Elena Shoshitaishvili

Wide Azimuth Streamer Imaging of Mad Dog; Have We Solved the Subsalt Imaging Problem?

Summary Forming an image of the Mad Dog field is critical for the efficient development of the reservoirs. BP has applied a variety of depth migration methods on standard surface streamer data to improve the image. However, the severe velocity discontinuities due to the complex salt which is proximate to the rugged water bottom causes gaps in the illumination of the structure that can not be filled with conventional surface streamer data. In 2004 BP implemented a “first of its kind” marine wide azimuth streamer survey. Depth migrating the wide azimuth seismic data yielded a substantially improved image of the reservoirs. This paper discusses the motivation behind the seismic acquisition redesign and some of the challenges of processing wide azimuth surface streamer data.

Practical approach to joint imaging of multicomponent data

Recent advances in the ocean-bottom cable (OBC) acquisition technology allow recording of high quality multi-component data in the marine environment. These data have been used to image reservoirs obscured by gas clouds (e.g., Valhall) and reservoirs with low P -wave impedance contrast that are hard to see in conventional streamer data (e.g., Alba). Further benefits of using multicomponent data in exploration may come from analyzing P -wave ( PP ) and converted-wave ( PS ) data jointly to obtain more information about a reservoir than is available from PP data alone. Joint interpretation of multicomponent data is complicated by the fact that PP data are traditionally imaged in PP time and PS data are imaged in PS time. Thus, the PP and PS images have different vertical scales. To reconcile these scales, an interpreter has to identify events in both images that correspond to the same reflector and then stretch one of the images to match the other. The event identification is not always straightforward because some interfaces generate PP and PS reflections of the same polarity and other interfaces of the opposite polarity. Thus, there may not be a natural choice of troughs or peaks to correlate. To eliminate the need to stretch PP and PS images and to facilitate joint interpretation, we developed a methodology for joint imaging of multicomponent data in depth. We image PP and PS data in depth by anisotropic pre-stack depth migration. For PS data, our migration algorithm combines P -wave propagation from a source and S -wave propagation from a receiver to a reflection point. The algorithm also handles the OBC acquisition geometry. Prestack depth migration of multicomponent data requires a complete transversely isotropic (TI) velocity model for the subsurface. This model consists of four parameters: P -wave velocity, S -wave velocity, and the Thomsen anisotropy parameters, …

Advanced Subsalt Imaging and 3D Surface Multiple Attenuation in Atlantis: A Case Study

Atlantis sub-salt seismic image quality ranges from poor to unreliable over 60% of the anticipated areal extent of the field. Initial field development will focus on the betterimaged southern segments. Development of sub-salt reserves will be contingent on additional appraisal drilling and early field performance. Seismic data quality is limited due to the following: surface related multiple contamination; uncertainty in the sediment and salt velocity model; complex illumination due to the overlying salt geometry and salt fingers and a steeply dipping water bottom associated with the Sigsbee Escarpment. Recent seismic imaging efforts at Atlantis include the application of wavefield migration techniques, enhanced velocity model building, raytrace illumination studies and the application of 2D and 3D Surface Related Multiple Elimination (SRME).