Seismic and Shear Wave

Dynamic loading problems in the offshore environment depend on either estimated or measured values of shear modulus. In practice, in situ determination of shear wave velocity on land has been used as the best approximation to the actual values for laboratory tests on samples (Richart, 1975). The techniques for using these seismic methods and data acquisition techniques to determine shear wave velocity for land-based applications have been well developed. The problem in the marine environment has been to develop methods to determine in situ shear wave velocity measurements both at the seabed surface and at known depths in the sediment column, which can be determined in a cost-effective manner. 

To solve the problem of measuring shear wave velocity in the soil column, a seismic cone penetrometer has been developed. The seismic cone contains a triaxial set of geophones (i.e., detectors) incorporated in a conventional in situ piezocone. It is typically pushed into the soil from the seabed or from the bottom of an advancing borehole. The source is typically a hydraulically driven spring hammer located on the seabed. It is ideally coupled to the sediment surface and preferentially generates horizontally polarized shear waves. It is important to decouple the drill rods and tools from the seismic cone prior to testing because compression wave energy may be transmitted. This allows the full characteristics of the soil in terms of shear modulus to be determined. 

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