Utility of the horizontal-to-vertical spectral ratio passive seismic method for estimating thickness of Quaternary sediments in Minnesota and adjacent parts of Wisconsin

Quaternary glacial and fluvial sediments in Minnesota and adjacent areas are a major source for groundwater, and they are important to many geologic investigations. Unfortunately, the thicknesses of these widespread sediments are often poorly known. The horizontal-to-vertical-spectral ratio (HVSR) passive seismic method is being increasingly used in the region to assist in mapping the thickness of these sediments and the topography of the underlying bedrock surface. HVSR results when control sites with known sediment thickness are used to derive calibration curves that in turn are used to estimate sediment thicknesses in areas lacking control. The HVSR method must be used with appropriate caution. Extreme variations in S-wave velocities of near-surface materials can complicate or even negate the use of calibration curves, and very irregular bedrock surfaces can produce weak, multipeak HVSR spectra that are difficult to interpret. Over soft bedrock, HVSR peaks may reflect intrabedrock features, and velocity contrasts within the glacial sequence can overwhelm HVSR bedrock signatures. Nonetheless, the HVSR method has proven a useful tool to investigate Quaternary geology and depth to bedrock in areas of the state where subsurface data are limited. Thickness estimate errors are usually less than 25%, which is adequate for many geologic applications. The addition of multilocation spectral profiles and cross sections anchored with bedrock control points has allowed us to use data with poor signal quality and to achieve good continuity away from control stations. HVSR methods provide a suitable and cheaper alternative to conventional seismic studies, and they help to target sites for Quaternary drilling programs, bedrock topography evaluation, and determining glacial sediment thickness for 3D mapping. In areas where conventional seismic profiling may be necessary, the HVSR method is useful in selecting and prioritizing targets. The HVSR method should have widespread application in the glaciated areas of the north-central mid-continent.