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Abstract

The results of the previous chapters clearly demonstrate that anisotropy in general and transverse isotropy in particular have a substantial influence on the traveltimes of reflected waves and their normal-moveout velocities. Because reflection moveout is the main source of information for velocity analysis, conventional (i.e., those based on the assumption of isotropy) algorithms are bound to produce distorted velocity models and images in the presence of anisotropy. For example, as shown in chapter 3, the difference between stacking (moveout) and vertical velocities in anisotropic media leads to misties in time-to-depth conversion and the wrong depth scale of seismic sections. In addition, anisotropy causes serious difficulties in imaging of dipping reflectors, such as fault planes (chapter 6; also, see Lynn et al., 1991, and examples below).

Once the importance of anisotropy in seismic processing is accepted, two further impediments to taking its presence into account must be overcome. First, processing algorithms that include anisotropy are more complex than those that ignore it. Second, estimating the anisotropy parameters required by these algorithms is a highly challenging task. The second problem has always seemed to be especially intimidating for exploration seismologists.

Even for the relatively simple VTI model treated in this chapter, P-wave propagation is governed by four stiffness coefficients or, alternatively, four Thomsen (1986) parameters (the vertical velocities VP0 and VS0 and the anisotropy coefficients ∊ and δ). This would seem to imply that velocity analysis for VTI media will not only be complicated in practice,

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