Polarization rotation upon reflection of direct shear waves in purely isotropic media
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The reflection process alters the polarization of a direct shear wave in a purely isotropic medium. I show that where the alteration occurs is governed by the shear reflection amplitude versus incidence angle for the reflecting interface. Specifically, I address the zero crossing in the SV--SV component's reflectivity (where the amplitude changes sign), and how the zero crossing governs the alteration's location. The severity of the deviation of the polarization of the source from the observed polarization of the reflection varies with the difference in azimuth between source position and the receiver location. I develop a correction based on the observed shear amplitude versus incidence angle, which corrects to the equivalent of nearangle polarization for mid and large incidence angle (large offset) data. When the exact parameters from the model data are used to create the correction, the reflected shear polarization matches the source polarization out to moderate offsets. A universal, model-independent correction does not perform as well as the exact correction, but it may be suitable for many applications where knowledge of regional geology is not complete. I also show preliminary results from applying the correction to anisotropic model data. The correction is most effective for small percentages of anisotropy (<5%), after which the anisotropy signature of the observed polarization dominates over the isotropic reflection distortion.