Velocity model building for tilted orthorhombic depth imaging

Recently, tilted transverse isotropic (TTI) imaging has become a standard practice in deep water Gulf of Mexico (GOM) to resolve the anisotropic effects of wave propagation in salt-withdrawal mini-basins. Compared with isotropic and vertical transverse isotropic (VTI) imaging, TTI prestack depth imaging generally provides flatter common image gathers (CIGs) for WAZ data, improves image focusing, and significantly reduces well/seismic misties (Huang et al., 2008). However, the presence of significant tectonic stress or uneven stress can cause fractures in thin-bed layers, which results in a directional velocity variation for seismic wave propagation, or azimuthal anisotropy (Lynn and Michelena, 2011). 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 (T-ORT), is needed to cope with azimuthal velocity variation in these complex geological settings. Wide-azimuth (WAZ) data has been acquired in most of the deep water blocks in the GOM. Combining WAZ data with existing sets of narrow azimuth data provides abundant azimuthal information for deriving the parameters of a tilted orthorhombic model.