Scott Michell

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.

Comparisons of adaptive subtraction methods for multiple attenuation

Coherent noise may be removed from seismic data by first making an approximate model of the noise, then producing an even better estimation of the noise by adaptively matching the modeled noise to the data. This modified model of the noise may then be subtracted from the data, eliminating most coherent noise. The success of this approach depends both on how well the initial model matches the true noise and the success of the adaptive matching in modifying the initial noise prediction to match the true noise. The adaptive matching step is complicated by the presence of the signal and other noise in the data. In this article, the noise of interest is surface-related multiples, although other types of coherent noise may be removed with this approach.

Dual-azimuth versus wide-azimuth technology as applied in subsalt imaging of Mad Dog Field—a case study

Deepwater subsalt discoveries along the Sigsbee Escarpment in the Gulf of Mexico are approaching 10 years old. Some of these fields have begun production, and as we have drilled more wells into the subsalt we have learned that there are still uncertainties in our structural maps. Distortions due to salt overburden and noise from multiples reduce our ability to accurately map structures and the detailed faulting which is required for optimal production of these fields. The industry needs to find better ways to use seismic data to reduce our subsurface uncertainty. This article is a case study of the progress made at the BP-operated Mad Dog Field.

Investigation of vendor demultiple technology for complex subsalt geology

A 2-D elastic dataset over a complex geologic model was used to investigate and benchmark demultiple processing capabilities at seven seismic processing vendors. Results show the free surface related multiple elimination (SRME) method to be superior to other methods. However, SRME did not fully solve the demultiple problem on this data. Radon methods were not an effective processing step. Other methods offer promise for extensions to 3-D, but at the risk of removing primaries. These results and the test dataset can be used by researchers to further improve subsalt demultiple algorithms.

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).

Processing of a novel deepwater, wide-azimuth node seismic survey

In the fall of 2005, BP commissioned Fairfield Industries to conduct a large, wide-azimuth survey over Atlantis Field in deepwater Gulf of Mexico. The purpose of the survey was wide-azimuth, subsalt imaging using P-waves. The seafloor topography is complex with scoured furrows at the base of the escarpment. A shallow salt body covers a large part of the survey area and rafts the seafloor at the escarpment, leading to very strong velocity contrasts. The water depth in the area varies from approximately 1300 m above the Sigsbee escarpment to 2200 m below it.

Case Study: A Large 3D Wide Azimuth Ocean Bottom Node Survey In Deepwater GOM.

In the fall of 2005 Fairfield Industries was the primary contractor of a large, wide azimuth survey (Ross and Beaudoin, 2006) for BP over the Atlantis field in the deepwater GOM. The water depth in the area varied from approximately 1400 metres to 2300 metres. The survey used 902 four-component ocean bottom nodes (Mitchell and Grisham, 2006), which BP commissioned Fairfield to build. The purpose of the survey was wide azimuth, subsalt imaging using P-waves (Beaudoin and Michell, 2006).

Taking advantage of dual‐azimuth analysis for model building and imaging over Mad Dog

We describe the advantages of using dual-azimuth streamer data for complex model building and imaging in the Gulf of Mexico Deep Water. We iteratively image two datasets separately while building a unique common velocity model. Tomography and salt interpretation can take advantage of two illumination patterns that complement each other while each migration is performed along the native shooting direction. The results show improvements compared to the monoazimuth exploration vintage image and provides a step change needed for the field development.

Footprint Analysis of Land and TZ Acquisition Geometries Using Synthetic Data

Summary 3-D seismic methods are increasingly being used to solve subtle problems for reservoir characterization and development. An increased seismic resolution and accuracy of the seismic amplitudes are the primary goals of these 3-D surveys. Seismic resolution and amplitude fidelity could be affected by acquisition footprints, which are the artifacts introduced in the final results due to the 3D acquisition geometry. In this paper we analyze the acquisition footprints using synthetic data. Several 3D geometries, currently used on land and transition zone are analyzed and compared in this study. The geometries include crossline spread, brick pattern, diagonal shooting, bin fractionation, swath, patch and random geometry. The preliminary results of this ongoing study show that most of the acquisition footprints are generated by the residual coherent noise leaked into the stacking process.

Refining 3‐D velocity models for depth migration using tomography: Application to rapid permafrost variations in Alaska's North Slope transition zone province

The presence of permafrost near the surface creates special problems for accurate imaging of seismic data. Permafrost thickness can exceed 2000 ft (600 m) onshore and completely disappear offshore, and the high contrast between permafrost velocity and unfrozen sediment velocity produces severe distortions in reflections observed in seismic data. Images from time migration have false structures and positioning errors that compromise their quality and increase uncertainty. In this type of environment 3-D depth migration should be strongly considered. In this study we use tomography to construct the velocity model of the near-surface. Tomography is an ideal tool for this estimation problem where refraction information is limited or completely absent due to the decrease of velocity with depth, and where lack of short offsets, irregular offset sampling and multiple azimuths compromise velocity resolution near the surface. The image obtained with velocities from tomography attenuates false structure introduced by the permafrost. This image is comparable to an alternative image obtained with a velocity field built using well and seismic information.