As interest in deep offshore exploration in West Africa, the Gulf of Mexico and Brazil continues to grow, clients have shown a enthusiastic reaction to CGG’s launch of its new A+ service for anisotropic processing of seismic data.
The long offsets on these surveys lead to very wide incidence angles, beyond standard assumptions of 30 degrees. The standard toolbox used by geophysicists is no longer adequate. CGG has therefore developed a unique methodology, known as A+, which modifies all the key stages in a data processing sequence by introducing anisotropy. The result is a clear improvement in the kinematic model (NMO 4th order, DMO, migration) and in the amplitudes (Newman or anelliptical geometrical spreading corrections).
Until recently, seismic data processing was solely based on the average velocities of geological layers. Following on from work by T. Alkhalifah and L. Thomsen, CGG has introduced the anellipticity parameter to optimize stack focusing. This more realistic description of kinematics extends the Dix formula to large offsets, making it possible to correct non-hyperbolic NMO and better focus reflectors dipping beyond 30° degrees. The reservoir image after migration is more accurate, particularly on strongly dipping events. In addition, the velocities calculated are closer to the geological reality.
 Angle stacks reveal high amplitudes beyond 30 degrees |
 Measured eta(color) surimposed on seismic show consistency with structures |
Now that a wider range of incidence angles can be exploited, interpreters have direct access to the elastic rock properties. By full taking into account the natural phenomenon of anisotropy, instead of trying to avoid this technical difficulty, seismic data can now be applied for reservoir characterization purposes. :
* AVO, AVA studies allow interpretation of DHI
* Elastic inversion can give access to porosity and saturation
* The anellipticity field helps a qualitative interpretation of geological variations on a regional scale.
Anistropic processing is now proposed on a routine basis at all CGG centers. The new processing platform in Redhill, UK can deliver sophisticated time imaging sequences combining Kirchhoff Prestack Migration and an integrated handling of anisotropy in ever shorter turnaround times. This means that geophysicists working on interactive workstations such as ChronoVista now have a good grasp of the information of direct interest to reservoir engineers and can therefore act on its quality.