Integrating shear-wave reflection and resistivity profiling to improve subsurface characterization of glacial sediments

In Illinois, both high resolution seismic reflection and 2-D resistivity profiling have become fairly common tools for mapping glacial sediments. Over the past several years we have been using the two methods in combination to improve the characterization of these sediments and to refine geologic and hydrogeologic models. We demonstrate our approach with several case studies where high-resolution shear (S)-wave reflection and electrical resistivity profiling surveys were acquired at the same locations in northern and central Illinois. The two geophysical methods measure the mechanical and the electrical properties of these sediments in the subsurface which together improve our understanding of the sedimentary sequences and the underlying bedrock surface. We present results from measurements made along co-located geophysical lines which vary in length from 400 m to greater than 2300 m. Typically, the S-wave reflection profiles have greater vertical resolution and provide better imaging of depositional surfaces than the corresponding resistivity profiles. On the other hand, the resistivity profiles better express the horizontal variability within individual units compared to the reflection profiles. The reflection profiles more reliably image the bedrock surface, whereas the resistivity profiles are more sensitive to variations in moisture content, mineral content and cementation within the glacial sediments. Together, the overall geometry of depositional packages can be inferred from reflection surfaces, while compositional variations within those packages can be inferred from variations primarily in the resistivity data and secondarily in the seismic data. Geophysical and geological interpretations are constrained by core data and downhole geophysical data from boreholes located on or near the geophysical profiles.