Local inversion for anisotropy from qP-wave slowness and polarization vectors

Ivan Psencik , Xuyao Zheng , Steve Horne & Scott Leaney


Data from multi-azimuthal walkaway VSPs are very useful for detection and determination of local anisotropy at individual receivers in a borehole. Various inversion techniques have been proposed. They differ by assumptions about properties of the medium surrounding receivers, by types of waves involved and by computational methods used for the inversion. We present a technique based on the first-order perturbation theory for anisotropic media. Basic equations of our technique are applicable to the data of qP waves propagating in laterally inhomogeneous, weakly anisotropic media of arbitrary symmetry. The data are components of polarization and slowness vectors of qP waves generated at surface sources and recorded at a given receiver. For each source, we thus have an equation. The SVD algorithm is used to solve an overdetermined system of equations for weak anisotropy (WA) parameters. From them qP-wave phase velocity can be reconstructed. We test the technique on a set of synthetic multi-azimuthal walkaway VSP data and on a set of walkaway VSP data measured in the Java Sea region. In the former case, true anisotropy of the model is known, in the latter case, the results of inversion can be compared with results of other techniques. In both cases good agreement is found.

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Expanded Abstracts of 7th Int. Congress of SBGf (Salvador), Brazilian Geophysical Society, 2001, submitted.
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