A comparison of different methods for estimating Thomsen's anisotropy parameters

Chunyan Xiao

ABSTRACT

Various anisotropic moveout velocity analysis methods are used to estimate anisotropy parameters in a VTI medium in combination with well-log data. Analyses of four reflection-traveltime inversions in weakly anisotropic media show that inversion accuracy is related to spread length and subsurface anisotropy parameters. Within its own offset range (hyperbolic, shifted hyperbolic, modified three-term Taylor series or Alkhalifah's), the accuracy of estimated δ decreases with offset and the accuracy of estimated ε increases with offset. The smaller the value of (ε-δ), the greater the accuracy of the estimated δ value. The results from the four reflection-traveltime inversions by semblance analysis for synthetic seismic examples demonstrate that in estimating δ, the Alkhalifah's and the shifted hyperbolic estimations are better than the modified three-term Taylor-series method. Only the Alkhalifah's approximation can be used to estimate the anisotropy parameter ε accurately. Hyperbolic estimation is only suitable for estimation of elliptical anisotropy.

Anisotropy parameters were obtained by anisotropic moveout velocity analysis performed on Blackfoot P-wave reflection-seismic data, in combination with sonic-log data. The results show that estimated values of ε and δ seem reasonable only if the time intervals of the layer are larger than about 200 ms. When the lower three layers are combined as one target layer, it exhibits relatively high values of ε and δ.

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