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