Since their introduction in the 1980s, multi-streamer marine 3D seismic surveys have been widely recognised as a key enabler for increasing drilling success rates during the exploration and development of offshore oil and gas fields.
Today, these surveys are typically acquired by a vessel equipped with between 8 and 16 streamers towed 50 to 100 m apart, each 3 to 8 km long. Each streamer contains hydrophone sensors, and spatial sampling of the data recorded along each streamer (inline) can be as fine as 3.125 m; however, the much greater distance between adjacent streamers means that sampling in the crossline direction can be 16–32 times sparser. Such coarsely sampled data cannot capture the whole 3D wave field, which limits its ability to accurately image the subsurface. While developments in seismic sources and sensors have improved the frequency bandwidth that can be input to the subsurface and subsequently recorded back at the surface, achieving truly 3D high-resolution images of complex subsurface structures also requires adequate spatial sampling in both the inline and crossline directions.
In 2012, WesternGeco publicised the launch of its IsoMetrix* marine seismic technology, which for the first time allows true measurement of 3D seismic wave fields recorded using towed streamers. This ability is the result of an far-reaching ten-year research and engineering program that has been the largest single engineering project ever undertaken by Schlumberger, the world’s leading provider of oilfield services. The new technology delivers high-fidelity point-receiver seismic data while overcoming spatial wavenumber bandwidth compromises that have limited previous marine seismic acquisition methods. The result is a reliable, continuous measurement of the full upgoing and downgoing notchless seismic wavefield sampled at a 6.25 m x 6.25 m point-receiver surface grid. This fine isometric sampling in both crossline and inline directions makes the data suitable for use in a wide variety of interpretation and modeling applications.
Isometric 3D sampling is enabled by the Nessie-6* point-receiver streamer system. This incorporates a new generation towed streamer design that combines measurements of wave field pressure and gradient—vertically and crossline. It uses point-receiver technology that combines hydrophones with calibrated point-receiver microelectromechanical system (MEMS) accelerometers that measure the full particle acceleration of the upgoing and downgoing seismic wavefield.
A new computer algorithm—which has been shown to be extremely robust in dealing with highly aliased data—performs simultaneous spatial reconstruction and receiver deghosting of the seismic pressure wavefield. The source ghost is addressed by the WesternGeco newly developed Delta calibrated marine broadband seismic source family of notchless seismic sources. The algorithm can compute the upgoing and downgoing separated wavefield at any desired position within a spread of streamers.
The step change in imaging is as profound as was the move from X-rays to full 3D scans in the world of medicine.
During August 2012, the 3D seismic vessel Western Pride, deploying eight full-length streamers in the North Sea, completed acquisition of its first commercial project on schedule. During September, commercial surveys were acquired for two other operators. The new technology has proved to be very robust during operations and is attracting considerable interest from oil and gas exploration and production companies of all sizes across West Africa and the rest of the world.