Processing for Tiv Media
Transverse isotropy with a vertical symmetry axis (VTI media) is the most common anisotropic model for sedimentary basins. Here, we apply P-wave processing algorithms developed for VTI media to a 2-D synthetic data set generated by a finite difference code. The model, typical for the Gulf of Mexico, has a moderate structural complexity and includes a salt body and a dipping fault plane. Using the Alkhalifah-Tsvankin dip-moveout (DMO) inversion method, we estimate the anisotropic coefficient η responsible for the dip dependence of P-wave NMO velocity in VTI media. In combination with the normal-moveout (NMO) velocity from a horizontal reflector [Vnmo(0), the argument “0” refers to reflector dip], η is sufficient for performing all P-wave time-processing steps, including NMO and DMO corrections, prestack and poststack time migration. The NMO (stacking) velocities needed to determine Vnmo(0) and η are picked from conventional semblance velocity panels for reflections from subhorizontal interfaces, the dipping fault plane and the flank of the salt body. To mitigate the instability in the interval parameter estimation, the dependence of Vnmo(0) and η on the vertical reflection time is approximated by Chebyshev polynomials with the coefficients found by “global” fitting of all velocity picks.
We perform prestack depth migration for the reconstructed anisotropic model and two isotropic models with different choices of the velocity field. The anisotropic migration result has a good overall quality, but reflectors are mispositioned in depth because the vertical velocity for this model cannot be obtained from surface -wave data alone. The isotropic migrated section with the NMO velocity Vnmo(0) substituted for the isotropic velocity also has the wrong depth scale and is somewhat inferior to the anisotropic result in the focusing of dipping events. Still, the image distortions are not significant because the parameter η, which controls NMO velocity for dipping reflectors, is rather small (the average value of η is about 0.05). In contrast, the isotropic section migrated with the vertical velocity has a poor quality (although the depth of the subhorizontal reflectors is correct) due to the fact that in VTI media Vo can be used to stack neither dipping nor horizontal events. The difference between vo and the zero-dip stacking velocity Vnmo(0) is determined by the anisotropic coefficient δ, which is greater than η in our model (on average δ ≈ 0.1).
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“This volume contains 25 papers that represent most of the best work in seismic anisotropy in 1998 and 1999. Fracture characterizations and processing of converted waves are the two main topics covered in this volume. They are addressed from both theoretical and practical viewpoints. Also included are papers describing the historical roots of seismic anisotropy.”