Marine seismic surveys with enhanced azimuth coverage: Lessons in survey design and acquisition

摘要

The azimuth from the source to the receiver in a marine seismic survey acquired with towed streamers is by nature usually close to the sail-line direction. Deviations in this recording azimuth, such as those caused by ocean currents, diminish the quality of the survey results so a substantial effort is made to minimize the damage. Conventional streamer surveys are now called narrow azimuth, or NAZ, even though the same thing was formerly called wide azimuth, and the quality of a "constant" offset (minimal) data set from such a survey has been debated (Padhi and Holley, 1997). There recently have been several surveys designed to increase the multiplicity of azimuths by sailing in multiple directions, called multi-azimuth or MAZ (Hegna and Gaus, 2002); by deploying sources on multiple vessels to widen the effective receiver array, which is known as wide azimuth or WAZ (Sukup, 2002); and by using a combination of both MAZ and WAZ, which is rich azimuth or RAZ (Howard, 2004). In 2006, WesternGeco acquired a RAZ survey for BHP Billiton, Hess Corporation, and Repsol YPF over Shenzi, a subsalt discovery in the Gulf of Mexico centered on Green Canyon block 653 (Figure 1). At the 2006 SEG Annual Meeting, fully processed images from a MAZ survey in the Nile Delta (Keggin et al., 2006), fast-track images from a WAZ survey at Mad Dog (Michell et al., 2006) and fast-track images from the RAZ survey at Shenzi (Howard and Moldoveanu, 2006) were compared to images from previous NAZ surveys. These surveys had a multiplicity in azimuth of 6, 16, and 18, along with increases in trace density of about 600%, 800%, and 600%, respectively. While it is possible the improvements shown in every case are from the increases in trace density, it will be taken here as a given that azimuth multiplicity can be considered as enhanced coverage, in contrast to merely allowing the azimuths to vary over a wider range. A companion paper (Howard et al., 2007) focuses on the studies that led to the Shenzi survey and documents some of the results. This paper further documents those results and offers some observations about these methods.