CHARACTERIZATION OF THE TROY VALLEY BEDROCK TOPOGRAPHY USING PASSIVE SEISMIC DATA

The Troy Bedrock Valley in Northeastern Illinois is one of the most deeply-incised bedrock valleys in Illinois. This valley is steeply incised into Ordovician-Cambrian bedrock and infilled with up to 150 meters of glacial sediments. Both the bedrock and glacial sediments are critical aquifer resources for local residents and municipalities. Improving our understanding of the bedrock valley topography will have important implications for understanding groundwater recharge to the Cambrian bedrock aquifers, which are regionally overused. Our research area is contained in north central Illinois with several data points in southern Wisconsin.

Using Tromino seismometers, 173 passive seismic soundings were systematically collected in the bedrock valley. The vertical and horizontal components of this seismic data were processed using Horizontal-Vertical Spectral Ratio (HVSR) methods. Data were processed using Grilla software to obtain peak frequency values. These values were input into a regression equation based on known values of bedrock depth from regional control points to estimate depth to bedrock at each point.

These estimated bedrock depths were compared to bedrock valley surfaces previously interpreted from water-well records. In many locations within the main Troy Bedrock Valley, our data were nearly identical to those from previous interpretations. However, sometimes bedrock depths from our data differed from those interpretations by up to 300 feet, particularly in the tributary valleys. Furthermore, our data give insights into new valley geometries and tributaries that may have been previously overlooked.

Our results have implications towards how the geometry of incised bedrock valleys controls recharge to shallow bedrock aquifers and, through improved flow modeling, can ultimately help decision-makers achieve short- and long-term goals for water resource use and management.