CONTINUING STUDY OF THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD FOR DETERMINING QUATERNARY SEDIMENT THICKNESS IN MINNESOTA

Work continues on using the horizontal-to-vertical-spectral ratio (HVSR) passive seismic method to determine the thickness of Quaternary sediments in Minnesota. The HVSR method is grounded on the primary resonant frequency of shear waves within loose sediments overlying rigid bedrock. This interface ideally produces a prominent peak in the HVSR spectra at the primary resonant frequency. This frequency (f₀) can be related to sediment thickness (z) through the simple relationship

z=af₀^b

where f₀is the estimated primary resonant frequency, and parameters a and b are calibrated empirically from control points representing a wide range of known bedrock depths.. The resulting power-curve relationship is then used to estimate depths at points without control.

As of autumn 2014, a total of 1647 passive seismic stations have been acquired around Minnesota, 527 of which are control points. For glacial sediments, 303 selected control points yield a fit with a and b parameters of 127 and -1.2644, respectively. For late- to post-glacial fluvio-lacustrine sediments along major streams, 37 selected control points yield a fit with a and b parameters of 83 and -1.232, respectively. Usually one of these parameter pairs is suitable for rough estimates of Quaternary sediment thickness, although such estimates may be improved locally. Analysis of fitted data indicates that errors are usually within 25%, which is adequate for many applications.

The HVSR method must be used with appropriate caution. Extreme variations in shear-wave velocities of near-surface materials can complicate or even negate use of calibration curves. Irregular bedrock surfaces can produce weak, multi-peak HVSR spectra that are difficult to interpret. For soft bedrock, HVSR peaks may reflect intra-bedrock features. Strong velocity contrasts within the glacial sequence can produce strong HVSR peaks that mask bedrock signatures. In spite of these limitations, the HVSR method has proved useful to many investigations of Quaternary geology, particularly with the addition of multi-location spectral profiles and cross sections anchored with bedrock control points. HVSR methods provide a suitable and much cheaper alternative to conventional seismic sounding and helps in prioritizing and targeting areas where conventional seismic sounding may be useful.