APPLICATION OF THE H/V PASSIVE SEISMIC METHOD IN MINNESOTA: SOME PRELIMINARY INSIGHTS

Determining the thickness of glacial deposits in large areas of Minnesota where these sediments are over 100 meters thick and few drill holes penetrate to bedrock requires a feasible geophysical method . Conventional seismic soundings are slow and costly, but recent technological advances in seismometers has created an opportunity for seismic analysis known as the horizontal-to-vertical -spectral ratio method (HVSR or H/V).

Developed for seismic risk studies in Japan and Europe, this method relies on ambient (passive) seismic noise, and has been adapted to estimating thickness of unconsolidated deposits.

The H/V method is used to estimate the primary resonant frequency of unconsolidated overburden. If the acoustic impedance (density*seismic velocity) at the overburden-bedrock contact differs by a factor of at least 2, the thickness (Z) of the unconsolidated materials can be estimated by the relationship:

Z=af₀^b

f₀ is the primary resonant frequency (shear wave) of the unconsolidated sequence, and a and b are parameters empirically determined for a given region.

In Minnesota, H/V seismic data have been acquired from over 180 sites, with encouraging results. Analysis of data from the 34 best sites with bedrock control from drill holes, has resulted in parameters for the above equation of a=130.96 and b=-1.3304). Estimates of glacial overburden thickness using these parameters generally have errors of 10-25% over depths ranging from a few meters to greater than 150 meters when compared to reference drill holes.

Like all geophysical methods the H/V method cannot be expected to work well in all situations. For example, poor H/V results are sometimes obtained where thick regolith and unusually dense tills occur. In addition, new parameters will probably be necessary for some situations. Although, the H/V method does not match seismic refraction profiling for accuracy and derivative information, the advantages of passive seismic for determining depth to bedrock are rapid data collection, much lower equipment and field costs, relative ease of data analysis and large number of samples collected. In addition, the rougher estimates from H/V analysis are quite acceptable for many applications. The H/V method is proving to be a useful tool for geologic investigations in glaciated terrain.