USING ELECTRIC RESISTIVITY TOMOGRAPHY TO EXPLORE SUBSURFACE FEATURES OF OLD MIDDLE CREEK LAKEBED, SNYDER COUNTY, PA

Middle Creek Lake, a 1-mile-long reservoir in Selinsgrove, Pennsylvania, was retained behind a dam for over 150 years, accumulating sediment and organic matter. These deposits rest atop the Trimmers Rock Formation, a Devonian-aged sequence of shale, siltstone, and fine-grained sandstone characteristic of central Pennsylvania. This study focuses on identifying the water table and the regional groundwater flow, determining the depth to bedrock, and confirming the presence of a thrust fault parallel to the northeast-southwest trending ridges and valleys of the region. A 238-meter-long survey was conducted using roll-along Dipole-Dipole Electrical Resistivity Tomography (ERT) with one-meter electrode spacing, extending from the southern boundary of the basin to Middle Creek in the north edge of the basin. Initial results revealed an upper layer of clay-rich sediments mixed with sand and gravel, ranging from 1 to 3 meters in thickness across the profile. Anomalous resistivity in the central transect suggests a possible bedrock thrust fault, marked by a sudden resistivity contrast within the Trimmers Rock Formation. This feature aligns with the compressional tectonic environment and may be part of the Appalachian fold-and-thrust belt system. Further surveys with larger electrode spacing are underway to explore deeper subsurface features.