In Reverse Time Migration incorporating Tilted Transverse Isotropy, known as TTI RTM, CGGVeritas has developed the algorithm of choice for wide azimuth. Its use in general production worldwide on narrow- and wide-azimuth data is providing considerable uplift in sub-salt imaging.
Wide-azimuth and multi-azimuth acquisition has highlighted the need for TTI algorithms in complex areas, as the greater range of azimuths available has shown TTI effects that were not apparent in narrow-azimuth data. Common Image Gathers (CIGs) with different azimuths in these areas cannot be flattened using a single isotropic, or VTI anisotropic, velocity model. Only a TTI model can flatten all the CIGs regardless of azimuth, with the resulting velocity models tending to produce more geologically viable sub-salt structures.
So as wide-azimuth data provides more reliable TTI parameters, TTI RTM is able to produce truly spectacular results. In particular the technique provides excellent focusing, illumination and positioning, with more accurate depth and amplitude control beneath and around complex salt bodies.
The superior sub-salt imaging provided by wide-azimuth acquisition, true 3D processing and TTI imaging has been well demonstrated in the Gulf of Mexico. CGGVeritas has acquired multi-client and proprietary data over several hundred Gulf of Mexico lease blocks using wide-azimuth techniques.
The Walker Ridge data library survey alone covers some 460 lease blocks. Several of the recent major discoveries in the Gulf of Mexico, such as Jack and Julia, Tonga, Heidelberg and BP's new elephant, Tiber, are covered by these surveys, which provide vastly superior images of the sub-salt structures than those previously available. Increasing amounts of this and proprietary data are being processed through TTI RTM, providing still further improvement.
In collaboration with BP, CGGVeritas has studied in detail the effects of the various rapidly developing depth imaging algorithms on the Puma and Mad Dog fields in the Green Canyon area of the Gulf of Mexico (Bowling et al 2009). Isotropic, VTI and TTI imaging were carried out sequentially for wide-azimuth towed-streamer data. This offered an excellent opportunity to understand the impact of different levels of anisotropy approximation, and why TTI RTM should be used for future wide-azimuth imaging.
Other TTI anisotropy and imaging studies in the Gulf of Mexico confirm these findings, such as the one performed for the Tonga field (T Huang et al 2009).
TTI RTM provides similar improvements in imaging elsewhere. Although it is more difficult to directly derive the anisotropy parameters, TTI RTM still has an impact on the imaging of narrow-azimuth surveys. Its current testing (Y Huang et al 2009) in the Santos Basin, offshore Brazil, after initial reprocessing with isotropic RTM, is producing promising results, despite the lack of publicly available well information in the area.
Ignoring the anisotropic effects in this area can incorrectly position salt flanks and distort the base of salt and pre-salt structures. Furthermore, the dip angles of some deep basins can reach more than 50 degrees. With such highly-dipping bedding, vertical transverse isotropy is significantly inaccurate.
The use of TTI RTM has also significantly improved the focusing and positioning of data from West Africa and the North Sea, and it is expected to be of benefit anywhere where imaging is required beneath dipping sedimentary layers. With the spread of wide-azimuth acquisition, the requirement for TTI RTM will also increase, in order to reap the full benefit of these datasets.
TTI RTM is part of the range of advanced imaging and true 3D wide-azimuth processing algorithms pioneered by CGGVeritas. These technologies, along with many others, are a key part of geovation, the unified CGGVeritas seismic processing & imaging platform.
Geovation is a significant consolidation of seismic data processing technology for CGGVeritas. It will be the industry reference in terms of geophysical technology, advanced interactive features and efficient processing of ever increasing wide-azimuth data volumes.
Advanced imaging, including TTI RTM, remains a research priority for CGGVeritas, with particular emphasis on the derivation of TTI parameters from wide-azimuth data, and extending full wavefield inversion for TTI media.
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