GEOMORPHIC SURFACE RECONSTRUCTION OF A GLACIAL STREAM TERRACE USING 3D GPR DATA, HADDAM MEADOWS STATE PARK, HADDAM, CONNECTICUT

Haddam Meadows State Park is situated on the Connecticut River floodplain, northeast of Haddam, Connecticut. The area underwent a multifaceted depositional history during the late Pleistocene and Holocene, resulting in complex glacial and postglacial deposits. As part of an annual Geophysical Field Experience program for summer interns run by the USGS Office of Groundwater, Branch of Geophysics in the spring of 2015, students collected three-dimensional (3D) ground penetrating radar (GPR) on the floodplain bordering the Connecticut River. Data were collected using a broadband antenna with a center frequency of 160 MHz on a 200 x 100 m grid with a 1-m two-dimensional (2D) line spacing. Comparison with previous studies shows three distinct stratigraphic facies imaged by the GPR (1) alluvial overbank sand deposits, representing the modern floodplain, in the upper 2-3 m; (2) a 3-m thick unit composed of coarse sand and gravel interpreted as glacial stream terrace deposits; and (3) an underlying unit consisting of dipping subaqueous glacial lacustrine delta foreset beds.

In this study, we use 3D GPR data processing to remove the overlying modern floodplain deposits in order to reconstruct the geomorphic surface of the glacial stream terrace. Following basic GPR data processing (time zero adjustment, band-pass filtering, and reflection-amplitude gain) the GPR reflection horizon corresponding to this interface was digitized line by line for all 200 individual 2D-line transects. The contact picks from the 2D lines were then interpolated to create a detailed continuous horizon within the 3D GPR data volume. The resulting surface shows an overall northeast dip toward the Connecticut River, and highlights the irregularity of the surface, including paleochannels incised into the stream terrace deposits. This analysis shows how GPR acquisition and 3D processing can be used to efficiently image subsurface geomorphology, providing insights into geologic processes active during the last deglaciation.