GEOPHYSICAL MAPPING OF BURIED PALEOCHANNELS AND WETLANDS HYDROSTRATIGRAPHY ALONG THE SUSQUEHANNA RIVER, NORTHUMBERLAND COUNTY, PENNSYLVANIA

A series of three-dimensional seismic, ground-penetrating radar (GPR) and electromagnetic (EM) data sets were acquired over a 1.2 km² palustrine wetland site in Northumberland County, Pennsylvania. The geophysical survey region is 900 m by 600 m and underlain by 3 to 10 m thick glaciofluvial outwash sediments deposited by the Susquehanna River. The goal of the surveys is to image the complex fluvial (cobble and sand) stratigraphy of the surficial aquifer and determine the hydrogeomorphic setting of the wetlands. The geophysical images and GPR data sets can be used to construct 3D models of the sedimentary architecture and help constrain fine-scale models of the hydrologic and geophysical parameter at the site.

The GPR data sets were acquired using 200 MHz, 100 MHz and 35 Hz antennas. Depth of penetration ranges from 2 m for the 200 MHz data to 15 m for the 35 MHz data. Well logs from test borings and groundwater monitoring wells were used to ground-truth the data. Processing significantly improves the reliability and interpretability of the images, especially in the fine-grained, organic-rich wetland sediments.

The data sets reveal the wetland are situated above a buried strath, or rock-cut, terrace, whose scarp parallels the Susquehanna River for over a kilometer. The wetlands are underlain by paleochannel backfill sediments, consisting a coarse lag deposits overlain by 2.5 to 3m of interbedded silty-sand, clay, and peat deposits. Time slices and vertical cuts through the GPR data volumes reveal the shape and orientation of the surrounding braided stream outwash deposits and enable one to accurately locate important hydrostratigraphic boundaries.