Use of Shear Waves in Seismic Refraction Surveying
ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MS GEOTECHNICAL LAB
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The nearly universal practice in surface seismic refraction surveying for engineering purposes is to use only data obtained from compression waves, or P-waves, which are always the first arrivals and are thus the most easily detected and identified. Most seismic refraction surveys used in geotechnical practice are concerned with identifying depths to various layer boundaries or to the bedrock surface. P-wave velocities are normally obtained in the course of such surveys, but are usually of only secondary interest in seismic analyses. Shear wave, or S-wave, velocities, on the other hand, are of primary interest in engineering practice when they are needed for use as input to seismic analyses of structures. The S-wave velocity of the surface layer is frequently measured with a field survey using a layout similar to that of a refraction survey. However, refracted S-wave signals are not normally used. In principle, S-wave returns obtained by refraction through subsurface strata should be useful for measuring the S-wave velocities of these strata. In practice, however, results are often disappointing. In this report, the current state of the art in generating and discriminating refracted S-waves at the ground surface is described, and some theoretical and practical considerations in the propagation and discrimination of refracted S-waves are discussed. The studies show that it is feasible to use refracted S-waves for the investigation of subsurface strata, at least to limited depths and under favorable conditions.
- Geology, Geochemistry and Mineralogy