Zhou, Jinjuan (University of Houston) | Davydycheva, Sofia (Schlumberger) | Wang, Hanming (Schlumberger) | Habashy, Tarek (Schlumberger) | Frey, Mark (Schlumberger) | Barber, Tom (Schlumberger) | Abubakar, Aria (Schlumberger)
Seismic anisotropy refers to seismic waves traveling with different velocity at different propagation angle, usually in consolidated, shale-ornes areas such as in gulf in mexico and West Africa. Conventional isotropic (velocity independent of angle) pre-stack depth migration often produces mis-positioned and distorted images in these areas. Instead of using a single migration velocity parameter as in the isotropic depth migration, the anisotropic parameters)often referred to as delta and epsilon) estimated from seismic data with well controls.
Gaudin, Brenda (Shell Exploration and Production Company) | Mc Rae, Mark (Shell Exploration and Production Company) | Lepre, Laura (Shell Exploration and Production Company) | Stopin, Alexandre (Shell Exploration and Production Company)
Reducing uncertainty at Deimos, a subsalt discovery in the Mars basin, has been limited by the difficulty in making a definitive interpretation of the reservoirs. Based on illumination studies, 3D VSP and Ocean Bottom Seismic Wide Azimuth (OBS WAZ) data were acquired in 2007 to better illuminate the subsalt section of the reservoir. To obtain the full potential of these new data an anisotropic velocity model (suitable for all acquisition azimuths) is built. Data from past surveys shot in two different acquisition azimuths is used. Anisotropy is assumed to be vertical transverse isotropy (VTI). The parameter h (Alkhalifah and Tsvankin, 1995) was determined using high order Residual Move-Out (RMO) and the parameter (Thomsen, 1986) was determined using well to seismic depth tie. After the velocity and anisotropy update the prestack offset gathers showed improved flatness, and the depth shifts between the seismic and the wells greatly reduced. The salt bag (including recumbencies) for the entire area of interest was reinterpreted on migrated data using the anisotropic model. Sonic velocities (corrected for well deviation) were also used to update the velocity model.
This final anisotropic model was then used to migrate with a one-way anisotropic wave equation algorithm. The results showed a dramatic improvement in the positioning and continuity of the subsalt events. The step change in imaging subsalt is due to the elliptic part of the anisotropy. Using dual azimuth data helped for the illumination of the model. The availability of older surveys enables the project to gain valuable lead-time.