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

**Ellen N.S. Gomes** ** , **
**Xuyao Zheng** ** , **
**Ivan Psencik** ** , **
**Steve Horne** ** & **
**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 surrounding the borehole receiver
array. 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. The inversion formula is
local, and has therefore potential to separate
effects of anisotropy from effects of
inhomogeneity. The data used are components
of the slowness vector along the receiver
array and polarization vectors. The inversion
is performed without any assumptions
concerning the remaining components of the
slowness vector. 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.
Inverted are the raw data as well as data,
in which weighting is used to reduce the
effect of outliers. 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.

### Whole paper

The reprint is available in
PDF (372 kB).

*Stud. geophys. geod.*, **48** (2004), 215-231.

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