Coil Shooting

3D marine seismic data have traditionally been acquired by a vessel sailing in a series of parallel straight lines. This configuration suffers from an inherent problem in that the seismic raypaths are aligned predominantly in one direction. In the presence of complex geology, ray bending can leave portions of the subsurface untouched by seismic waves and only a narrow range of source-receiver azimuths is recorded. This led to the development of wide-azimuth (WAZ), rich-azimuth (RAZ) and multi-azimuth (MAZ) acquisition configurations, which deliver better illumination of the subsurface and higher signal-to-noise ratio in challenging imaging areas such as beneath complex salt bodies.

Coil Shooting single-vessel full-azimuth (FAZ) acquisition is a technique which extends conventional multi- and wide-azimuth survey capabilities by acquiring marine seismic data using a single vessel sailing in a series of overlapping, continuously linked circles. This delivers a greater range of azimuths and offsets than parallel WAZ geometries, and there are no non-productive intervals for line changes. The WesternGeco application of this technique is enabled by the Q-Marine point-receiver marine seismic system:

• The Q-marine system’s fin-steering devices accurately control the depth and lateral position of the streamers.
• A fully braced acoustic positioning network provides accurate positioning information for all in-sea equipment.
• Single-sensor recording and proven noise attenuation methods handle the full range of observed crossflow noise without harming signal fidelity.

MAZ and WAZ are proven, successful methods of imaging beneath salt and basalt, and other complex geological situations. These surveys involve a number of vessels offset from each other, some towing sources, others towing sources and streamers. The result is better target illumination thanks to greater azimuthal coverage and higher signal-to-noise ratios.

Typically, WAZ surveys are conducted using three or four vessels, each traveling in parallel lines. However, while conducting a project in the Gulf of Mexico, WesternGeco achieved high-quality full-azimuth recordings with a single vessel. WesternGeco continues to lead the way, streamlining and enhancing techniques with client needs in mind. In the second half of 2010 WesternGeco added a further advanced technique of Coil Shooting—Dual Coil Shooting. This technique uses multiple source and receiver vessels sailing in interlinked circles and offers ultra-long offset data as well as full-azimuth data.