Investigating coarse-grained glacio-fluvial outwash with Ground Penetrating Radar (GPR), Harpursville, Susquehanna Valley, New York

The relationship between Ground Penetrating Radar (GPR), Earth Resistivity Ground Imaging (ERGI) and sedimentary facies was examined in coarse-grained glacio-fluvial outwash exposed in two quarries in the Susquehanna River Valley between Harpursville and Windsor, NY. GPR profiles, ERGI profiles and outcrop studies were conducted along exposed quarry faces. GPR Common Offset Profiles (COP) were taken with 110, 225, and 450 MHz antennae along the top of quarry faces, and processed with Seismic Unix software that was modified for application at the scale of GPR-reflections. Both GPR and ERGI were able to image large-scale geometries of the deposits and GPR was able to image reflections that correspond to large-scale inclined (cross) strata. Both geophysical methods were not able to image all scales of sedimentary stratification, large, medium and small, needed to adequately interpret depositional environments. Outcrop studies combined with GPR and ERGI reveal deposits consisting of multiple superimposed scales of stratification relating to the migration of bars, dunes bedload sheets and ripples. Geometry, texture and orientation of sedimentary structures indicate the deposits were formed in a braided river and Gilbert-type deltaic environment, both common in proglacial valley deposits. The success of GPR and ERGI investigations in delineating the sedimentary structure of silt to cobble sized outwash sediments strongly depends on the electrical contrasts of sedimentary structures. Specifically, the water content of the deposits, the amount of silt, and the presence of point scatterers within the deposits controls resolution attained in a GPR or ERGI investigation.