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EAGE 63rd Conference & Technical Exhibition — Amsterdam, The Netherlands, 11 - 15 June 2001

AbstractThe effect of azimuthal anisotropy on reflection seismic data is significant to the extent that the imagingof such data is often poor if azimuthal isotropy is assumed. An analysis of azimuthal anisotropy can bebased on the sign flipping of the transverse component of wide azimuth multicomponent data. Followingthe analysis, the data are imaged properly by taking the anisotropy into account. We have applied suchprocessing to a wide-azimuth 3-D multicomponent survey from the North Sea (Harding).The benefits of proper processing, taking azimuthal anisotropy into account, are improved imaging and a3-D estimate of azimuthal anisotropy direction and magnitude, which is likely to provide importantattributes for fracture and stress characterization.

IntroductionAzimuthal anisotropy (AA) is not a new discovery. Even a minimal review of the state of the art isimpossible within the scope of this abstract. A recent reprint series on anisotropy (MacBeth and Lynn,2000) contains an impressive collection of papers. The general theory of anisotropy is formidable. It tooksome admirable contributions by a few authors to simplify the theory and enable practical applications. Itwas predicted and observed that shear waves very often split due to AA. Yet, a few 3-D multicomponentsurveys were processed ignoring azimuthal anisotropy with some success. Why wasn’t AA a showstopper? Perhaps, AA is not strong everywhere, but an important fact is that the first few 3-D 4-Csurveys were acquired in places where conventional data had severe problems such as gas clouds, smallPP impedance contrast, or severe multiples. In such cases, seabed data provided good value even if AAwas ignored. Also, we could not fully address AA until we had resolved the vector fidelity issue (Bagainiet al, 2000).In this paper we describe the application of AA analysis and data processing to a wide-azimuth cross-spread survey in the Harding area of the North Sea. We show that imaging is significantly improved ifAA is not ignored. Also, a 3-D model of AA direction and magnitude is produced and can be used infurther interpretation, such as fracture and stress characterization.

Converted waves: raw data observationsAzimuthal anisotropy can be detected very early on raw data. 45o common-azimuth gathers can easily beextracted. On such gathers, X- and Y-component data should be identical providing there is perfectvector fidelity and azimuthal isotropy. The time shift between the X- and the Y-component in Figure 1 isan indication of shear wave splitting.

Converted waves: 2-D observationsFurther detection of AA is provided by 2-D data. We recommend acquiring such data in conjunction withany 3-D data, and we have done so in the Harding survey.If the earth is azimuthally isotropic converted-wave energy should be polarized in the radial direction. Inparticular, Y-component 2-D images should be much weaker than X-component images. Figure 2 showsthat the shallow part is azimuthally isotropic but the deeper layers are azimuthally anisotropic.

N-06 AZIMUTHAL ANISOTROPY—MORE THAN JUSTTHEORY

S. RONEN, M. JARVIS and A. PROBERTWesternGeco, Schlumberger House, Gatwick RH6 0NZ, UK

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Figure 1: 45o common-azimuth gather. Left: the geometry for a cross-spread example. Right: thewavelet difference between the X- (in red) and the Y-component (in blue) is due to shear wave splitting.

Figure 2: Left: X-component stack. Right: Y-component stack. Note the lack of image on the Y-com-ponent stack above 1 s two-way travel time, and the comparable image below 2.4 s.

Azimuthal anisotropy analysis for converted wavesThe polarity of PS converted waves is radial at theconversion point C (in Figure 3). If the upgoing shearwaves pass through an azimuthally anisotropicoverburden they will split into two S1 and S2 waves. Thismay cause energy to be recorded on the transversecomponent. If the overburden is azimuthally isotropicthere will be no splitting and the polarity of the waveswill remain radial at all times. Another possible reasonfor a lack of energy on the transverse component is if theshot-receiver azimuth happens to be exactly in one of themajor directions S1 or S2. It is unlikely that a 2-D, or anarrow azimuth survey will happen to be entirely in a

