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`Ex. PGS 1080
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`EX. PGS 1080
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`V021
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`4D SEISMIC REPEATABILITY OVER THE GULLFAKS
`FIELD — SOURCE AND RECEIVER POSITIONING ISSUES
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`_
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`JAMES KEGGIN and BJZRN OLAV EKREN
`Statoi/ and BP R & D Al/iance, Rotvo/l, Postuttak, 7005 Trondheim, Norway
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`Introduction
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`Reservoir monitoring using repeat 3D surveys is potentially a very powerful tool for
`identifying remaining reserves in existing and new fields. Today the method is in its infancy and
`is, as yet, not fully proven. The success of the method will depend, amongst other factors, upon
`the quality and repeatability of our 3D datasets. The challenge for the geophysicist is to ensure
`that the change in seismic response due to changes in the reservoir rock properties is greater than
`the inherent errors and lack of repeatability of our seismic data.
`In a perfect world, repeat
`seismic data would be acquired with identical equipment under identical environmental
`conditions. Since these ideal requirements are never met, it is essential that we understand the
`magnitude of any errors that might be introduced in our data. This study examines the effect of
`navigation errors and the effect of mis-matched source-receiver azimuths on marine seismic
`repeatability. The work was carried out as part of a reservoir monitoring project on the Gullfaks
`field in the Norwegian North Sea. 3D datasets from 1985 and 1995 were used.
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`Navigation errors
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`If we are to use the 4D seismic method on our existing fields, we will almost certainly be
`using data from existing 3D surveys. Although today's positioning systems are usually accurate
`to a few metres, significant errors are to be found on older datasets. Before the advent of GPS,
`active tailbuoys and acoustic positioning systems in the late 1981)'s, errors of over 1(')(')m on the
`far offsets and 20m on the near offsets were common.
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`To examine the effect of positioning errors on pie-stack seismic data, we extracted traces
`from a 1985 and a 1995 3D dataset that had nearly identical source and receiver locations.
`These traces were from an area where we expected no change in rock properties between 1985
`and 1995. Figure 1 shows the near offset cable position of a matching pair of shot records (the
`solid line '85 port cable matches the hashed line '95 inner port cable). Corresponding source and
`receiver positions are within 10m of each other. The traces from these partial shot records are
`shown in Figure 2. The 1985 shot was then subtracted from the adjacent 1995 port side shots to
`illustrate mispositioning. The surrounding 1995 shot records are shown in Figure 3 and the
`resulting difference data are shown in Figure 4. The amplitudes of the difference data were
`measured over a large window and displayed graphically in Figures 5 and 6.
`It is clear that the
`best match is occurring where expected. This suggests that both navigation datasets are free
`from large errors.
`It is also comforting to see that the two datasets are very similar. Most
`striking of all is the sensitivity of the difference plot to the simulated mispositioning of the 1985
`data. A 50m bulk shift in position produces a difference amplitude of the same magnitude as
`the input. Similar experiments on far offset data showed cross-line positional errors of over
`50m. Given that errors of 5(lm and Over are known to exist,
`it will be impossible to produce
`meaningful pre—stack difference data that can be processed and interpreted. This sensitivity to
`positioning error will be directly related to geological complexity - these data were shot over an
`area of rapidly varying geology. Clearly,
`in areas of simple layercake geology, the difference
`surface shown in Figure 5 will probably have a much broader minimum.
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`
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`EAGE Winter Symposium, Reservoir Geophysics: The Road Ahead — Venice Lido, Italy, 27 » 30 October 1996
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`Ex. PGS 1080-1
`EX. PGS 1080-1
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`Azimuthal variations between two datasets.
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`Hidden in every post stack volume and every fold of coverage map are the shot-receiver
`azimuths of the pre-stack traces. Although a pair of repeated CMPs may have identical fold and
`offset distribution, they will almost certainly contain traces with different azimuths (assuming
`the data has been recorded with surface towed cables). The key questions are: How do mis-
`matched azimuths effect repeatability ? How big are these effects ? and can these effects be
`compensated for in processing ? All of these issues will be discussed using data before and after
`stack. An extreme case of this problem is shown in Figure 7 where 1985 data with very small
`feathering angles are compared with undershoot data from 1995. Since our repeat seismic
`surveys will be shot over producing fields, the undershootin g of surface obstructions will cause
`widespread problems. The degree to which we can correct for mis-matched azimuths will be a
`key factor when designing acquisition techniques for 4D seismic. If we use fixed receiver
`techniques,
`it will be possible to repeat the location of shot and receiver as well as CMP
`position. Fixed receiver acquisition schemes (seabed cable or vertical cable for example) will be
`much less troubled by surface obstructions than the conventional surface tow method.
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`Conclusions
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`1. Positioning errors cause significant errors on pie-stack difference data. These large errors
`suggest that pie—stack difference data cannot be used in a meaningful way on older 3D datasets.
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`2. Azimuthal differences cause significant errors on pre-stack difference data. These are
`difficult to eliminate during processing.
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`3. Although the pre-stack differenced data are extremely sensitive to source and receiver
`position, it should still be possible to draw sensible conclusions from repeat datasets by using
`other interpretation methods. This point has been well illustrated in several case histories (eg
`Watts et. a1. 1995).
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`4. For this datset, the repeatability errors introduced as a result of mis-matched source and
`receiver coordinates are greater that those introduced by differing acquisition hardware (source
`signature, cable response, recording systems).
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`5. The problems of inaccurate positioning and azimuth mis-match will be most acute for
`surface towed cable data. Fixed receiver techniques (seabed cable for example) should not
`suffer from these problems provided that the source and receiver positions are accurately
`measured. If other problems such as stronger multiples, asymmetric raypaths, coupling effects
`and adequate receiver coverage can be overcome, the fixed receiver type of acquisition looks
`very attractive for repeat 3D surveys.
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`Acknowledgments.
`Thanks to Ian Jack, Andy Morton and Neil Philip for their advice, criticism and practical help.
`We thank the partnership of Statoil, Saga and Norsk Hydro for releasing these data for
`publication.
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`References:
`
`Manin, Boucquacrt, Reghnaudin, chnault & Thevcnot, Recent developments in source and receiver positioning,
`First Break Vol 6, No. 6, June 1988.
`Mark Houston, Cable positioning using compasses, tuitbuoys and acoustic devices. 53rd Mtg EAEG abstracts
`1991.
`
`Manin, Can we improve binning from the study of seismic inutge.v?, 48th Mtg EAEG abstracts 1986.
`Watts, Jizba, Gawith, Gutteridge, Reservoir monitoring of the Magnusfield through. 4D time-lapse seismic analysis,
`Presented 7th EAPG annual conference June 1995.
`
`David E Lurnley, Seismic time—lapse numitoring of subsurface fluidflow, PhD Thesis, Stszord Exploration Project,
`Dec 1995.
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`Ex. PGS 1080-2
`EX. PGS 1080-2
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