to increase spatial resolution. These surveys are being done in a cornfield, which makes logistics difficult.