GEOPHYSICAL DATA ACQUISITION METHODS FOR ESTIMATING DEPTH TO BEDROCK: ELECTRICAL RESISTIVITY IMAGING AND THE PASSIVE SEISMIC METHOD

Knowledge of the depth to bedrock is essential for both rural and urban land use decisions. For example, depth to bedrock is referenced more than 600 times in Wisconsin’s Natural Resources code. At the Wisconsin Geological and Natural History Survey, two geophysical methods are commonly used to determine depth to bedrock: Electrical Resistivity Imaging (ERI) and the Horizontal-to-Vertical Spectral Ratio (HVSR) acquired by the Passive Seismic technique.

These geophysical methods rely on physical differences between the bedrock and overlying sediment. ERI utilizes the contrast between less resistive overlying sediment and the commonly more resistive bedrock to create a profile of electrical resistivity with depth. We compared our ERI output with rock quarry walls and observed that prominent lateral and vertical variations in both bedrock and sediment character could be seen.

With HVSR, low frequency passive seismic “noise” is measured for ~20 minutes at a location in order to determine the resonant frequency of the overlying sediment. An empirical equation with constants that slightly vary with different sediment relates the acquired resonant frequency to sediment thickness (depth to bedrock). We ground-truthed several different Wisconsin HVSR data sets with depth to bedrock values determined from drilling.

ERI provides a profile of the depth to bedrock as well as information about lateral variations in the bedrock. We expect this method to gain use in areas where we suspect karst features such as sinkholes or fracture zones to be present. We have used HVSR to augment depth to bedrock data for several groundwater study areas, and provide better constrained sediment thicknesses in the Wisconsin River valley. Both methods are fast, low cost, non-invasive, and have provided reliable estimates of depths to bedrock in locations where the depths were previously unknown.