Estimation of anisotropy from VSP data
Published:January 01, 2011
The previous chapters were devoted to estimation of anisotropy from the kinematics of reflected waves. Extending Thomsen notation to symmetries lower than transverse isotropy made it possible to build realistic models of the azimuthally anisotropic subsurface. The success of the traveltime inversion was ensured by our ability to identify the combinations of the stiffness coefficients that control such key signatures measured from wide-azimuth data as the NMO ellipse and nonhyperbolic moveout. Still, because reflection traveltimes are insensitive to velocity and anisotropy variations on a fine scale, moveout inversion produces models that have relatively low spatial resolution.
In principle, there are two options for improving the resolution of subsurface velocity fields. One is to include information about seismic amplitudes either directly (see Chapter 8) or via full-waveform inversion of reflection data (e.g., Tarantola, 1987; Virieux and Operto, 2009). Realizing the potential of full-waveform inversion, however, largely belongs to the future because only preliminary (albeit encouraging) results for anisotropic media are currently available (Chang and McMechan, 2009). The second option is to make use of acquisition geometries that differ from conventional surface recording. Here, we discuss application of vertical seismic profiling (VSP) geometries in anisotropic parameter estimation and demonstrate their ability to resolve in situ anisotropy with the spatial resolution close to the dominant seismic wavelength.
The term “VSP” refers to observations of elastic waves excited by sources located at or near the earth’s surface and recorded by geophones placed in a borehole. A string of such geophones tracks the evolution
Figures & Tables
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.