GROUND-BASED GEOPHYSICS TO COMPLEMENT AIRBORNE TIME-DOMAIN ELECTROMAGNETICS FOR AQUIFER MAPPING

Ground-based geophysics methods were used to constrain helicopter time-domain electromagnetic (HTEM) data in east-central Illinois. HTEM profiles were flown to characterize high-resolution geology and variability within a regional sand and gravel aquifer system. We deployed 2-D electrical resistivity tomography (ERT), 2-D seismic reflection and passive-seismic (horizontal-to-vertical spectral ratio (HVSR)) methods across the study area to assess HTEM interpretations of bedrock geology and topography, glacial sediment lithology and variability, and lithostratigraphic frameworks. At some locations, ground-based methods were coupled with one another to target geologic frameworks and stratigraphy most effectively. These results helped resolve variability of aquifer geometry and character and helped improve interpretations of HTEM profiles in areas with sparse subsurface data.