Local determination of weak anisotropy parameters from a walkaway VSP data

Xuyao Zheng , Ivan Psencik , Steve Horne & Scott Leaney


We apply the inversion scheme of Zheng and Psencik (2001) to the walkaway VSP data. The most important difference from the application in Zheng and Psencik (2001) is that the data are collected along a single profile so that we are able to describe anisotropy only in the vertical plane containing the profile. Another important aspect is that we know only vertical components of polarization vectors and we have to assume that the vectors are confined to the vertical plane containing the profile. We test first the inversion scheme on synthetic data. We consider two models of transversely isotropic (TI) medium with axis of symmetry deviating from vertical. The qP-wave anisotropy of the models is 8% and 16%. We show that the vertical sections of qP-wave phase velocity can be recovered very reliably for both anisotropies even if 10% noise is added to data. Then we apply the inversion scheme to the walkaway VSP data of Horne and Leaney (2000) collected in the Java Sea region. The data are inverted first without making any assumption about the medium, then assuming that the medium is transversely isotropic with vertical axis of symmetry (VTI), i.e., with the assumption also made by Horne and Leaney (2000) in their inversion. In both cases anisotropy of slightly lower strength is obtained than by Horne and Leaney (2000). In the first case, the recovered medium is close to TI but its axis is tilted.

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In: Seismic Waves in Complex 3-D Structures, Report 11, pp. 43-68, Dep. Geophys., Charles Univ., Prague, 2001.
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