Bertram Nolte

Subsalt 3D VSP imaging at Deimos Field in the deepwater Gulf of Mexico

At Deimos Field in the deepwater Gulf of Mexico, surface seismic imaging cannot fully image sedimentary structure near the steeply dipping base of salt. 3D vertical seismic profile (VSP) imaging was introduced to the deepwater GOM to improve seismic imaging of complex structural and stratigraphic features (Ray et al., 2003; Hornby et al., 2006) and recently has been used for imaging “blind spots” in surface seismic coverage (Hornby et al., 2005; Hornby et al., 2007). With this method, 3D surveys are conducted using a surface source vessel and downhole geophones. Here we investigate the use of 3D VSP imaging to complement the surface results with additional coverage beyond that of the surface seismic adjacent to the complex salt body.

Mad Dog TTIRTM: Better than Expected

Summary In 2009, BP initiated a complete reprocessing of the Mad Dog WATS (wide-azimuth towed-streamer) dataset using TTI (tilted transverse isotropy) Pre-stack Depth Migration (PSDM) technology developed by CGGVeritas. This project would build upon the 2008 VTI (vertical transverse isotropy) model built by CGGVeritas by improving key areas that were imaged poorly. The TTI reprocessing was extremely successful. In fact, the uplift for the subsalt target area was qualitatively larger than the uplift observed between isotropic and VTI. Mad Dog TTI reprocessing further confirmed that by honoring the stratigraphic layering of the geologic structure, a TTI velocity model is a better approximation of the subsurface than both isotropic and VTI models. The Mad Dog WATS data combined with a previously recorded NATS (narrow azimuth towed streamer) dataset allowed the ability to detect and account for anisotropy in different azimuths, which is not possible with a single NATS survey. TTI reverse time migration (RTM) improved the ability to define salt geometry and thereby obtain a good subsalt image. TTI RTM should be considered for future imaging of fields with complex structures such as Mad Dog.

Gaining Insights About Imaging Uncertainty: TTI 3D Synthetic Case Study

Application of the WesternGeco uncertainty workflow to a TTI 3-D synthetic model produced by BP revealed new insights on the scale dependency of the anisotropic nullspace and its impact on structural uncertainty. This analysis was done by using tilted transverse isotropy (TTI) offset ray tracing through the model for an ocean-bottom station (OBS) mirror acquisition geometry: no synthetic seismic data were involved.

Application of one‐way wave‐equation migration in tilted coordinates to salt‐model building at Atlantis

We apply shot-record wave-equation migration in tilted coordinates to 3D OBS data from the Atlantis field in the Gulf of Mexico for the purpose of salt-model building. This technique produces images on which steeply-dipping salt flanks can be picked with confidence, which is not possible on images computed with conventional one-way waveequation migration because of its inherent steep-dip limitations.

A crossed‐dipole reciprocity “paradox”

The principle of seismic reciprocity states that the roles of sources and receivers in seismic experiments can be interchanged, and the same signal as a function of time will be recorded in both cases. In a multicomponent zero‐offset shear‐wave experiment, the sources and receivers are at the same location, and the reciprocal experiment is the same as the original. Reciprocity thus predicts that the recorded XY (inline source to crossline receiver) and YX (crossline source to inline receiver) sections should be identical. This should be true regardless of anisotropy, attenuation, or heterogeneity in the Earth.

Wide Azimuth Anisotropic Imaging At Tubular Bells Field In the Gulf of Mexico

BP, with co-owners Chevron and Hess, acquired a Wide Azimuth Towed Streamer (WATS) survey over the Tubular Bells field in the deepwater Gulf of Mexico to address challenges of imaging subsalt reservoirs. Finite-difference modeling shows that we need to incorporate anisotropy to properly image subsalt reservoirs at Tubular Bells. We present the processing and model building work flow; in particular, how we use Reverse Time Migration (RTM) to image steep dips that results in a better velocity model. The final migrated dataset provides a significant improvement in image quality over the conventional narrow azimuth (NATS) data, and is being used to reinterpret the area.