COMPARING SEISMIC REFRACTION, ELECTRICAL RESISTIVITY TOMOGRAPHY AND GROUND PENETRATING RADAR IN THE EXPLORATION OF MINE SHAFTS: CASE STUDY AT CENTRALIA, PA

Near surface geophysical methods have been heavily used in mapping geological structures, karst topography, identification of mine shafts and underground void spaces. In this study, a site in Centralia, PA was selected as an experimental place to study the relative effectiveness of Seismic Refraction (SR), Electrical Resistivity Tomography (ERT), and Ground Penetrating Radar (GPR) in the ability to detect abandoned mine shafts. Centralia is well known for its extensive mine fire, and many abandoned mines with countless subsurface mine shafts. All surveys were performed along the same transect and the resulted output profiles from the three methods were compared. SR and ERT were the preferred methods of mine shaft detection, as these two surveys provided the most information available. Dipole-Dipole and Wenner ERT inversions detected the presence of 4 mine shafts with their respective placement in the subsurface. Standard Seismic Refraction Tomography (SRT) methods could not easily identify the mine shafts; however, using a simple travel-time curve from a single energy source made identification of the mine shafts much easier. In travel-time curves, deviations from anticipated resultant velocities were indicative of p-wave propagation in void spaces. These were seen over both forward and reverse SR transverses. The SR was able to detect 3 subsurface voids. The study showed that simple travel-time curves of a field seismogram can sometimes identify a mine shaft more effectively than an SRT profile. Due to the high signal-to-noise ratio in 2D-GPR tomography, point by point GPR scanning was used instead and 5 shafts were revealed. The study showed that a combination of ERT and SR seemed to be more effective for this survey than any other combination among the three methods used in this study.