Moveout inversion of multicomponent data for lower symmetries
Published:January 01, 2011
2011. "Moveout inversion of multicomponent data for lower symmetries", Seismology of Azimuthally Anisotropic Media and Seismic Fracture Characterization, Ilya Tsvankin, Vladimir Grechka
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The results of Chapter 5 demonstrate the benefits of combining P-wave data with reflection moveout of shear or mode-converted waves in parameter estimation for TI media. The advantages of multicomponent velocity analysis are even more crucial for lower-symmetry models described by a larger number of independent anisotropy parameters. This chapter is focused on moveout inversion of wide-azimuth, multi-component data from layered orthorhombic (sections 6.1-6.3) and monoclinic (section 6.4) media.
Velocity model building using PP and PS traveltimes is especially attractive because it does not require shear-wave excitation. We begin the first section by reviewing the properties of reflection moveout of converted waves for models composed of horizontal layers with a horizontal symmetry plane. As demonstrated in Chapter 1, PS traveltimes for such media are reciprocal with respect to the source and receiver positions and can be described by the conventional t2(x2) series. Furthermore, there exists a simple relationship between the NMO ellipses of pure and converted waves which, combined with the generalized Dix differentiation, makes it possible to compute the interval NMO ellipses of the split shear waves S1S1 and S2S2 from PP and PS (PS1 and PS2) data. Then the interval PP- and SS-wave NMO ellipses are inverted for the parameters of orthorhombic or monoclinic layers. This algorithm is tested on multicomponent (PP and PS) physical-modeling data acquired in several azimuthal directions over a block of orthorhombic composite material. The computed symmetry-plane NMO velocities of PP- and SS-waves are combined with the known reflector depth to estimate eight.
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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.