Interval Anisotropic Parameter Estimation Using Velocity-independent Layer Stripping
Moveout analysis of long-spread P-wave data is widely used to estimate the key time-processing parameter η in layered VTI media. Inversion for interval η values, however, suffers from instability caused by the tradeoff between the effective moveout parameters and by the subsequent error amplification during Dix-type layer stripping. Here, we propose an alternative approach to interval parameter estimation based on the velocity-independent layer-stripping method of Dewangan & Tsvankin (2006). This method makes it possible to compute the interval moveout in a layer of interest without knowledge of the medium parameters in the overburden. Then the interval traveltimes are inverted for the normal-moveout velocity (Vnmo) and η using the single-layer nonhyperbolic moveout equation. Thus, the layer stripping of η in the conventional method is replaced by the much more stable stripping of reflection traveltimes. We also develop the 3D version of the layer-stripping method and apply it to interval parameter estimation for orthorhombic media using wide-azimuth P-wave data. The superior accuracy and stability of our algorithm is illustrated on ray-traced synthetic data for typical layered VTI and orthorhombic models. Even small correlated noise in reflection traveltimes produces substantial distortions in the effective η, and for some models, even in the effective Vnmo. As a result, the inter val η values estimated by the conventional Dix-type method may be highly inaccurate. In contrast, the output of our layer-stripping algorithm proves to be insensitive to mild correlated traveltime errors. The algorithm is also tested on wide-azimuth P-wave reflection data recorded above a fractured reservoir at Rulison field in Colorado. The interval moveout parameters estimated by the velocity-independent layer stripping in the shale layer above the reservoir are more plausible and stable than those obtained by the Dix-type method.
Velocity analysis using nonhyperbolic moveout in transversely isotropic media
Transversely isotropy(TI) is a general kind of media. When the length of CMP is great larger then the depth of the reflector, the information included in the Time-Distance curve can be used to get the anisotropy parameters of the TI media. Several kinds of velocity with TI media are discussed. An anisotropy media model and an isotropy media model is tested for the double-parameter Velocity analysis.
Quartic moveout coefficient: 3D description and application to tilted TI media
Nonhyperbolic (long-spread) moveout provides essential information for a number of seismic inversion/ processing applications, particularly for parameter estimation in anisotropic media. Here, we present an analytic expression for the quartic moveout coefficient A4 that controls the magnitude of nonhyperbolic moveout of pure (nonconverted) modes. Our result takes into account reflection-point dispersal on irregular interfaces and is valid for arbitrarily anisotropic, heterogeneous media. All quantities needed to compute A4 can be evaluated during the tracing of the zero-offset ray, so long-spread moveout can be modeled without timeconsuming multioffset, multiazimuth ray tracing. The general equation for the quartic coefficient is then used to study azimuthally varying nonhyperbolic moveout of P-waves in a dipping transversely isotropic (TI) layer with an arbitrary tilt” of the symmetry axis. Assuming that the symmetry axis is confined to the dip plane, we employed the weak-anisotropy approximation to analyze the dependence of A4 on the anisotropic parameters. The linearized expression for A4 is proportional to the anellipticity coefficient·…†i‐ and does not depend on the individual values of the Thomsen parameters. Typically, the magnitude of nonhyperbolic moveout in tilted TI media above a dipping reflector is highest near the reflector strike, whereas deviations from hyperbolic moveout on the dip line are substantial only for mild dips. The azimuthal variation of the quartic coefficient is