Inversion of azimuthally dependent NMO velocity in transversely isotropic media with a tilted axis of symmetry
Just as the transversely isotropic model with a vertical symmetry axis (VTI media) is typical for describing horizontally layered sediments, transverse isotropy with a tilted symmetry axis (TTI) describes dipping TI layers (such as tilted shale beds near salt domes) or crack systems. P-wave kinematic signatures in TTI media are controlled by the velocity V{sub PO} in the symmetry direction, Thomsen's anisotropic coefficients {xi} and {delta}, and the orientation (tilt {nu} and azimuth {beta}) of the symmetry axis. Here, the authors show that all five parameters can be obtained from azimuthally varying P-wave NMO velocities measured for two reflectors with different dips and/or azimuths (one of the reflectors can be horizontal). The shear-wave velocity V{sub SO} in the symmetry direction, which has negligible influence on P-wave kinematic signatures, can be found only from the moveout of shear waves. Using the exact NMO equation, the authors examine the propagation of errors in observed moveout velocities into estimated values of the anisotropic parameters and establish the necessary conditions for a stable inversion procedure. Since the azimuthal variation of the NMO velocity is elliptical, each reflection event provides them with up to three constraints on the model parameters. Generally, the five parameters responsible for P-wave velocity can be obtained from two P-wave ellipses, but the feasibility of the moveout inversion strongly depends on the tilt {nu}. While most of the analysis is carried out for a single layer, the authors also extend the inversion algorithm to vertically heterogeneous TTI media above a dipping reflector using the generalized Dix equation. A synthetic example for a strongly anisotropic, stratified TTI medium demonstrates a high accuracy of the inversion.
- Research Organization:
- Colorado School of Mines, Golden, CO (US)
- Sponsoring Organization:
- US Department of Energy
- OSTI ID:
- 20015463
- Journal Information:
- Geophysics, Journal Name: Geophysics Journal Issue: 1 Vol. 65; ISSN GPYSA7; ISSN 0016-8033
- Country of Publication:
- United States
- Language:
- English
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