Correction for distortion in polarization of reflected shear-waves in isotropic and anisotropic media
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The progressive growth of onshore shale production (both gas and liquids) to replace depleting and aging oil fields may benefit from the use of surface seismic shear wave data analysis for full characterization of shale reservoir properties and lead to optimum development of these resources. This includes descriptions of azimuthal anisotropy (HTI - transverse isotropy with a horizontal symmetry axis) for characterization of fractures and internal fracture systems. The objective of this study is to document a predicted distortion in polarization of propagating seismic shear waves upon reflection at a subsurface interface and to propose a correction to this distortion. The polarization distortion occurs even in wholly isotropic media. This correction is based on an understanding of shear amplitude behavior as a function of the reflection incidence angle, particularly differences in the reflection angle relation for different shear components. This study includes a demonstration of the efficacy of the proposed correction by applying it to simulated and real direct shear-wave source data. Such corrections should result in a minimized polarization distortion in the reflection process. The apparent consistency of a null value (zero crossing) of the SV-SV reflectivity (near 20-24 degrees) for common density and velocity contrasts as well as the remarkably regular behavior of the SV-SV and SH-SH reflectivity curves following a linear relation in sin2 and tan2 of the incidence angle and offers the opportunity for a simple and stable correction with minimal sensitivity to detailed knowledge of contrasts in velocity and density. The only independent information required for the correction is the angle of incidence where the SV-SV and SH-SH reflections vanish and the range of these angles is typically quite limited. Some key questions were addressed in gaining an understanding of shear wave polarization distortion upon reflection for varying model data: 1) how do we address reflected polarization distortion for purely isotropic medium for varying incidence angles? 2) How do we apply this correction for an isotropic and anisotropic media for both simulated and actual field data 3) How do we address applications to real data and how distorted amplitudes can be corrected to identify actual subsurface HTI anisotropy. Significantly, the polarization distortion correction is implemented as a simple extension of the established Alford rotation for normal incidence shear reflections of varying polarization. This extension leads to the improved analysis of direct shear-source 3D data with inherently distorted polarization. Thus, analysis may be applied to estimate HTI anisotropy previously not realizable in finite offset data subject to polarization distortion. Example applications to actual field data are included. Note that the polarization correction does remove the AVO effects often exploited in analysis of P-P data where polarization is not an issue that is, the AVO amplitude effect is essentially removed from the SV-SV and SH-SH oriented direct shear-wave profiles, which permits proper analysis of the polarization. Further, additional analysis of the polarization correction on field data with documented anisotropy will be required to fully develop the usefulness of this proposed correction.