Extracting shear wave polarizations from different source orientations: Synthetic modelling

Abstract

Shear wave splitting, diagnostic of some form of seismic anisotropy, has been identified in the past, principally by examining polarization diagrams and by rotating the axes of recordings to separate the shear wave polarizations. We introduce a new technique for identifying polarizations of shear waves when the data has been recorded with more than one source orientation. The technique, suitable for vertical seismic profiles (VSPs) or reflection data, averages over an entire shear wavetrain, eliminating source related effects while retaining the anisotropy related polarizations. In shear wave reflection experiments and shear wave VSPs the distribution of displacement directions depends on the elastic properties of the medium (anisotropy), the source orientation, and the effects of scattering. We linearly combine seismograms from different source polarizations in order to simulate seismograms for any source polarization. This allows us to show the distribution of displacement directions as a function of source polarization in a high‐relief display. The diagrams show characteristic features in the presence of anisotropy which could help to determine the polarizations of the two split shear waves in difficult circumstances. The techniques are tested with synthetic seismograms.