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