Abstract

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, epsilon₁₅ and epsilon₃₅ 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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