Abstract
A method for determining the ocean current normal to and tangentially to a seismic streamer is introduced. High resolution real time estimation of ocean currents is valuable for optimal lateral steering of seismic streamers. With optimal steering, crossflow noise caused can be reduced. Additionally, improved steering can reduce line-change times. Further, estimated ocean currents can contribute to world current oceanographic models.
A PID controller approach is used in the estimation process. The controller adjusts the ocean current normal to the streamer tangent for minimizing offsets between simulated and measured streamer positions. The streamer model is 3-dimensional and incorporates cable stretch, tension, steering devices and tailbuoy.
The current estimation method has been tested on 50 line-change data from a seismic survey. Especially on line-changes, cable steering, and hence current estimation, contribute to steering efficiency. Line-changes are time consuming, typically 3 or more hours. Efficient lateral steering could reduce this time, resulting in time and cost savings for seismic surveys. Unfortunately, steering wing angles were not available in the study data set; hence the estimation assumes zero wing angles. However, only large currents could account for the magnitude of streamer displacement relative to the zero current model result. This implies that the algorithm is capturing a significant current effect and translating it into current force quite well, as simulations given the estimated currents show good agreement with the actual streamer positions. Computational time of the algorithm is much less than real time. Hence current predictions can be made available to the streamer control system with little delay. Further, streamer prediction can be performed using the simulator with the updated ocean currents. Additionally, the estimated current can be transmitted in near real time to oceanographic research institutes.
To the authors' knowledge the present approach of current estimation has not been attempted previously. Since there exist small scale local variations in current, measuring the current only at the vessel and some stationary positions has limited value. As the current estimation algorithm can be run continuously in real time, it would provide more accurate and higher resolution information to the steering system, facilitating improved steering efficiency.
