Joint processing of PP and PS data
Published:January 01, 2011
In the presence of anisotropy, supplementing P-wave reflection data with shear waves is often needed for estimating even the parameter set responsible for P-wave kinematics. For example, reflection moveout of P-waves in laterally homogeneous VTI media generally constrains only two parameter combinations - the zero-dip NMO velocity Vnmo, P and the anellipticity parameter η (Alkhalifah and Tsvankin, 1995). As discussed in Chapter δ, addition of reflection traveltimes of SV-waves to P-wave moveout helps resolve the vertical P- and S-wave velocities and the anisotropy parameters ε and δ, provided the reflector has a mild dip and the data are acquired for a wide range of azimuths. Moreover, joint inversion of P- and S-wave data can be effectively used in lower-symmetry orthorhombic and monoclinic media (see Chapter 6).
Therefore, finding practical ways of combining P- and S-waves is critically important for anisotropic velocity model-building. In theory, such joint processing algorithms might seem to be easy to implement because the techniques discussed in previous chapters are equally valid for both PP and SS pure-mode reflections. (Here we use the double indices “PP” and “SS” to emphasize that the downgoing and upgoing segments of ray trajectories correspond to the same wave type.) The applicability of algorithms originally designed for PP-waves to SS data is ensured by the reciprocity of pure-mode traveltimes with respect to the source and receiver positions. As a result, moveout of any pure-mode reflection in common-midpoint (CMP) gathers can be described by the traveltime series
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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.