PASSIVE SEISMIC METHODS TO REFINE BEDROCK VALLEY GEOMETRIES IN NORTHERN ILLINOIS

Passive-seismic methods (horizontal to vertical spectral ratio, HVSR) in northern Illinois has improved the understanding of bedrock valley geometries and their implications for regional aquifer connections. Deeply-incised bedrock valleys in north-central Illinois are filled with glacial sediments that are often largely composed of coarse-grained outwash deposits. These deposits likely provide pathways for groundwater connection to bedrock aquifers that subcrop in the bedrock valleys. Detailed geometries of these valleys and their tributaries are limited by sparse borehole data. We collected over 75 HVSR calibration points within various geologic settings to optimize data processing and interpretation. We also collected multi-kilometer transects of high-resolution HVSR data that extended across a regional bedrock valley and its tributaries. The results test previous interpretations of the bedrock topography and help develop new frameworks of local and regional geology.