Subsalt Imaging Using Tilted Orthorhombic Reverse Time Migration
Authors: Mothi Sabaresan , Huang Tony , Mcgill Patrick 
Published: May 2013

Abstract
In recent years, it has become the standard practice in deep water Gulf of Mexico (GOM) to perform seismic imaging assuming tilted transverse isotropy (TTI) symmetry to describe the anisotropic effects of wave propagation in salt-withdrawal mini-basins. When compared to isotropic and vertical transverse isotropic (VTI) imaging, TTI prestack depth imaging generally provides flatter common image gathers (CIGs) for wide azimuth data, improves image focusing, and significantly reduces well/seismic mis-ties. This anisotropy is thought to arise from the geometry of sedimentation processes, with the " tilt" applied by subsequent tectonic activity. However, the presence of significant tectonic stress or uneven stress can cause fractures in thin-bed layers, which results in a further directional velocity variation for seismic wave propagation, or azimuthal anisotropy around the bed normals. In these cases, the transverse isotropic assumption is insufficient to explain conflicting residual moveouts among CIGs of different azimuths from TTI imaging. A more general anisotropic model, tilted orthorhombic (TOR), is needed to cope with azimuthal velocity variation in these complex geological settings.
In this paper, we apply TOR model building and migration to an area in the Green Canyon area of central GOM with the aim of improving the subsalt image. Two orthogonal GOM surveys, one narrow azimuth towed streamer and one wide azimuth towed streamer, are used to derive both TTI and TOR models. With the TOR model, we observe improved gather flatness among azimuths, better well ties, and improved salt imaging - all of which lead to more accurate delineation of salt geometries and, consequently, better imaging beneath the salt. We infer that tilted orthorhombic provides better a representation of an overburden with fractures and uneven stress. Through improved overburden velocities, TOR reverse time migration produces better subsalt images.

Introduction
Considering available NATS surveys and the recent surge of WATS acquisition, the Gulf of Mexico (GOM) is now crisscrossed by abundant data. WATS data is often combined with existing NATS data to form Multi-Azimuth (MAZ) coverage that provides additional azimuthal information. This abundant information can improve subsalt illumination by providing better constraints for deriving accurate anisotropy parameters.
Pre-stack depth imaging workflows incorporating Tilted Transverse Isotropy (TTI) with MAZ are now common practice in deep water Gulf of Mexico for compensating for the anisotropic effects of salt-withdrawal mini-basins (Lynn and Michelina, 2011). TTI RTM (Zhang et al., 2011) improves image focusing, particularly along steeply dipping salt flanks and the subsalt region underneath mini-basins (Huang et al., 2009, and Bowling, 2010). However, TTI is a simplified representation of elastic media; it assumes the wave propagation speeds are the same in all directions in the transverse plane (Thomsen, 1986). In fact, this may be an oversimplification in some GOM areas with significantly uneven stress and fractures. Despite our best TTI-compliant practices in these areas, we observe inconsistent residual moveout in common images gathers (CIGs) along different azimuths.