## Uncertainty in local determination of anisotropy
parameters from a walkaway VSP

Ellen S. Gomes,
**Ivan Psencik**,
Greg Ball
### Summary

A walkaway VSP consisting of four walkaway lines at three different
VSP tool depths is analyzed to estimate the uncertainty in the local
determination of anisotropy parameters. The estimated polarizations and
vertical slownesses from wavefield separation for down-going and
up-going compressional waves incident at the eight 3-component geophones
in the VSP tool are used in the analysis. The analysis is performed
assuming isotropy, transverse isotropy with a vertical symmetry axis,
and general anisotropy. For the
walkaway line with the best data quality, the analysis clearly shows
that the shale in which the VSP tool is placed is anisotropic and that
the symmetry axis is inclined from vertical. Nevertheless, the
uncertainty in the estimates of the anisotropy parameters is large. The
most reliably determined parameter is the compressional-wave velocity
along the symmetry axis. The analysis shows the competing effects of
adding the reflected up-going compressional waves in the analysis and
the level of noise in these data.

### Keywords

Weak anisotropy, P waves, slowness vector, polarization
vector, local inversion, walkaway VSP.

### Whole paper

The
image of the paper in GIF 150dpi (610 kB)
is designed for an instant screen preview.

The paper is available in
PostScript (2 347 kB!)
and GZIPped PostScript (403 kB).

In: Seismic Waves in Complex 3-D Structures, Report 17,
pp. 63-77, Dep. Geophys., Charles Univ., Prague, 2007.

SW3D
- main page of consortium ** Seismic Waves in Complex 3-D Structures **.