GROUND PENETRATING RADAR INVESTIGATIONS OF SINKHOLE DEVELOPMENT DUE TO MINING OPERATIONS IN GOLD HILL, NORTH CAROLINA

Subsurface profiles generated by ground penetrating radar (GPR) surveys reveal potentially unstable ground and shallow voids in the Gold Hill Historic Park, Gold Hill, North Carolina. In February of 2014, a deep sinkhole (> 15 meters) developed in a high traffic area of the Gold Hill Historic Park. Efforts to fill the opening were thwarted by subsequent collapse in April 2014 and thereafter, which highlighted continued risk to people and property within this historic mining district. A GSSI SIR^® 3000 GPR system fitted with a 200 MHz antenna was employed during the summer of 2015 to survey near-surface risks and disturbances in two locations where recent subsidence has occurred. In these locations, GPR revealed subsurface features to a maximum depth of approximately 4 meters. This investigation included a series of single transects that were used to produce two-dimensional subsurface profiles, and a series of transects to produce a three-dimensional grid that allowed for construction of a cross-sectional model. All data were processed using GSSI Radan 7 software.

GPR data were used to identify near-surface geometries including bedrock margins and areas that were backfilled after mining operations ceased. Fill consists of boulders mixed with sand to clay-size material. Shafts and stoped area margins were found to be discernable by abrupt changes in reflection styles within the GPR profiles. The previously unknown location of the Reservoir Shaft is potentially revealed by development of the 2014 sinkhole. The grid survey of an adjacent area where gradual subsidence has been observed was used to construct a three-dimensional profile, which indicates a relatively wide (approximately 10 meters²) area of fill material. These GPR surveys have been shown to be effective for identifying near-surface risks related to historic mining operations.