PASSIVE SEISMIC PROFILES TO REFINE DETAILED BEDROCK-VALLEY GEOMETRIES

The use of 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 recharge to bedrock aquifers that subcrop in the deepest portions of bedrock-valley incision. Regional bedrock-valley locations are well understood, but interpretations of their detailed geometries are limited by sparse borehole data. We collected multi-kilometer transects of high-resolution HVSR data that extended across a regional bedrock valley and its tributaries (measurements approximately every 300 meters). The data have tested previous interpretations of the bedrock topography and suggest differing degrees of incision depth and breadth, which have important implications for the degree of contact between shallow outwash deposits and bedrock aquifer rocks.