The development of reverse time migration (RTM) and the availability of wide-azimuth data have significantly increased our ability to image subsalt. Much of this potential, however, remains to be developed by seismic imagers. One area for future development is the incorporation of anisotropy in subsalt imaging. Most anisotropic imaging involves vertical transverse isotropy (VTI), while tilted transverse isotropy (TTI) is generally overlooked. Shale that overlies the dipping salt flanks can cause TTI anisotropy issues. This type of geometry is common in the Deepwater Gulf of Mexico, particularly around salt-withdrawal minibasins. Ignoring the tilted symmetry not only causes image blurring and mis-positioning of the salt flank, but also degrades and distorts the base of salt and subsalt images. RTM for isotropic and VTI has been routinely used, but the applications of RTM in a 3D heterogeneous TTI medium is still in its infancy. This lag is the result of difficulties in numerical formulations for non-vertical symmetric axes and the subsequent instabilities. The TTI implementation also carries much higher computational costs than those of isotropic and VTI cases. With the demonstration of its benefits and the advances in computing power, the usage of TTI RTM is expected to increase significantly. Initial applications of TTI RTM used narrow-azimuth, towed-streamer data. The lack of azimuthal information limited the ability to derive the velocity and corresponding anisotropic parameters. More accurate TTI parameters were derived and the benefits of TTI imaging were obtained only when two orthogonal narrow-azimuth datasets were processed simultaneously. The advent of wide-azimuth data in the Deepwater Gulf of Mexico further opens the door for TTI imaging. This is because the wide-azimuth data contains more abundant azimuthal information than either narrow-azimuth or multiple narrow-azimuth datasets. Topics related to TTI RTM remain a focus within the seismic imaging community. Improving the derivation of TTI anisotropic parameters from wide-azimuth data and extending full wavefield inversion for TTI media are among the most active areas of study.
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