Carpenter, Philip J.
M.S. (Master of Science)
Department of Geology and Environmental Geosciences
Karst--Illinois--Quincy||Coal mines and mining--Illinois--Quincy||Mines and mineral resources--Illinois--Quincy
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 was to assess various geophysical methods that can identify karst cavities and associated features in bedrock to depths of up to 30 m. Two-dimensional electrical resistivity tomography (ERT), azimuthal resistivity (AR), electromagnetic (EM) conductivity, seismic refraction, 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, whereas AR was used to investigate electrical anisotropy in the subsurface. GPR surveys were conducted over the GPS location of the cavity. Seismic refraction surveys attempted to determine depth to bedrock in the vicinity of the karst feature. Nearby soil pipes and fractures may be a surface manifestation of this feature. The ERT surveys used a dipole-dipole array with maximum electrode spacing of 6 m and total line length of 114 m to achieve an approximate depth penetration of 24 m. AR measurements consisted of Wenner array soundings along lines of varying azimuth. EM surveys, performed with 40 m coil spacing, achieved a maximum signal response from 20-30 m depth in the vertical dipole mode. GPR surveys were made with 25 and 50 MHz antennas. Seismic experiments used a sledgehammer source and a 24-channel seismograph with 5 m geophone spacing. 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, suggesting 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 of the existing mine. 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. AR measurements support the orientation of the karst feature inferred from EM data, but interpretation of AR results involves complicated theory. 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). Seismic surveys suffered from noisy signal, rendering them unreliable. A more energetic seismic source may solve this problem. Overall the EM and 2D resistivity methods worked the best, and these methods are recommended for future surveys.
Morozov, Vitaliy Alekseyevich, "Geophysical characterization of karst features above an underground mine near Quincy, Il" (2016). Graduate Research Theses & Dissertations. 3282.
ix, 111 pages
Northern Illinois University
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