Depth‐domain velocity analysis in VTI media using surfaceP-wave data: Is it feasible?

Abstract

The main difficulties in anisotropic velocity analysis and inversion using surface seismic data are associated with the multiparameter nature of the problem and inherent trade-offs between the model parameters. For the most common anisotropic model, transverse isotropy with a vertical symmetry axis (VTI media), P-wave kinematic signatures are controlled by the vertical velocity V 0 and the anisotropic parameters and δ. However, only two combinations of these parameters-NMO velocity from a horizontal reflector V nmo (0) and the anellipticity coefficient η-can be determined from P-wave reflection traveltimes if the medium above the reflector is laterally homogeneous. While V nmo (0) and η are sufficient for time-domain imaging in VTI media, they cannot be used to resolve the vertical velocity and build velocity models needed for depth migration. Here, we demonstrate that P-wave reflection data can be inverted for all three relevant VTI parameters (V 0 , and δ) if the model contains nonhorizontal intermediate interfaces. Using anisotropic reflection tomography, we carry out parameter estimation for a two-layer medium with a curved intermediate interface and reconstruct the correct anisotropic depth model. To explain the success of this inversion procedure, we present an analytic study of reflection traveltimes for this model and show that the information about the vertical velocity and reflector depth was contained in the reflected rays which crossed the dipping intermediate interface. The results of this work are especially encouraging because the need for depth imaging (such as prestack depth migration) arises mostly in laterally heterogeneous media. Still, we restricted this study to a relatively simple model and constrained the inversion by assuming that one of the layers is isotropic. In general, although lateral heterogeneity does create a dependence of P-wave reflection traveltimes on the vertical velocity, there is no guarantee that for more complicated models all anisotropic parameters can be resolved in a unique fashion.

Seismology of Azimuthally Anisotropic Media and Seismic Fracture Characterization

“…Le Stunff et al (2001) demonstrated that the success of the tomographic inversion procedure was ensured by the reflection traveltimes for the bottom reflector near the left edge of the model, where the intermediate interface is dipping. Indeed, P-wave moveout in VTI media is controlled by the zero-dip NMO velocity and parameter η only if the medium above the reflector is laterally homogeneous.…”

“…Following Le Stunff et al (2001), we consider P-wave moveout inversion for a model composed of two homogeneous layers (VTI and isotropic) separated by a curved interface ( Figure 2.11). P-wave reflection traveltimes for both interfaces were calculated by anisotropic ray tracing; the modeling parameters are listed in Table 2.1.…”

A practical method for estimating effective parameters of anisotropy from reflection seismic data

“…However, the anisotropy parameters to use are not easy to determine. Transverse isotropy is often described in terms of the Thomsen parameters ε and δ (Thomsen, 1986), and there has been considerable published work addressing the question of Presented how to extract these parameters from surface seismic data, focusing primarily on the application of nonhyperbolic velocity analysis and traveltime inversion (e.g., Alkhalifah and Tsvankin, 1995;Grechka and Tsvankin, 1998a, b;Grechka and Tsvankin, 1999;Grechka, Pech, et al, 2001;Le Stunff et al, 2001) or on the analysis of joint P-and converted-wave data (Grechka, Theophanis, and Tsvankin, 1999;Sayers, 1999;Grechka, Pech, and Tsvankin, 2002;van der Baan and Kendall, 2002). We present an independent method of estimating the effective anisotropy parameters from surface P-wave reflection seismic data.…”

VTI anisotropic corrections and effective parameter estimation after isotropic prestack depth migration

“…In practice, this is not the case, especially when well data are not available. Earlier studies showed that if only surface P-wave data are available, it will be difficult if not impossible to resolve accurately the three quantities V z , , and ␦ ͑Le Stunff et al, 2001;Sarkar and Tsvankin, 2004͒. When relying on surface data only, we suggest using the near offsets in the IMO domain to perform a standard isotropic interval-velocity analysis ͑such as depth residual moveout and tomography͒ prior to the estimation of and ␦.…”