Multisource multireceiver method and system for geophysical exploration

Alford, R. M.

doi:10.1121/1.398187

Cited by 5 publications

(9 citation statements)

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“…Assume that a conventional stack of a CMP gather forms a trace which is an accurate surrogate for a normal-incidence, multiple-free, noisereduced trace. Although this assumption is not one to be taken casually, the results of Alford (1986b) and Willis et al (1986) suggest that it is acceptable in the present context. Hence, the discussion in this section applies only to vertical raypaths; in Figure 1, the rays are normal to the page.…”

Section: In Azimuthally Anisotropic Mediamentioning

confidence: 83%

“…. "-A series of papers presented at the 1986 SEG Convention (Thomsen, 1986b;Rai and Hanson, 1986;Lynn and Thomsen, 1986;Alford, 1986b;Willis et al, 1986;and Martin et al, 1986;cf., also Alford, 1986a, andAlford et al, 1986) provided arguments and data to show that these difficulties at the free surface may be overcome by appropriate data acquisition and analysis techniques. Taken as a group, these papers assert that the effects of azimuthal anisotropy on surface shear-wave (Swave) data are profound and that the implications of the effects are far-reaching.…”

Section: Introductionmentioning

confidence: 99%

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Reflection seismology over azimuthally anisotropic media

Thomsen¹

1988

GEOPHYSICS

236

137

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Recent surveys have shown that azimuthal anisotropy (due most plausibly to aligned fractures) has an important effect on seismic shear waves. Previous work had discussed these effects on VSP data; the same effects are seen in surface recording of reflections at small to moderate angles of incidence. The anisotropic effects one different polarization components of vertically traveling shear waves permit the recognition and estimation of very small degrees of azimuthal anisotropy (of order 2 1 percent), as in an interferometer. Anisotropic effects on traveltime yield estimates of anisotropy which are averages over large depth intervals. Often, raw field data must be corrected for these effects before the reflectors may be imaged; two variations of a rotational algorithm to determine the "principal time series" are derived. Anisotropic effects on moveout lead to abnormal moveout unless the survey line is parallel to the fractures. Anisotropic effects on reflection amplitude permit the recognition and estimation of anisotropy (hence fracture intensity) differences at the reflecting horizon, i.e., with high vertical resolution,

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Section: In Azimuthally Anisotropic Mediamentioning

confidence: 83%

Section: Introductionmentioning

confidence: 99%

Reflection seismology over azimuthally anisotropic media

Thomsen¹

1988

GEOPHYSICS

236

137

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“…4. Here, the angle calculated from the Alford algorithm (Alford 1986a) is plotted, sample by sample, for two time windows. The first window encompasses the Austin Chalk interval, the second window encompasses the entire trace.…”

Section: A T a Acquisition A N D Alford Rotationmentioning

confidence: 99%

Prediction of lateral variability in fracture intensity using multicomponent shear-wave surface seismic as a precursor to horizontal drilling in the Austin Chalk

Mueller

1991

Geophysical Journal International

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S U M M A R Y A method of multicomponent multisource shear-wave interpretation, utilizing the output of the Alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media, yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling. The method exploits a fundamental difference between the two particle motion stacks when vertical, aligned fractures are the source of the shear-wave splitting. The fast shear wave (Sl) section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures; hence, reflectivity on the S1 CDP-stacked section should be likewise insensitive to fractures. The slow shear wave (S2) section, however, possesses particle motion perpendicular to the fracture planes, so variation in lateral fracture intensity will affect velocity and reflectivity. Examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the S1 section for the Austin Chalk, the horizon of interest, but laterally inconsistent reflector strength for the same horizon on the S2 section in the study area. Reflector dimming on the S2 section was interpreted as indicating fracture intensity maxima. Horizontal drilling confirmed both fracture azimuth and intensity predictions, and resulted in significantly higher initial hydrocarbon production than for surrounding wells.

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“…The Expanded Abstract of that SEG presentation (Alford, 1986) is one of the more widely cited references in geophysical literature. A detailed explanation of the logic and physics embedded in the coordinate transformation procedure was later published as U.S. Patent 4,817,061 (Alford et al, 1989). Figure 8.2.…”

Section: Alford Rotationmentioning

confidence: 99%

Improving the Monitoring, Verification, and Accounting of CO<sub>2</sub> Sequestered in Geologic Systems with Multicomponent Seismic Technology and Rock Physics Modeling

Alkan¹,

DeAngelo²,

Hardage³

et al. 2012

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Research done in this study showed that P-SV seismic data provide better spatial resolution of geologic targets at our Appalachian Basin study area than do P-P data. This finding is important because the latter data (P-P) are the principal seismic data used to evaluate rock systems considered for CO 2 sequestration. The increase in P-SV 1 resolution over P-P resolution was particularly significant, with P-SV 1 wavelengths being approximately 40-percent shorter than P-P wavelengths. CO 2 sequestration projects across the Appalachian Basin should take advantage of the increased resolution provided by converted-shear seismic modes relative to P-wave seismic data.In addition to S-wave data providing better resolution of geologic targets, we found S-wave images described reservoir heterogeneities that P-P data could not see. Specifically, a channel-like anomaly was imaged in a key porous sandstone interval by P-SV 1 data, and no indication of the feature existed in P-P data. If any stratigraphic unit is considered for CO 2 storage purposes, it is important to know all heterogeneities internal to the unit to understand reservoir compartmentalization. We conclude it is essential that multicomponent seismic data be used to evaluate all potential reservoir targets whenever a CO 2 storage effort is considered, particularly when sequestration efforts are initiated in the Appalachian Basin.Significant differences were observed between P-wave sequences and Swave sequences in data windows corresponding to the Oriskany Sandstone, a popular unit considered for CO 2 sequestration. This example demonstrates that S-wave sequences and facies often differ from P-wave sequences and facies and is a principle we have observed in every multicomponent seismic interpretation our research laboratory has done. As a result, we now emphasis elastic wavefield seismic stratigraphy in our reservoir characterization studies, which is a science based on the concept that the same weight must be given to S-wave sequences and facies as is given to P-wave sequences and facies. This philosophy differs from the standard practice of depending on only conventional P-wave seismic stratigraphy to characterize reservoir units.The fundamental physics of elastic wavefield seismic stratigraphy is that Swave modes sense different sequences and facies across some intervals than does a P-wave mode because S-wave displacement vectors are orthogonal to Pwave displacement vectors and thus react to a different rock fabric than do P waves. Although P and S images are different, both images can still be correct in terms of the rock fabric information they reveal.ii

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Multisource multireceiver method and system for geophysical exploration

Cited by 5 publications

References 0 publications

Reflection seismology over azimuthally anisotropic media

Reflection seismology over azimuthally anisotropic media

Prediction of lateral variability in fracture intensity using multicomponent shear-wave surface seismic as a precursor to horizontal drilling in the Austin Chalk

Improving the Monitoring, Verification, and Accounting of CO<sub>2</sub> Sequestered in Geologic Systems with Multicomponent Seismic Technology and Rock Physics Modeling

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