A WINDOW INTO THE SUBSURFACE OF THE CUMBERLAND VALLEY OF CENTRAL PENNSYLVANIA: GROUND PENETRATING RADAR (GPR) INVESTIGATION OF KARST FEATURES IN THE SHIPPENSBURG VALLEY QUARRY

Karst features are developed in carbonate bedrock as a result of chemical weathering. Such processes form sink holes, caves, enlarged fractures, solution cavities, bedrock pinnacles, etc., all of which can be hazardous. This study GPR in an attempt to identify the signature of karst features characteristic of the fold and thrust belt as developed in the Cumberland Valley of Central PA. As GPR is a non-invasive technique that detects anomalies and discontinuities in the subsurface, the signature of anomalies produced by GPR and visualized in radargrams are 2D abstractions of the real 3D morphologies of subsurface features. In most scenarios, interpretation and “truthing” of radargrams is dependent on the experience of the geologist and can be subject to interpretation. Unfortunately, the weathering of limestone often leads to a thick, clayey, conductive soil that is difficult for GPR to penetrate. This often obscures deeper features (see studies such as Doolittle 1998, Hoover 2003). This can limit accurate characterization of the features below the surface, and prevent identification of potential geologic hazards.

This study looks specifically at an excavated and back-filled karst landscape developed in a quarry NE of Shippensburg, Pennsylvania in order to gain insights in order to more accurately interpret GPR returns elsewhere in the valley. The exposure was made by the quarry operator to produce a cleaner, soil-free limestone prior to blasting. Once cleared of soil, the pinnacled limestone was visually inspected and this allowed for characterization of the landform morphology. The area was then backfilled with crushed-stone before GPR (Mala X3M, 100 Mhz and 250 Mhz) surveys were taken. Results show that steeply-dipping bedrock returns are the most subtle features identified, as are orthogonal joint sets. Prominent features include dolomitized fractures, and a series of reflectors that appear to be steeply-dipping void spaces formed near the top of bedding planes of purer limestone beds. Some of these appear to be partially filled with recrystallized minerals. The morphology of the highly-pinnacled limestone observed prior to backfilling was detected where pinnacles came close to the surface, but were less distinct where crevasses extended more than a few meters below the surface.