Computation of Principal Directions of Azimuthal Anisotropy From P-Wave Seismic Data

Subhashis Mallick; Kenneth L. Craft; Laurent J. Meister; Ron E Chambers

doi:10.1071/EG997379

ISSN: 0812-3985
E-ISSN: 1834-7533

Computation of Principal Directions of Azimuthal Anisotropy From P-Wave Seismic Data
Authors Subhashis Mallick^a, Kenneth L. Craft^a, Laurent J. Meister^a and Ron E Chambers^a
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^a Western Geophysical, , P.O. Box 2469, Houston, TX, 77252, USA, . (713) 963 5762, (713) 963 5757
Publisher: Australian Society of Exploration Geophysicists
Source: Exploration Geophysics, Volume 28, Issue 4, Sep 1997, p. 379 - 382
DOI: https://doi.org/10.1071/EG997379
- Published online: 01 Sep 1997

Abstract

Principal directions for an azimuthally anisotropic medium are the directions along which the quasi P and quasi S waves propagate as pure P and S modes. For azimuthal anisotropy, induced by a single set of oriented vertical fractures, two of these principal directions are the directions parallel to and perpendicular to the fractures.

To investigate if conventional P-wave data could be used in fracture detection, we computed synthetic data for an isotropic Taylor shale over a fractured Austin chalk model. We found that the modelled P-wave reflection amplitude variation with offset has different slopes along different directions. We also found that the reflection amplitude at fixed offsets is periodic in 2ϕ, where ϕ is the orientation angle of the shooting direction with respect to one of the principal directions. For fracture induced anisotropy, this principal direction corresponds to the direction parallel to or perpendicular to the fractures. We use this periodic azimuthal dependence to obtain fracture orientation and a qualitative measure of the fracture density from the azimuthal P-wave data.

We applied our technique to real P-wave data, collected over a wide source-to-receiver azimuth. Computations using our method gave an orientation of the principal direction consistent with the general fracture orientation in the area as inferred from other geological and geophysical evidence.

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References

Alford, R.M., 1986, Shear data in the presence of azimuthal anisotropy, Dilley, Texas: 56th Ann. Internat. Mtg. Soc. Expl. Geophys., Expanded Abstracts, 476-479.
Allen, J.L., and Peddy, C.P., 1993, Amplitude variation with offset: Gulf coast case studies: Soc. Expl. Geophys.
Chang, C.H., and Gardner, G.H.F., 1993, Effects of vertically aligned fractures on reflection amplitudes, an amplitude-versus-offset study: 63rd Ann. Internat. Mtg. Soc. Expl. Geophys., Expanded Abstracts, 769-771.
Lefeuvre, F, 1994, Fracture related anisotropy detection and analysis: "and if the P waves were enough": 64th Ann. Internat. Mtg. Soc. Expl. Geophys., Expanded Abstracts, 942-945.
Mallick, S., and Frazer, L.N., 1990, Computation of synthetic seismograms for stratified azimuthally anisotropic media: J. Geophys. Res 95, 8513-8526.
Mallick, S., and Frazer, L.N., 1991, Reflection/transmission coefficients and azimuthal anisotropy in marine seismic studies: Geophys. J. Int. 105, 241-252.
Neidell, N.S., and Cook, E.E., 1986, Seismic method for identifying low velocity subsurface zones: US Patent No 4,571,710; Feb. 18, 1986.
RUeger, A., and Tsvankin, I., 1995, Azimuthal variation of AVO response for fractured reservoirs, 65th Ann. Internat. Mtg. Soc. Expl. Geophys., Expanded Abstracts, 1103-1106.
Thomsen, L., 1988, Reflection seismology over azimuthally anisotropic media: Geophysics 53, 304-313.

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Article Type: Research Article

Keyword(s): AVOAnisotropy; azimuthal anisotropy; fractures

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Computation of Principal Directions of Azimuthal Anisotropy From P-Wave Seismic Data

Abstract

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