The proportion of deepwater acreage in oil company portfolios increased considerably over the past decade. New subsurface imaging challenges were encountered and required technology breakthroughs to meet business objectives. In many of these deepwater areas, the imaging challenges were increased due to complex overburden that distorted the seismic signal at the reservoir. Additionally, the requirement to extract more resources from mature basins put increased demand on seismic data quality requirements for field development and production. Conventional narrow azimuth marine seismic acquisition was reaching a technical limit in these environments and new approaches were necessary. Both towed streamer and ocean bottom seismic solutions were required to improve reservoir imaging along the value chain, from exploration to production and from shallow to deep water. Wide azimuth techniques, combined with state of the art processing, velocity modeling and depth migration became the tools of choice for seismic surveying to address the emerging subsurface imaging challenges Success in field trials led to early business uptake. New technology development, both in acquisition equipment and processing algorithms, combined with innovations in survey design and operational efficiency, enabled at-scale implementation. As a result, the last five years has seen considerable wide azimuth data coverage in some major marine basins by both operators and contractors. The momentum continues to build with the increased use of wide azimuth methods opening up new opportunities and spurring new technology development. Introduction Since 2005 there have been significant developments in marine wide azimuth seismic acquisition which are applicable to a range of subsurface imaging challenges across the value chain from exploration to production. BP has been an industry leader in this area of technology across a spectrum of activity as described by Barley & Summers (2007). At-scale field trials of towed streamer and ocean bottom techniques over fields in a number of basins proved the concept of new robust acquisition designs. This led to widespread application of these methods and enabled illumination of reservoirs previously invisible using conventional methods. The ability to model and analyse cost effective and appropriate acquisition designs along with learning from application has developed considerably in recent years. As such, the scale of implementation across the industry has had considerable business impact. The key to effective usage is the ability to understand the relative merits of different acquisition techniques to a particular business problem and evaluate parameterization for that situation.
BP's exploration success in deepwater Block 31, offshore Angola, has been driven by conventional narrow-azimuth 3D seismic data coupled with the latest available imaging algorithms. However limitations in these data are now apparent and the data is deemed insufficient for the appraisal and development of the subsalt discoveries in the western part of the block. 3D acoustic finite-difference modelling was applied to Block 31 to evaluate the potential data quality uplift from a wide-azimuth towed streamer (WATS) survey. Results showed that a significant improvement in data quality is possible. The modelling also investigated key acquisition variables (acquisition direction, sail line separation, number of tiles, cable length) to arrive at a solution that optimized both data quality and cost. Acquisition of this survey began in December 2008 and it is expected to complete in August 2009. This is the first WATS seismic survey outside the Gulf of Mexico and the first in Angola.
SUMMARY Magnetic and seismic refraction investigations of two areas on the outcrop of the Whin Sill in western Northumberland with obscure geological relationships have served to clarify these relationships. In the area east of Great Chesters, a 0.9 km wide gap in outcrop of the sill is shown to have been produced by a channel eroded 30 m into the sill and filled by glacial drift. In the area of Limestone Corner the transgressive nature of the sill is demonstrated, but the two leaves of the sill are shown not to be connected by a continuous dyke.
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