Inversion of P-wave NMO ellipses
Published:January 01, 2011
In this chapter, the formalism introduced in Chapter 1 is used to develop inversion and processing techniques for conventional-spread P-wave data from azimuthally anisotropic media. Stacking-velocity analysis for wide-azimuth 3D surveys often ignores the azimuthal dependence of normal moveout from horizontal reflectors, which may lead to distortions in the processing results (Lynn et al., 1996). A single value of stacking (NMO) velocity applied to the whole CMP gather causes underestimation of Vnmo for source-receiver azimuths near the “fast” direction of the NMO ellipse and overestimation near the “slow” direction. Hence, mixing of different azimuths impairs the performance of the moveout correction and, therefore, the quality of the stacked section. These distortions can be avoided by reconstructing the best-fit NMO ellipse and applying the correct stacking velocity for all azimuthal directions.
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Seismology of Azimuthally Anisotropic Media and Seismic Fracture Characterization
Traveltimes of reflected waves (reflection moveout) in heterogeneous anisotropic media are usually modeled by multioffset and multiazimuth ray tracing (e.g., Gajewski and Pšenčĺk, 1987). Whereas anisotropic ray-tracing codes are sufficiently fast for forward modeling, their application in moveout inversion requires repeated generation of azimuthally-dependent traveltimes around many common-midpoint (CMP) locations, which makes the inversion procedure extremely time-consuming. Also, purely numerical solutions do not give insight into the influence of anisotropy on reflection traveltimes.
This chapter is devoted to analytic treatment of conventional-spread reflection moveout in anisotropic media. For models with moderate structural complexity and spreadlength-to-depth ratios close to unity, traveltimes in CMP geometry are welldescribed by normal-moveout (NMO) velocity defined in the zero-spread limit (Tsvankin and Thomsen, 1994; Tsvankin, 2005). Even in the presence of nonhyperbolic moveout, NMO velocity (Vnmo) is still responsible for the most stable, conventionaloffset portion of the moveout curve. The description of Vnmo given here provides an analytic basis for moveout inversion, helps evaluate the contribution of the anisotropy parameters to reflection traveltimes, and leads to a significant increase in the efficiency of traveltime modeling/inversion methods.