Integration of Surface and Borehole Geophysical Methods to Develop a Bedrock Model

Abstract

A multi‐method geophysical survey was performed at a contaminated facility in New Jersey to update the subsurface geological model for the project area, in order to advance the understanding of hydrogeologic conditions and constrain the placement of additional borings. Anomalous saprolite thicknesses, permeable stratigraphic contacts, and overturned stratigraphy introduced geologic complexities inhibiting the progress of the project. More than 16,000 linear feet of geophysical data were collected over a survey area approximately 9 acres in size. GPR was selected as the primary method for modeling the saprolite and bedrock elevations, as well as profiling fill and other soil horizons above the saprolite. Seismic reflection and DC resistivity methods were used to obtain data to confirm saprolite of variable thickness, competent bedrock surfaces, geologic fault structures, and other geophysical anomalies. Seismic and resistivity data were integrated within common depth intervals to emphasize bedrock structures detected by both methods as well as those unique to each method. Resistivity profiles also assisted in mapping the spatial distribution of different geologic units. Borehole geophysical log data, obtained a year earlier, were integrated with the surface seismic data and provided key information for deciphering complex reverse fault structures. The data collected by these three complementary geophysical methods were used to create bedrock and saprolite topographic models. The bedrock models were used to provide a better understanding of the spatial extent of older and younger units and possible structural contacts between them due to reverse faulting.