Z037 A 3D Wide-azimuth Field Test with Simultaneous Marine Sources

Transcription

1 Z037 3D Wide-azimuth Field Test with Simultaneous Marine Sources W.H. Dragoset* (WesternGeco), H. Li (WesternGeco), L. Cooper (WesternGeco), D. Eke (WesternGeco), J. Kapoor (WesternGeco), I. Moore (WesternGeco) & C. easley (WesternGeco) SUMMRY We acquired a small 3D wide-azimuth (WZ) survey using simultaneous dithered sources in a geologically complex area that had been previously acquired with a standard WZ configuration. The simultaneous-source data were processed without application of any explicit separation procedure. Various results were compared to the results from the standard WZ data set. ased on these comparisons, we concluded the following: 1) Velocity model building (based on angle gather quality) and 3D SRME multiple attenuation are possible with non-separated simultaneous-source data. 2) Migrated images for the two data sets were essentially identical, except that 3) because of better spatial sampling, the simultaneous source image had a better signal-to-noise ratio at depth.

2 Introduction Within the last five years, wide-azimuth (WZ) 3D marine surveys have become an accepted method of improving subsurface illumination and seismic data quality. lthough a wide variety of such survey designs are possible, they share a common trait: a constraint imposed by the relationship between the number of sources, boat speed, shot spacing, and record length. Eliminating that constraint would add a degree of freedom to WZ survey design, thereby enabling faster, more efficient surveys, better illumination, and improved data quality. Shooting marine sources simultaneously rather than sequentially is a method of relaxing that survey design constraint. n interfering marine source has traditionally been considered to be noise that must be removed in processing. However, easley et al. (1998) and ealsey (2008) showed that the interfering source could be separated in processing and hence used as signal. This approach exploits spatial separation of the sources and geometry-related filters such as DMO and prestack migration to accomplish the separation. nother approach to simultaneous impulsive sources considered recently by several authors has its roots in efforts to deal with seismic crew interference (Lynn et al. 1987; Haldorsen and Farmer 1989). s Lynn et al. stated mis-synchronization of the survey and noise sources gives rise to time misalignment of the crew noise when the data are sorted into common-midpoint (CMP) gathers for stacking. this misalignment is fundamental to our ability to suppress the noise. More recently, others have expanded upon these concepts and a number of field tests have been acquired and processed (erkhout et al. 2008; Hampson et al. 2008; Fromyr et al. 2008; Moore et al. 2008). This paper describes a small 3D WZ field test that was acquired in 2008 using two pairs of simultaneous sources. The test was performed in the Gulf of Mexico over an area that had been acquired earlier using a standard, four-sequential-source WZ survey design. This allowed a direct comparison of the final simultaneous-source (hereafter abbreviated as SimSrc) and sequential-source images. The purpose of the test was to evaluate SimSrc technology in a WZ setting. Specifically, we aimed to answer these questions: 1) How are key prestack steps in the data processing flow, such as velocity model building and multiple attenuation, affected by applying them to non-separated SimSrc data? 2) Can we achieve acceptable images using only a geometric filter (prestack migration in this case) to untangle the interference between the simultaneous sources? 3) How do the signal-to-noise ratios of the two final images compare? Following this introduction, we describe the SimSrc field test, examine the data processing results, compare them to the results from the sequential-source survey, and then discuss the three questions. Field test description The field test covered about six OCS blocks that were acquired as an eight-swath extension of a WesternGeco WZ survey, hereafter referred to as WZ 4 (Figure 1). The acquisition sequence was as follows: 1) The seismic crew sailed to the northeast, acquiring a standard WZ swath at the western edge of WZ 4. 2) t the boundary between WZ 4 and WZ 2 (a survey previously acquired), the crew switched over to simultaneous shooting and recorded an additional 25 km. 3) The crew turned around and acquired another SimSrc swath while sailing to the southwest. 4) t the boundary between surveys, the crew switched back to standard acquisition and continued on, recording a WZ 4 swath as it sailed southwest. Figure 2 shows the details of the WZ acquisition configuration. The seismic crew consisted of three source-only boats and a cable-plus-source boat. Over the WZ 4 area, the sources fired in the sequence,, C, D. Over the WZ 2 area, the sources fired in the pattern shown in the figure. The older WZ 2 survey was acquired in the same fashion as the standard WZ 4 survey. Note that, compared to the WZ 4 lines, the SimSrc lines had twice as much seismic energy entering the subsurface and half the inline source interval. The firing times for the second source in each SimSrc pair were perturbed (dithered) in an incoherent pattern

