A case study of azimuthal AVO analysis with anisotropic spreading correction
Geomechanical properties of tight, low-porosity reservoirs are largely governed by natural fracture networks. Hence, reliable estimation of fracture density and orientation is extremely important for cost-effective hydraulic completion and hydrocarbon production. Direct information about fracturing can be obtained using borehole methods, such as image log analysis, which provide estimates of fracture counts and orientations on various scales. The main shortcoming of borehole measurements is that they are sensitive only to formation properties in the immediate vicinity of the well. In some cases, the spatial distribution of fractures can be inferred from fault maps obtained by coherence analysis of surface seismic data. However, the correlation between areas of high fracture density and fault locations is not always straightforward.
Estimation of interval anisotropy parameters using velocity-independent layer stripping
Moveout analysis of long-spread P-wave data is widely used toestimatethekeytime-processingparameter inlayeredtransversely isotropic media with a vertical symmetry axis VTI. Inversion for interval values, however, suffers from instability caused by the trade-off between the effective moveout parameters and by subsequent error amplification during Dix-type layer stripping. We propose an alternative approach to nonhyperbolic moveout inversion based on the velocity-independent layerstripping VILS method of Dewangan and Tsvankin. Also, we developthe3DversionofVILSandapplyittointervalparameter estimation in orthorhombic media using wide-azimuth, longspreaddata.Iftheoverburdenislaterallyhomogeneousandhasa horizontal symmetry plane, VILS produces the exact interval traveltime-offset function in the target layer without knowledge of the velocity field. Hence, Dix-type differentiation of moveout parameters used in existing techniques is replaced by the much morestablelayerstrippingofreflectiontraveltimes.Theinterval traveltimes are then inverted for the moveout parameters using the single-layer nonhyperbolic moveout equation. The superior accuracy and stability of the algorithm are illustrated on raytraced synthetic data for typical VTI and orthorhombic models. Even small correlated noise in reflection traveltimes causes substantial distortions in the interval values computed by conventional Dix-type differentiation. In contrast, the output ofVILS is 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 VILS in the shale layer above the reservoir are more plausible and less influenced by noisethanthoseobtainedbytheDix-typemethod.
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