Inversion of moveout velocities for horizontal transverse isotropy
- Colorado School of Medicine, Golden, CO (United States)
Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model used to describe fractured reservoirs that contain parallel vertical cracks. Here. I present an exact equation for normal moveout (NMO) velocities from horizontal reflectors in HTI media and apply it to invert moveout data for the anisotropic parameters. The azimuthally dependent NMO velocity of P-waves, that can be obtained from 3-D surveys, provides enough information to determine the principle direction of the anisotropy (crack orientation) and the P-wave vertical velocity, as well as an effective anisotropic parameter equivalent to Thomsen`s coefficient {delta}. An important parameter of fracture systems is the crack density that is usually estimated through the traveltimes or reflection amplitudes of the split shear waves at vertical incidence. The formalism developed here makes it possible to obtain the crack density using the NMO velocities of P and shear waves from horizontal reflectors. Furthermore P-wave data alone (combined with an estimate of one of the shear-wave vertical velocities) are sufficient to find the crack density if either the P-wave velocity along the symmetry direction is known or dipping events are available. Also, P-wave moveout can be used to constrain the crack density in the special case of the vanishing parameter {epsilon} corresponding to thin cracks and negligible equant porosity. Seismic processing algorithms developed for vertical transverse isotropy remain entirely valid in the symmetry plane of HTI media that contains the symmetry axis. The anisotropic coefficients recovered from moveout data can also be used for processing in off-symmetry planes that requires a more elaborate treatment.
- DOE Contract Number:
- FG02-89ER14079
- OSTI ID:
- 542996
- Report Number(s):
- CONF-951013--
- Country of Publication:
- United States
- Language:
- English
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