Improve Subsalt Image with Wide Azimuth Data in the Gulf of Mexico

Lin, Dechun¹

¹US Imaging, CGGVeritas, Houston, TX.

Since the first wide azimuth (WAZ) dataset was acquired in the Mad Dog area, a large number of WAZ surveys have been shot in the Gulf of Mexico (GOM), generating a significant wave of excitement in the seismic industry. It is generally observed that WAZ data have better sub-surface illumination and enhanced multiple suppression. However, our experiences in processing WAZ data tell us that there are a lot of remaining problems that need to be addressed during the processing phase.First, we still see a lot of illumination holes in WAZ data. Due to complex salt geometry, there are numerous sub-salt regions that have limited illumination from a WAZ dataset. We can minimize such illumination holes by adding narrow azimuth (NAZ) datasets to the processing flow.Second, multiples are a lingering problem. The stacking power of WAZ data is in some cases unable to sufficiently suppress the multiples. Nonetheless, they can be effectively attenuated by WAZ 3D SRME. Third, an accurate salt geometry is still the key to a clear sub-salt image. Even with the sizeable amount of data from a WAZ survey, we could still get a poor image, or even worse, an incorrect image underneath a salt body if the salt geometry is inaccurate. In some cases, we have to try several salt scenarios to determine the right one. This involves iterative migrations and salt interpretations, or iterative imaging. To be effective, iterative imaging requires a fast and robust migration engine, a good interpretation tool, and expertise in both processing and interpretation. Finally, we need to utilize the latest technology, such as RTM and TTI imaging, to take full advantage of WAZ data. Due to its capability of imaging steep dips and capturing multi-arrival ray paths, RTM is well-suited for imaging salt geometry and sub-salt events. Furthermore, it could be cheaper to run RTM than Kirchhoff migration on WAZ data. TTI imaging, including TTI RTM, has recently gained momentum in imaging deep water GOM. It helps remove some of the velocity artifacts that are observed in the bottom of deep basins after isotropic/VTI velocity tomographic updates. Also, it can more accurately position the salt boundaries, and thus produce superior sub-salt images. In short, WAZ data certainly provides a good opportunity for better imaging the sub-salt structures in deep water areas of the GOM. Even so, quality data processing and high-end imaging technologies are still essential in producing the best possible WAZ image.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil