Azimuthal anisotropy: Is it really ubiquitous?

Azimuthal anisotropy: is it really ubiquitous? Bryan DeVault, Vecta Oil & Gas, Ltd

Summary Following the groundbreaking work of Crampin (1985), Alford (1986), Lynn and Thomsen (1990) and Lewis et al (1991), it quickly became conventional wisdom in the industry that substantial shear-wave azimuthal anisotropy is nearly ubiquitous in the subsurface. Crampin and Zatsepin

(1995) made theoretical arguments that implied that most of the subsurface is azimuthally anisotropic. As industry’s ability to measure and quantify azimuthal anisotropy at a variety of scales has grown, this assumption has remained largely unexamined. Looking at over a dozen datasets, including surface 9-C seismic, 9-C VSPs, and dipole sonic logs at a number of locations in the Western US and Alberta, I have seen little unambiguous evidence of

azimuthal shear-wave anisotropy despite polarization analysis of each dataset.

Introduction The discovery of shear-wave splitting on exploration seismic data and its application to reservoir characterization generated a great deal of interest by oil and gas companies

and universities beginning in the late 1980s. The practical insight into shear-wave propagation this discovery provided was so deep that most subsequent analysis of 9-C datasets neglected the possibility of no or only trivial azimuthal anisotropy in the subsurface. Frequently, surface shear-wave data quality was too poor to unambiguously recover the orientation of the principal axes (e.g. Roche, 1997; Blott, 1997) but other data such as 9-C VSP surveys suggested a data fast direction and this information was

used to rotate the data into a principal-axis frame prior to

conventional surface processing. Simmons and Backus (2001) suggested that a radial-transverse frame was a natural domain to perform analysis of 9-C surface data and that processing 3D 9-C data in field coordinates is a bad idea under almost all circumstances. If the predominant type of subsurface anisotropy is vertical transverse isotropy (VTI) rather than azimuthal anisotropy (HTI or lower-order

symmetries), the radial-transverse domain is the correct one for data processing and analysis. Few polarization analyses have been performed on real 9-C datasets, however, to determine which of these two alternatives is most appropriate for data processing and analysis. For other symmetry systems, such as weakly orthorhombic anisotropy, some offset-dependent combination of the two may even be necessary.

Vecta and its partners have acquired numerous 9-C 2D and 3D datasets throughout the Western US and the Western Canadian Sedimentary Basin (Figure 1), at locations in West Texas, Southwest Kansas, Southeastern Colorado, North Dakota, and Southern Alberta. In many of these areas, we also acquired 9-C VSP surveys and cross-dipole sonic logs to further aid in characterizing the elastic

parameters of the subsurface.

Figure 2: Cross-dipole sonic log segment from a Midland Basin

well. Note near-complete lack (<1%) of azimuthal anisotropy over

entire length of logged interval.

Figure 1: Location map of datasets analyzed for shear-wave

splitting

1009SEG/San Antonio 2007 Annual Meeting

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Azimuthal anisotropy: where is it?

Analysis techniques The data studied included a mixture of 2D and 3D 9-C surface data, 9-C VSPs, and cross-dipole sonics. Some projects had only 2D 9-C seismic, while others had 9-C 3D

surface seismic, a cross-dipole sonic, and a VSP. For the dipole sonics, which were run with orthogonal transmitters and receivers, analysis was limited to examination of the amount of splitting between the fast and slow shear modes, as well as the consistency and quality of the fast-axis solution. Near-offset and walkaway VSP polarizations were also examined, as well as the amount of time-delay splitting between the shear-wave modes. For the surface

seismic, I examined the off-diagonal components of 2D lines for coherent reflection events after common processing was applied to all four field data components. To minimize the chance that the 2D lines were fortuitously acquired along the principal axes of the symmetry system, analysis focused on sets of 2D lines acquired at oblique angles to each other.

For the 3D 9-C datasets, polarization analysis focused on examining flattened shot and receiver stacks to enhance the shear signal. These proved to be very sensitive indicators of azimuthal anisotropy and allowed determination of the fast axis orientation to within five degrees where azimuthal anisotropy was present. Another useful technique was comparison of radial-transverse rotation and fixed-angle rotation scan results on azimuth-sectored substacks of the

3D data. Because the (q)SH and (q)SV modes have very different AVO behavior (the latter is characterized by strong phase reversals in the mid angle range, while the SH mode usually has very weak amplitude changes with offset that are entirely controlled by the shear-wave velocity contrast across each reflection interface), analysis of prestack amplitudes is also a very sensitive polarization indicator:

data that is incorrectly rotated to a radial-transverse frame will have AVO behavior on the “SH” mode that is very different from what isotropic models predict. This kind of amplitude behavior on data that are rotated into a radial-transverse frame indicates that the data should be analyzed in a principal-axis frame. Shear data quality permitted this analysis on several of the datasets examined.

West Texas datasets Several 9-C 2D and 3D datasets, dipole sonics, and a 9-C VSP were acquired on the Eastern Shelf and in Midland Basin of West Texas. Only one of the many 2D lines acquired had evidence of anisotropy when Alford analysis was performed.

Additionally, all of the dipole sonics had very little or no

observable shear-wave splitting (Figure 2). A small 3D 9-C survey acquired in Schleicher County also had no evidence of azimuthal anisotropy, excellent SH data quality, and SH AVO behavior completely characteristic of a flat-layered isotropic subsurface from the surface to

acoustic basement (Figure 3). No consistent time delay was observed between the SH and SV stacks, and 2D 9-C

data acquired nearby exhibited no coherent energy on commonly-processed off-diagonal sections.

Southeast Colorado/Southwest Kansas datasets

Several 2D and 3D 9-C datasets, dipole sonics, two 9-C VSPs, and a 9-C walkaway VSP scattered over an area of more than 10000 km2 exhibited no evidence of azimuthal anisotropy. Of all the areas studied, this one exhibited the least evidence of azimuthal anisotropy. Extensive analysis of one high-quality 9-C 3D dataset indicated that shear-wave polarizations for the entire dataset were best matched by an isotropic (or weakly VTI)

model of radial and transverse shear propagation. Figure 4 shows the four horizontal components for a brute stack inline for a single azimuth for the best Alford scan result and with the data rotated into radial-transverse coordinates. The radial-transverse rotation has no coherent energy on the off-diagonal components, in contrast to the best fixed-angle rotation, which had a considerable amount of residual reflection energy. Repeating this analysis for different

azimuths, line locations, and line orientations revealed that

Figure 3 SH angle gathers (3 to 48 degrees in 5-degree increments)

from 9-C 3D survey acquired on the Eastern Shelf of West Texas

portion of Permian Basin. Near-constant SH amplitude behavior

with offset is inconsistent with significant shear-wave azimuthal

anisotropy.


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radial-transverse rotation performed better than any fixed-angle rotation in minimizing off-diagonal energy. A 9-C walkaway VSP collected at a location almost 200 km to the southeast in Stanton County, Kansas also showed

no evidence of azimuthal anisotropy. Polarization analyses of direct shear arrivals from offset shots unambiguously indicated a radial-transverse polarization for the pure shear modes (Figure 5).

North Dakota and Alberta Data A 9-C 3D dataset acquired in the eastern Williston Basin in

North Dakota exhibited weak azimuthal anisotropy (Stevens and DeVault, 2005). Although little time-delay splitting was observed (generally < 10 ms of splitting between the surface and the main Lower Paleozoic reservoir objectives), shot and receiver reflection stacks exhibited clear evidence of azimuthal anisotropy.

Azimuthal anisotropy was also evident on azimuth-sectored subsets of the data (Figure 6), where clear nulls are visible on the off-diagonal components of the 4-C horizontal data matrix in the principal directions. Although azimuthal anisotropy is clearly present on the data, it is very weak, as

indicated by the very minor amount of splitting observed

on the data; as described in Stevens and DeVault (2005), the subsurface here is best characterized as weakly orthorhombic. A set of 9-C 2D surface seismic datasets acquired in the

Western Canada Sedimentary Basin east of the Alberta Foothills, by contrast, showed very little evidence of azimuthal anisotropy despite very good seismic data quality. Almost no coherent energy was observed on the off-diagonal components of the data (Fig. 7), which were collected at several azimuths. Additionally, SH and SV

Figure 4 Horizontal components for Southeast Colorado 3D inline

from single-azimuth brute stack showing best fixed angle result (top) and radial-transverse data rotation (bottom).

Figure 5: Southwest Kansas Walkaway VSP SH (top) and SV

(bottom) receiver gathers from 3500-foot tool level after horizontal

rotation; typical hodogram for a single SH shot is shown in inset.

Note complete absence of radial-component energy on SH gather and transverse energy on SV gather.


