Prestack amplitude analysis of wide-azimuth data
Published:January 01, 2011
In Chapters 2, 3, 5, and 6 we discussed moveout inversion of wide-azimuth reflection data for the parameters of transversely isotropic, orthorhombic, and monoclinic media. Essential information for anisotropic parameter estimation and fracture characterization can also be obtained from azimuthally dependent reflection coefficients. Analysis of prestack amplitude variation with offset and azimuth (often called azimuthal AVO analysis or AVAZ) is one of the most effective tools for seismic characterization of fractures and in situ stress field (e.g., Mallick et al., 1998; Lynn et al., 1999b; Bakulin et al., 2000a, b). The main advantage of amplitude methods compared to traveltime inversion is their superior vertical resolution, which makes AVO analysis suitable for relatively thin reservoirs. Also, body-wave amplitudes are highly sensitive to azimuthal velocity variations associated with vertical fracture systems and nonhydrostatic stresses.
We start by reviewing the reflection/transmission problem and plane-wave reflection coefficients for a boundary between azimuthally anisotropic halfspaces. For orthorhombic models with the same orientation of the vertical symmetry planes above and below the reflector, the symmetry-plane reflection coefficients can be directly adapted from the corresponding VTI equations. Application of the weak-contrast, weak-anisotropy approximation shows that the azimuthal variation of the P-wave AVO gradient in orthorhombic and HTI media is controlled by the shear-wave splitting coefficient and the parameter responsible for the eccentricity of the NMO ellipse.
Because AVO algorithms aim to operate with at the target horizon, an important element of AVO processing
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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.