An algorithm for local evaluation of weak anisotropy (WA) parameters from measurements of slowness vector components and/or of particle motions of qS waves at individual receivers in a borehole in a multi-azimuthal multiple-source offset VSP experiment is proposed. The formulae are derived under assumption of weak but arbitrary anisotropy of the medium and under assumption that the two qS waves can be recorded separately. In contrast to qP-wave inversion, in which knowledge of a single component of the slowness vector is sufficient for performing an inversion in a medium with an arbitrary overburden, when using qS waves, two components of the slowness vector or an additional information must be available. On the other hand, use of qS-wave data alone or together with qP-wave data may lead to the determination of complete set of 21 WA parameters specifying the medium. General inversion equations are specified for the case of a walkaway experiment. Preliminary results of application of these equations to the walkaway VSP data collected in the Java Sea region are presented.
Local inversion, qS waves, slowness vector, polarization vector, weak anisotropy, walkaway VSP
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