## Local determination of weak anisotropy parameters
from a walkaway VSP data

**Xuyao Zheng** **,**
**Ivan Psencik** **,**
**Steve Horne** **&**
**Scott Leaney**
### Summary

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.

### Whole paper

The paper is available in
PostScript (1277 kB !)
and GZIPped PostScript (309 kB).

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

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