EVALUATION OF THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD FOR BEDROCK DEPTH DETERMINATION IN MICHIGAN

The horizontal-to-vertical spectral ratio (HVSR) passive seismic geophysical method allows one to determine glacial drift thickness (bedrock depth) if there are strong enough acoustic impedance contrasts between the drift and underlying bedrock. The HVSR method uses naturally occurring seismic noise (wind, waves, flowing water, distant weather) and man-made noise (vehicles, industry) as an energy source. A single station, three-component seismometer (two horizontal and one vertical) is used to record the ambient seismic noise.

Michigan has the thickest drift on land in North America, but the thickness is quite variable and the underlying bedrock surface is very irregular. Many places have a poor distribution or quality of bedrock depth control points. The HVSR method has been evaluated to Michigan for the last two years. HVSR calibration readings at wells of known bedrock depth have been gathered in several areas of the state. The calibration readings are compiled for a given area with the same geologic setting resulting in local and regional calibration curves and curve equations. HVSR exploration readings are taken at locations of unknown bedrock depths and the data inserted into the local calibration equation solving for bedrock depth. The HVSR method has been successfully used in several parts of the state to determine bedrock depths, map bedrock topography, fill in data gaps, to confirm or deny anomalous bedrock depths, and to better define bedrock valleys, highs and scarps.

The HVSR method has many advantages including low cost, ease of use, one man operation, single station, portability, noninvasive, quick, minimal data processing, its specificity to a single interface (bedrock surface) and its ability to be used in culturally noisy areas. The HVSR technique is currently being used in geological mapping, groundwater investigations, and mineral exploration. It has great potential in geotechnical and engineering investigations and utility excavations. Additionally it can be used as an independent depth calibration for modeling with other geophysical survey methods. The HVSR technique may aid in petroleum exploration by supplying an independent bedrock elevation profile along seismic reflection lines providing higher quality bedrock elevations to assist in traditional static corrections.