## Local determination of weak anisotropy parameters from walkaway
VSP qP-wave data in the Java Sea region

**Ellen Gomes**, **Xuyao Zheng**,
**Ivan Psencik**,
**Steve Horne** and **Scott Leaney**
### Summary

We apply the inversion scheme of Zheng and Psencik
(2002) to the walkaway VSP data of Horne and Leaney (2000) collected in
the Java Sea region. The goal is a local determination of parameters of
the medium in a vicinity of a receiver situated in a borehole. The
inversion scheme is based on linearized equations expressing *qP*-wave
slowness and polarization vectors in terms of weak anisotropy (WA) parameters.
It thus represents an alternative approach to Horne and Leaney (2000),
who based their procedure on inversion of the Christoffel equation using
a global optimization method. The presented inversion scheme is independent
of structural complexities in the overburden and of the orientation of the
borehole. No assumption of local homogeneity around the receiver is
necessary. The data used are
vertical components of the slowness and polarization vectors. The inversion
is performed without any assumptions concerning the orientation of these
vectors. The inversion is made (a) assuming arbitrary anisotropy, i.e.,
without any assumptions about symmetry of the medium, (b) assuming
transverse isotropy with a vertical axis of symmetry and (c) assuming
isotropy of the medium. Results of inversions of the raw data are compared
with results in which weighting is used to reduce the effect of outliers
in the data. It is found that the WA parameters *epsilon*_{z},
*epsilon*_{15} and *epsilon*_{35} are considerably more stable than
the parameters *epsilon*_{x} and *delta*_{x}. The latter two parameters
are also found to be strongly correlated. Weaker correlation is also found
between the mentioned two parameters and *epsilon*_{z}. The results of
inversion show clearly that the studied medium is not isotropic. They
also seem to indicate that the studied medium does not possess
the VTI symmetry.

### Keywords

Weak anisotropy, weak anisotropy (WA) parameters, *qP* waves,
slowness vector, polarization vector, local inversion, walkaway VSP

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In: Seismic Waves in Complex 3-D Structures, Report 13,
pp. 53-71, Dep. Geophys., Charles Univ., Prague, 2003.

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