Microtremor-based determination of shear wave velocity of soft soils using the SPAC method

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Stephenson, W.R.; Barker, P.R.; Beetham, R.D. 2011 Microtremor-based determination of shear wave velocity of soft soils using the SPAC method. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/73 62 p.

Abstract: The SPAC method as currently implemented at GNS can provide good estimates of the thickness and shear wave velocity of a well-defined near-surface layer of soil, together with a value of Vs30. It uses an equilateral triangle of three-component geophones with each side having a length of a few tens of metres, recording microtremors for a period of half an hour. The surface layer must be well-extended, and have a strong velocity contrast with the underlying material. By evaluating the Nakamura ratio (which is available from SPAC data that have been recorded using three-component geophones) and incorporating it in the analysis, a good depth-to-rock value may be obtained in favourable cases. Shear-wave velocity values for deeper materials can be improved by employing two triangular arrays of slightly different aperture, simultaneously. More development is needed to make this approach routine. In the event that a SPAC coherency interpretation is not clear, it would be well to try another technique – ReMi, SCPT then VSP (with a cased hole), in order of preference. The presence of even a small amount of incoherent noise makes the inversion of coherency to dispersion inaccurate. Improving the inversion of coherency data to layer properties is the area most likely to yield advances for a given effort. (auth)