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

Extensive knowledge of subsurface structure is required for safe underground mining. Fractures and unexpected caves may make mining dangerous and/or impossible. The purpose of this study is to assess various geophysical methods that can identify karst cavities and associated features to depths of up to 30 m. Two-dimensional electrical resistivity tomography (ERT), electromagnetic (EM) conductivity, and ground-penetrating radar (GPR) surveys were made over a known cavity in the Mississippian Haight Creek dolomite near Quincy, IL. The dolomite is covered by approximately 20-30 m of glacial deposits. ERT and EM methods were used along multiple lines to detect spatial changes in subsurface structure. GPR surveys were conducted over the GPS location of the cavity. Nearby soil pipes and fractures may be a surface manifestation of this feature.

The ERT surveys used a dipole-dipole array with 6 m spacing and total line length of 114 m to achieve an approximate depth penetration of 24 m. EM surveys performed with 40 m coil spacing achieved a maximum depth penetration of 60 m (with maximum signal response from 20-30 m depth) in the vertical dipole mode. GPR surveys were made with 25 MHz and 50 MHz antennas.

The EM conductivity method proved to be the most effective at delineating the target feature. Distinct high-conductivity anomalies were detected in the vicinity of the solution cavity, indicating that it is filled with soil and/or water. Additional high-conductivity anomalies were also observed and may mean that undiscovered karst features are present to the south, southeast and east. Some ERT results also showed apparent discontinuities in the sedimentary layers, which could be indicative of fractures or cavities in the subcropping bedrock. These discontinuities were recorded below the soil pipes/fissures 25 m to the northeast from the cavity, but not above the cavity itself, which suggests that the cavity may be connected to the surface via a dipping conduit, fault or fracture. GPR surveys with a 50 MHz antenna imaged what appears to be the water table, while surveys with a 25 MHz antenna may have detected bedrock surface with a depression above the cavity. However, GPR response was weak due to severe signal attenuation in silty, clayey glacial sediments, warranting future surveys with an even lower frequency antenna (such as 12.5 MHz).