SHALLOW GEOPHYSICAL ASSESSMENT OF TWO CAVE SITES IN THE CAMBRIAN-ORDOVICIAN LIMESTONES OF CUMBERLAND VALLEY OF SOUTH-CENTRAL PENNSYLVANIA: RESULTS OF AN EMBEDDED STUDENT MAPPING ASSIGNMENT FROM THE PASSHE GEOLOGY FIELD COURSE

In karst environments, solution cavities, sinkholes, and caves are common, and can result in environmental hazards. Throughout Pennsylvania and elsewhere, construction sites underlain by karst features may be delayed or made significantly more expensive due to sinkhole prevalence. The hazards posed by subsurface karst features can be greatly minimized by proactive mapping and identification of vulnerable locations to guide development projects. Geophysical methods provide an efficient, cost-effective, and environmentally friendly means to reasonably detect and map subsurface karst features. This study addressed two goals: 1) to use geophysical technology and map two known cavern sites in the Cambro-Ordovician limestones of the Cumberland Valley of south-central Pennsylvania; and 2) to support training and skill development of future geologists during part of a 5-week summer geology field camp.

Two geophysical methods, ground penetrating radar (GPR) and magnetometry, were used for the study. The two cavern sites surveyed, the Black Coffey Cavern (BCC) and the Carnegie Cave (CC) are prominent sites that were mapped in the subsurface in the 1950s and have “Smeltzer” maps showing substantive cavern networks especially at the BCC. At both sites, the surrounding landscapes are notably pockmarked with closed depressions, fluted landforms, grikes, and disappearing streams. While the cavern networks exist on maps, there is no information for either site on how far beneath the land surface the cave branches are located. Thus, the specific goal of this geophysical survey was to determine the architecture and depth of cave branches at each site. An X3M Mala GPR system, utilizing the 100 Mhz and the 250 Mhz antennas was used alongside a Geometrics G-858 dual-sensor Magnetometer (arranged for gradiometric survey) to collect data at each site. Both instruments produced anomalies consistent with those for known caves elsewhere, successfully addressing the first study goal. The second goal of this course was fulfilled by providing students with the time to explore BCC to gain first-hand knowledge of the architecture of cave passages. After learning about cave systems underground, students were given the opportunity to use their knowledge of geology and geophysical tools to engage high-tech methods to image the subsurface and ground truth.