INVESTIGATION OF SHEAR WAVES

Abstract

A series of measurements was made to investigate some of the fundamental properties of shear waves and to explore the possibility of using horizontally polarized (SH) shear waves for reflection prospecting. A special source was devised to produce a shearing motion which was detectable as far as 400 feet vertically and 1,000 feet horizontally. Direct, refracted, and reflected SH and SV (vertically polarized) shear waves were identified on a series of surface and subsurface recordings. A strong, highly dispersive surface wave, which satisfies the theoretical criteria of Love waves, was also observed. Certain anomalous features of the data which did not conform to the predictions of simple isotropic theory were readily explained by considering the stratified section under observation to be transversely isotropic. It was found that horizontal SH velocity exceeded vertical SH velocity by 100 percent whereas the corresponding compressional wave velocities differed by only 12 percent. SV anisotropy was manifested by a complex variation of velocity at intermediate directions of travel. Other theoretical predictions were confirmed in detail by the experimental data. An evaluation of SH reflection recording was made in four different areas. It was possible at one location, using multiple horizontal geophones and the shear source, to obtain an SH reflection from the base of a thick weathered layer. However, the results in general indicate that the method is not likely to have much practical importance.