Moveout inversion of multicomponent data for TI media
Published:January 01, 2011
Continued progress in acquiring and processing high-quality multicomponent data has provided clear evidence of the influence of anisotropy on reflection traveltimes and moveout inversion. In particular, conventional isotropic imaging methods routinely produce depth misties between PP and PS (converted-wave) sections (e.g., Nolte et al., 2000), which can be removed by joint anisotropic velocity analysis of PP and PS data volumes. In this chapter, PP-wave reflection moveout is combined with traveltimes of mode-converted (PS) and shear (SS) waves in parameter estimation for transversely isotropic media.
We begin by examining joint inversion of PP and PS (PSV) data for the simple model of a horizontal VTI layer. Although the addition of PS traveltimes makes it possible to obtain the ratio of the vertical velocities of P- and S-waves and the shear-wave NMO velocity, inversion for the Thomsen parameters and reflector depth remains nonunique, even for uncommonly large spreadlength-to-depth ratios. Reconstruction of the depth scale of horizontally layered VTI models from surface data requires generation of shear waves and recording of wide-angle SS reflections, as demonstrated by Tsvankin and Thomsen (1995). In the second section we extend P-wave stacking-velocity inversion (tomography) described in Chapter 2 to the combination of NMO ellipses, zero-offset traveltimes, and reflection time slopes of PP- and SS-waves (SS traveltimes can be computed from PP and PS data). Application of the inversion algorithm to a homogeneous TI layer above a dipping reflector shows that for a range of dips and tilt angles of the symmetry axis conventional-spread
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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.