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AVO behavior were entirely characteristic of layered isotropic media. This lack of azimuthal anisotropy was generally confirmed by a cross-dipole sonic acquired in a well drilled on one of the 2D lines, where only sporadic evidence of weak azimuthal anisotropy was observed over

several thin intervals.

Conclusions Datasets acquired at numerous locations in the Western US and Canada at well log, VSP, and seismic scales showed only scant evidence of azimuthal anisotropy although we diligently searched for it in all of the data. Most of the data

were of sufficient quality to allow unambiguous detection of any azimuthal anisotropy. Where present, azimuthal anisotropy was weak and invariably averaged less than 1% from the surface to the target reservoirs. These somewhat unexpected results contrast with several

case studies from the Western US (Roche, 1997, Kendall, 1996, Shuck, 1994, Davis, 2006) where azimuthal anisotropy, although generally weak, was clearly present. When processing multicomponent data, it would seem

prudent to begin with the simplest model of shear propagation, which is radial-transverse shear polarization, using more complex anisotropic models only when the data themselves suggest their necessity. This data-driven approach to shear-wave polarization analysis is likely to result in a higher-quality processed output than one which ignores the alternatives for data rotation during processing.

Acknowledgments I would like to thank the management and staff of Vecta Oil and Gas, particularly John Beecherl, John Suydam and Miguel Garcia, for their valuable assistance, support, and

patience. Geocenter, particularly John Stevens, is also due many thanks for his analysis of many of the datasets shown here. The Bureau of Economic Geology at the University of Texas, particularly Paul Murray and Bob Hardage, provided insight and many excellent discussions of the data shown here. I would also like to thank Ran Zhou of Read Well Services for processing the walkaway VSP, and Vecta’s many partners for their ongoing support.

Figure 7 Southern Alberta 2D SH stack (top) and SHSV (bottom)

stack. Note lack of coherent energy on off-diagonal stack.

Figure 6 North Dakota horizontal components from 3D inline

plotted as a function of source-receiver azimuth. Note prominent

nulls and amplitude maxima on 12 and 21 for fast and slow directions


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EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2007 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Alford, R. M., 1986, Shear data in the presence of azimuthal anisotropy: Dilley, Texas: 56th Annual International Meeting, SEG,

Expanded Abstracts, 476–479. Blott, J. E., 1997, Morrow Valley-Fill sandstone reservoir characterization with 3-D, 3-C seismology, Sorrento Field, Colorado:

Ph.D. thesis, Colorado School of Mines. Crampin, S., 1985, Evaluation of anisotropy by shear-wave splitting: Geophysics, 50, 142–152. Crampin, S., and S. V. Zatsepin, 1995, Production seismology: The use of shear waves to monitor and model production in a

poro-reactive and interactive reservoir: 65th Annual International Meeting, SEG, Expanded Abstracts, 199–202. Davis, T. L., 2006, Multicomponent 4-D seismic reservoir characterization of tight gas sands, Rulison Field, Colorado: 76th

Annual International Meeting, SEG, Expanded Abstracts, 1143–1147. Kendall, R., and J. Kendall, 1996, Shear-wave amplitude anomalies in south-central Wyoming: The Leading Edge, 15, 913–920. Lewis, C., T. L. Davis, and C. Vuillermoz, 1991, Three-dimensional multicomponent imaging of reservoir heterogeneity, Silo

Field, Wyoming: Geophysics, 56, 2048–2056. Lynn, H. B., and L. A. Thomsen, 1990, Reflection shear-wave data collected near the principal axes of azimuthal anisotropy:

Geophysics, 55, 147–156. Roche, S. L., 1997, Time-lapse, multicomponent, three-dimensional seismic characterization of a San Andres shallow shelf

carbonate reservoir, Vacuum Field, Lea County, New Mexico: Ph.D. thesis, Colorado School of Mines. Shuck, E. L, 1993, Multicomponent, three-dimensional seismic characterization of a fractured coalbed methane reservoir, Cedar

Hill Field, San Juan County, New Mexico: Ph.D. thesis, Colorado School of Mines. Simmons, J., and M. Backus, 2001, Shear waves from 3-D-9-C seismic reflection data: Have we been looking for signal in all the

wrong places?: The Leading Edge, 20, 604–612. Stevens, J., and B. DeVault, Shear-wave azimuthal velocity anisotropy in a Williston Basin 9-C 3-D survey: 75th Annual

International Meeting, SEG, Expanded Abstracts, 901–903.


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Azimuthal anisotropy: Is it really ubiquitous?