OFR14-01, Evaluation of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for estimating the thickness of Quaternary deposits in Minnesota and adjacent parts of Wisconsin

Abstract

Horizontal to vertical spectral ratios (HVSR) of ambient seismic noise may be used to estimate the thickness of sediment over bedrock, based on empirically-derived, power-curve relationships between sediment thickness and primary resonant frequency of shear-waves. The primary resonant frequency can be deduced from prominent peaks or troughs in the HVSR spectra, provided that the sediment-bedrock interface is reasonably flat, and is associated with a strong acoustic impedance contrast. Several recent geologic investigations in southern Minnesota have provided an opportunity to evaluate the HVSR method as a way to estimate the thickness of Quaternary sediments for a variety of bedrock and sediment conditions. Wherever Quaternary sediments cover the bedrock in Minnesota and adjacent areas, the HVSR method will be a useful supplement to geological and other geophysical investigations, provided that appropriate cautions are heeded. Although, the HVSR method does not match conventional seismic studies in the level of interpretive detail such as modeling a surface, it offers distinct advantages of rapid data collection, much lower equipment and staff costs, ease of data analysis and the large number of samples that can be collected within an area. The HVSR method can also be readily applied in areas of significant cultural noise, where conventional seismic data is difficult or impossible to obtain.