MULTIPLE GEOPHYSICS METHOD APPROACH TO SUBSIDENCE POTENTIAL IN A HISTORIC GOLD MINE SITE

UNC Charlotte and the US Forest Service initiated a geological and geophysical subsidence investigation on the site of a historic gold mine in the Uwharrie National Forest, North Carolina. The first goal in this investigation is to locate the “Little Lead”, a possible drift shown on the original site map. It is not known if the lead was ever opened. The investigation was a class project for a near-surface geophysics class. Participants evaluated the site map and other documents to determine the feasibility of using various geophysical methods to detect the drift. On site, we flagged the approximate trend of the lead. A gravel road crossed the projection of the lead at a point where the drift would have been a few m deep. Because conditions of the target were unknown, the investigators decided to use three tools: (1) a GSSI SIR-3000 GPR, (2) a Geometrics G-858 cesium magnetometer, and (3) a GSSI Profiler EM. There were three possible target conditions: (1) The lead was never opened, and therefore only bedrock would be present. (2) The lead was opened and left as a void. (3) The lead was opened, and some objects such as metal or wooden devices were left in the drift. Each scenario would produce a different geophysical signature. Investigators first ran a 400 MHz GPR profile across the lead. That profile detected the tops of convex upward mounds with internally stacked reflections. A second profile was run with a 100 MHz antenna that produced a penetration depth of 10-13 m. This profile imaged several mounds, interpreted as debris piles from mine workings, and also several vertical bands of low frequency reflections topping at a depth of 6 m under a reflective broad hyperbola. The low frequency GPR zone coincided with a magnetic anomaly of about 250 nT and with well defined negative anomalies in all EM IP frequencies. Overall, the team determined that multiple geophysical methods confirm a high probability that there is a void on the profile where it crossed the lead. A second anomaly was located several m from the first, but that one produced a magnetic/EM high and no GPR low frequency band. It may be a rock debris pile, or could even be a pipe or rail track. This class investigation demonstrated how multiple methods may, or may not, agree on a single interpretation, and illustrated for the students how their work became part of a larger, on-going research program.