f Dual coil – long offset full azimuth towed streamer acquisition

Over the last twelve years significant advances have been made to mitigate for the limited azimuth range of towed streamer geometries and subsequently improve imaging in areas of high geological complexity, such as the Gulf of Mexico pre-salt provinces. These advances include acquiring the survey multiple times in different azimuths (MAZ) and acquiring the survey using multiple vessels (WAZ). More recently Coil Shooting has been deployed which utilizes a single towed streamer vessel following a circular pre-plot. However, even with these sophisticated surface geometries, recent experience has shown that the most complex of Gulf of Mexico geologies are still non-optimally imaged. Through a series of modeling efforts, using ray tracing and two-way wave equation methods, the WesternGeco technical team in Houston determined that to solve these most complex imaging problems a ultra-long offset full azimuth measurement was required. The same technical team used these new geometry requirements to develop a new acquisition technique of which Dual Coil is the result. The Dual Coil technique uses four vessels (two recording vessels and two source vessels), sailing in 12.5-km-diameter slightly offset circles. The source vessels sail at the tail of the respective recording spreads and the two recording vessels maintain 180 degrees separation. Ultra-long offsets are achieved by sources shooting across the coil (Moldoveanu and Kapoor 2009; Brice 2011). These circular “lines” are repeated in both directions to build up fold, offset distribution and azimuth distribution. At the time of writing, three dual coil surveys have been acquired in the Gulf of Mexico covering over 550 OCS blocks (13,000km 2). Each with different acquisition and imaging challenges. The final images will take over a year to complete using an intensive data processing flow including variable water velocity correction, 3D Surface Related Multiple Attenuation (SRME) and tilted transverse isotropy model building and imaging. However fast track images were generated on-board and delivered less than a week after completion of acquisition operations. Even using this preliminary model and Isotropic assumptions there is a clear improvement in imaging, when compared to the underlying WAZ image. As well as a general improvement in illumination and signal to noise, the sand beds terminating against the salt body are much easier to interpret. While the Dual Coil technique has been developed to solve specific imaging problems in the Gulf of Mexico associated with large complex salt bodies, there is no reason to think that it would not be appropriate for other difficult-to-image areas around the world such as where thick layers of basalt are present or where carbonate layers distort the ray-paths.