VOID DETECTION VIA MAGNETOMETRY AT A HISTORICAL SILVER MINE SITE

Silver Mine Park, a municipal park in Pequea, PA was home to active mining for silver and lead for several decades, starting in the 1860s. Although some mine workings are known and mapped, the full extent of mine excavation is not known. In addition, the two dominant host rocks in the area are calcite (Conestoga Formation) and dolomite (Vantage Formation), and karstification has produced several sinkholes in the park in the last two decades.

Because of the potential safety issues associated with subsurface voids, park personnel were interested in delineating potential void locations in active areas of the park, to aid in planning and management. A magnetic survey using a proton precession magnetometer was conducted in the main areas of interest using a 2 m grid spacing. Magnetic field strength data were contoured for visual analysis to identify zones of low field strength possibly indicative of voids.

Despite the complex geology (including faulting, folding, and potential unmined ore deposits) and areas with metal fencing and subsurface pipes, the magnetic survey identified several potential void locations. Although these zones have not been confirmed as voids, and funding for definitive delineation (e.g., boreholes) is unlikely, the locations could be mapped with another geophysical method (e.g., GPR) to support or refute the presence of voids; in the meantime, park personnel can use the potential void locations to maximize the safety of park users.