Acquisition of Highly Repeatable Seismic Data Using Active Streamer Steering
Tony Curtis1, Patrick Smith1, Leendert Combee1, Willy Olafsen1
(1) WesternGeco, Gatwick, United Kingdom
Time-lapse seismic has emerged as a key technology in reservoir management. The success of time-lapse seismic depends upon the repeatability of data, including precise repetition of acquisition geometry. For surveys acquired using conventional towed marine systems this will rarely be achieved, because variable sea currents affect the dynamic behavior and position of streamers.
Since early 2001, vessels equipped with a system for streamer steering have been in operation. The streamer control system contains three components, a) a full streamer acoustic positioning system, b) streamer steering devices, and c) a spread-control module within the navigation system. These components work in a closed-loop manner. Highly accurate positions computed along the streamers are compared with desired positions by the spread control module. Commands that control the streamer steering are then sent to the steering devices, which are a novel type of "bird" having two independent wings. By introducing a small splay between the wings, the device will roll, and in addition to the vertical force needed for depth keeping, will generate a lateral force which moves the streamer towards the desired position.
Data acquired in the North Sea demonstrate the effectiveness of streamer steering. Two adjacent 6-streamer swaths were re-acquired some days later using consistent steering control. Analysis of the navigation data shows streamer re-positioning was achieved within +/-10m. The seismic data were processed using independent deterministic processes, with no cross-equalization between the baseline and repeat data. Difference displays with an absence of visible signal energy confirm the repeatability of the seismic data.