SS: The Future of Seismic Imaging; Reverse Time Migration and Full Wavefield Inversion - Subsalt Imaging Using TTI Reverse Time Migration
Authors: Huang Tony , Zhang Yu , Zhang Houzhu James , Young Jerry 
Published: July 2009

Abstract
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 mispositioning 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.

Introduction
Seismic imagers routinely apply TTI depth imaging technology to image structures that lie beneath dipping, anisotropic overburden in the Canadian Foothill (Vestrum and Vermeulen, 2004), North Sea (Hawkins et al, 2002) and Offshore West Africa (Ball, 1995). Until now, ray-based migration algorithms served as the only choice for TTI imaging, because upgrading ray-based imaging algorithms for TTI is straightforward and incurs minor additional computational cost. Unlike ray-based algorithms, TTI wave-base algorithms are difficult to formulate and their implementations are often unstable and computationally intensive. Unfortunately, ray-based algorithms perform poorly in comparison to wave-based algorithms in imaging structures beneath such complex overburden as the subsalt in the Gulf of Mexico. This impedes the use of TTI imaging for subsalt in the Gulf of Mexico. The lack of appropriate checkshots and offset VSPs that can be used to constraint anisotropic parameters further discourages the utilization of TTI imaging in the Deepwater Gulf of Mexico.