DETECTING A SUSQUEHANNA RIVER PALEOCHANNEL ALONG THE NORTH BRANCH NEAR MAINVILLE, PENNSYLVANIA: A MULTI-METHOD GEOPHYSICAL SURVEY TO DETERMINE DEPTH TO BEDROCK

Pennsylvania has experienced many episodes of glaciation in the last two million years. Early Pleistocene glaciers extended far into Pennsylvania, damming, and possibly altering, parts of the Susquehanna River (Ramage et al. 1998). A geologic survey was conducted to find depth to bedrock across a valley adjacent to the North Branch of the Susquehanna River in Mainville, Pennsylvania to determine if this area was once a pathway for the Susquehanna River. Three geophysical methods were used to explore the characteristics of the subsurface along two profiles that run perpendicular to the axis of the valley and determine whether these characteristics indicate the presence of a paleochannel. Profile 1 is located near the topographic high of the valley, and Profile 2 is located at a lower elevation close to the junction of the current path of the river and the potential paleochannel. Variations in the corrected gravity field provided a broad overview of the characteristics of the subsurface due to differences in densities or thicknesses of materials while seismic refraction and electrical resistivity provided a more detailed picture of the subsurface.

Seismic refraction and electrical resistivity data were analyzed to determine the thickness of the subsurface layers as well as the depth to bedrock along the two profiles. Seismic data along the two profiles indicate a shallow interface (3.6 m on Profile 1 and 5.7 m on Profile 2) between slower seismic velocity material (800 m/s at Profile 1 and 400 m/s at Profile 2) overlying a material with seismic velocity of ~1700 m/s for both profiles. However, electrical resistivity data along Profile 2 indicates an interface at 5.7 m between a more resistive (150 to 400 ohm-m) layer overlying a less resistive (90 ohm-m) material. These preliminary results do not seem to indicate bedrock within the upper 10m. Ongoing analysis will use data from all three geophysical techniques to provide a detailed picture of the subsurface across the study area using a noninvasive and inexpensive survey method.