HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHODS FOR DETERMINING QUATERNARY SEDIMENT THICKNESS AND BEDROCK ELEVATION IN MINNESOTA: AN UPDATE

The horizontal-to-vertical-spectral ratio (HVSR) passive seismic method continues to be used in Minnesota to help map bedrock topography and thickness of Quaternary sediments. During the last year 551 passive seismic stations were acquired in several widespread areas of Minnesota, bringing the Minnesota total to 2243 stations. Most of this data has been acquired through the County Geologic Atlas (CGA) program at the Minnesota Geological Survey (MGS). We have made increased use of multi-station spectral cross-sections that are converted into generalized elevation sections. This conversion uses a power-law calibration that is based on observed HVSR results at 303 widely distributed control points where bedrock depth and elevation are known. More refined sections may become possible using locally derived calibrations, but the generalized approach produces useful sections where lateral correlation of HVSR features can be made within a framework of geologic control.

HVSR soundings have been used, in conjunction with existing geologic control, to map the trace and depth extent of buried bedrock valleys in Dodge, Hennepin, Isanti and St. Louis Counties. These areas, which include a variety of bedrock and glacial sediment conditions, yielded good HVSR results and no follow-up seismic refraction soundings were deemed necessary within the scope and scale of the CGA projects.

The use of multi-station cross-sections was especially helpful for HVSR interpretation in Becker, Cass, Hubbard, and Wadena Counties of north-central Minnesota, where both the HVSR results and the geologic control are not optimal. Seismic refraction profiling was used to test interpretations from low-quality HVSR signals. In several regions weak but perceptible HVSR peaks occur at low frequencies of 0.5-0.8 Hz, equating to depths of about 550-1000 ft. These signals are interpreted as depressions that locally contain Cretaceous strata. Neither the HVSR results nor the refraction profiling could discriminate the soft Cretaceous rocks from overlying, clay-rich, glacial sediments, and the low frequency signals are interpreted to be Precambrian rock underlying the Cretaceous strata. We found that the HVSR soundings saved considerable time and expense in prioritizing targets for both refraction soundings and Quaternary drilling.