3 within a 300-ms window that was designed for optimal source separability when using the separation algorithm of Moore et al. (2008). Figure 1 Plan for the 3D SimSrc WZ field test. Prior to the test, the field crew was shooting WZ 4 using a standard source configuration. The western-most WZ 4 lines were extended into the WZ 2 area (outlined by thick red lines) and shot with simultaneous sources (see Figure 2). The area covered by the test has a typical deepwater Gulf of Mexico salt geology. Firing sequence Nominal inline coordinates Source &C 0 m Source D Source &D 37.5 m 1200 m Source &C 75 m Source &D m Source C Etc m Source 660 m Source 1200 m 1200 m Figure 2 SimSrc test acquisition configuration. The WZ 4 lines (see Figure 1) were acquired with the four sources shooting sequentially. s the crew crossed into the area of the test, the shooting configuration was changed to pairs of simultaneous sources using the firing sequence shown in the table at the top-left of the figure. Processing results and comparisons ecause of the salt-related steep dips present in the data, the relatively large inline sampling interval (75 m per source pair), and the spatial aliasing-related assumptions of Moore et al. s algorithm (2008), good separation for this test was not possible using that method. Instead, we chose to process the data as if simultaneous sources were not present, thereby relying on only the imaging step to perform the source separation as described in easley et al. (1998). The migration of the SimSrc data was accomplished as follows: 1) Duplicate the recorded simultaneous-source shot records. 2) Put the geometry from one source in the trace headers of one copy and the geometry from the other source in the other copy. 3) pply the dither time statics to the copy with the dithered source geometry. 4) Merge the two data sets. 5) Migrate the merged data. Figure 3 shows a comparison of depth-migrated images for the SimSrc test and the WZ 2 survey. oth data sets had standard noise attenuation and wavelet processing steps applied, but neither had any multiple attenuation applied. They were then migrated with the WZ 2 velocity model using identical migration apertures. s expected, the SimSrc image has a better signal-to-noise ratio. side from that, the two images are comparable, which suggests that the imaging step was successful at directing the energy from the simultaneous sources to its proper location. In a production survey, one usually will not have access to a preexisting velocity model. One important question, then, is what impact will non-separated simultaneous sources have on the velocity-building procedure. We tested this by comparing angle gathers for both surveys (Figure 4). The gathers from the SimSrc data are of lesser quality, but nevertheless, they do contain clearly pickable events.

4 Depth (kft) Figure 3 Comparison of Kirchhoff prestack depth migrations for the standard WZ survey () and the 3D SimSrc test (). Note the improved signal strength in, best seen at the lower right. 10 Depth (kft) Figure 4 Kirchhoff prestack depth migration angle gathers. ) WZ 2 survey. ) SimSrc survey. The gathers from the WZ 2 survey are less noisy; nevertheless, the gathers from the SimSrc survey are pickable. 40 nother question is how well multiple attenuation will perform on non-separated SimSrc data. We tested this by applying a version of 3D SRME (Moore and Dragoset 2008) to the non-separated data using the same steps as those used for the migration, except that step 5 was to predict and subtract the multiples. We expected the massive stack involved in 3D SRME multiple prediction to remove the contribution of the source incoherencies to predicted multiples. Figure 5 shows the SimSrc result and that of applying the same 3D SRME algorithm to a similar line from the WZ 2 survey. The two results are of comparable quality. Conclusion We acquired a small 3D WZ survey using simultaneous dithered sources in a geologically complex area that had been acquired previously with a standard WZ configuration. The SimSrc data were processed without application of any explicit separation procedure. Various results were compared to the results from the standard WZ data set. ased on these comparisons, our answers to the questions posed in the introduction are: 1) Velocity model building (based on angle gather quality) and 3D SRME are possible with non-separated SimSrc data. ngle gather quality (and the quality of results of other prestack processes), however, might be a concern in other survey areas. 2) Migrated images for the two data sets were essentially identical, except that 3) because of better spatial sampling, the SimSrc image had a better signal-to-noise ratio at depth. We believe that our test is highly encouraging for SimSrc technology, especially because we expect that future advances in the acquisition of SimSrc data and explicit separation procedures will improve upon the results shown here.

5 3- Time (s) 5-7- C Figure 5 Unmigrated stacks before and after 3D SRME. rrows indicate the water-bottom multiple. ecause the two lines were recorded at different times, they are near each other, but are not identical. ) WZ 2 before SRME. ) SimSrc before SRME. C) WZ 2 after SRME. D) SimSrc after SRME. cknowledgments We acknowledge David Wilson, Tor Ommundsen, Morten Svendsen, and the crews of the Neptune, Kylie Williams, Odyssey, and Snapper for their contributions to the SimSrc field test. We also thank WesternGeco management for their support and for permission to publish. References easley, C., Chambers, R. and Jiang, Z. [1998] new look at simultaneous sources. 68th SEG International Exposition and Meeting, Expanded bstracts, easley, C.J. [2008] new look at marine simultaneous sources, The Leading Edge, 27, No. 7, erkhout,.j., lacquiere, G. and Verschuur, E. [2008] From simultaneous shooting to blended acquisition. 78th SEG International Exposition and Meeting, Expanded bstracts, Fromyr, E., Cambois, G., Loyd, R. and Kinkead, J. [2008] Flam Simultaneous Source Wide zimuth Test. 78th SEG International Exposition and Meeting, Expanded bstracts, Haldorsen, J. and Farmer, P. [1989] Suppression of high-energy noise using an alternative stacking procedure. Geophysics, 54, No. 2, Hampson, G., Stefani, J. and Herkenhoff, F. [2008] cquisition using simultaneous sources. The Leading Edge, 27, No. 7, Lynn, W., Doyle, M., Larner, K. nd Marschall, R. [1987] Experimental investigation of interference from other seismic crews. Geophysics, 52, No. 11, Moore, I. and Dragoset,. [2008] General surface multiple prediction: a flexible 3D SRME algorithm. First reak, 26, September, Moore, I., Dragoset,., Ommundsen, T., Wilson, D., Ward, C. and Eke, D. [2008] Simultaneous source separation using dithered sources. 78th SEG International Exposition and Meeting, Expanded bstracts, D