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Shot line

Cable

S

R

P

S

SS

C

Figure 3

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major S1 or S2 direction. However, in wide azimuthsurveys there are traces in all azimuths and some traceswill happen to have their shot-receiver azimuth in amajor AA direction. Garrota (1988) and Li (1997)predicted that the transverse component will vanish in amajor directions. In Figure 4 we show data from Hardingsorted by azimuth. It is apparent that the transversecomponent flips sign every 90o. The directions in whichthe transverse component flips sign are the majoranisotropy (S1 and S2) directions. Using such data, foreach location, time, and offset, we calculated theprobability of each azimuth to be a major direction. Atime slice produced from the result of this process isshown in Figure 5. For each cross-spread location a“SNOWFLAKE” is produced. The S1 and S2 directionscan be picked from the SNOWFLAKES. Once the S1and S2 directions are known, S1 and S2 traces aregenerated (Figure 6) and the time delays are picked. Wethus produce S1 and S2 directions and time delays foreach analysis location and for each time.For a certain time, or horizon, these 3-D attributes can be displayed on a map (Figure 7). These azimuthalanisotropy attributes are potentially indicative of fractures and stress.

Radial Transverse

0 90 180 270 0 90 180 270Figure 4: Radial and transverse datasorted by shot-receiver azimuth. Notethe sign flips of the transverse data.

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Figure 6: S1 and S2 traces showingthe time delay due to shear wavesplitting increasing with time.

Figure 7: S1 direction scaled by S1 toS2 shear wave splitting time lag. Thelength of each is proportional to thetime lag.

Figure 5: SNOWFLAKES.The probability that eachazimuth is in an azimuthalanisotropy major (S1 or S2)direction.

EAGE 63rd Conference & Technical Exhibition — Amsterdam, The Netherlands, 11 - 15 June 2001

Imaging under an azimuthally anisotropic overburdenFollowing the SNOWFLAKES analysis, the data are processed taking the AA into account. It is possibleto image the S1 and S2 data separately. However, in Harding with AA in the overburden, we chose toshift the S2 data and add them to the S1 data. We then imaged a single converted wave data set. The effecton the image quality is very significant. Figure 8 shows a comparison between data processed with andwithout AA compensation.

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Figure 8: A cross section through the 3D image without azimuthal anisotropy compensation on the leftand with azimuthal anisotropy compensation on the right. The 3-D image produced with AAcompensation is much more coherent and interpretable. For example, note the reflectors at 1.5 s and thestructure on the right hand side of the sections at 3 s.

ConclusionsWe have applied azimuthal anisotropy analysis and processing to 4-C data from the Harding area of theNorth Sea. The analysis was based on sign flipping of the transverse component. The improved imagingis very significant. In addition, the process has provided a meaningful 3-D azimuthally anisotropicanalysis. We speculate that this analysis can provide useful attributes for fracture and stresscharacterization.

AcknowledgementsWe have relied on and benefited from contributions of many people. Dave Tilling, Dave Underwood,Bjorn Olofsson, Chris Cunnell, Richard Bryan, and Richard Bale have contributed to the developmentand group-experience of the SNOWFLAKES analysis method. Claudio Bagaini and Joffrey Brunellierehelped with vector fidelity analysis and correction. Rupert Hoare provided objectives (includingdeadlines!), guidance, and encouragement.

References

MacBeth, C., and Lynn, H.B., 2000. Applied Seismic Anisotropy: Theory, Background, and FieldStudies. Geophysics reprint series no. 20. Ed. Ebrom, D.A.Bagaini, C., Bale, R., Caprioli, P., Muyzert, E., and Ronen, S., 2000. Assessment and calibration ofhorizontal geophone fidelity in seabed 4C using shear-waves. EAGE meeting in Glasgow.Garotta, R. and Granger, P. Y., 1988. Acquisition and processing of 3C x 3-D data using convertedwaves: Annual Meeting Abstracts, Soc. Expl. Geophys., Session S13.2.Li, X-Y, 1997. Fractured reservoir delineation using multicomponent seismic data. GeophysicalProspecting, 45, 39-64.