BEDROCK TOPOGRAPHY MAPPING USING THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD – CASS COUNTY MICHIGAN

The bedrock topography, bedrock valleys and drift thickness, and were mapped in Cass County Michigan as part of a larger surficial geological mapping project. Due to the limited number of bedrock control points in the county, a Tromino passive seismic instrument was used along with existing oil and gas test wells and other wells to determine the depth to bedrock and the bedrock topography.

This technique uses the horizontal -to-vertical spectral ratio (HVSR) method to record ambient seismic noise with 3-component (two horizontal and one vertical) geophones at a single station. A local HVSR calibration curve and equation created by taking readings at wells of known bedrock depth allows bedrock depth estimations at HVSR reading locations with unknown bedrock depths.

In Cass County the mostly Mississippian and Devonian aged shale bedrock surface ranges from approximately 10 feet below sea level at the bottom of a deep bedrock valley in the NW part of the county to 730 feet above mean sea level.

As series of generally NW trending bedrock valleys occurs mostly in the north half of the county. This bedrock valley system is an extension of a series of deep bedrock valley that runs SE-NW across Van Buren County to the north. One of the main NW trending bedrock valleys underlies a large tunnel valley and an inferred tunnel valley extension.

The glacial deposits in the mapped area average 300 feet thick and range from 106 to 830 feet thick over a deep bedrock valley. The 830 feet of glacial drift is the thickest known in the southern half of the Lower Peninsula of Michigan.

The previously unmapped bedrock valleys are significant because even in a prolific drift aquifer system with significant high-capacity irrigation, in most places only the upper half or less of the glacial deposits has been characterized or are being used for aquifers. In at least one of the bedrock valleys, drilling has confirmed a previously unknown gravel aquifer.