Source Steering System
The ability to re-occupy a predefined source location is particularly relevant for 4D surveys, where repeatability is crucial to detect and measure small changes in the seismic response as a result of oil and gas extraction from the reservoir. It also has benefits for standard 2D and 3D acquisition.
Time-lapse seismic surveys consist of a series of 3D monitor surveys that aim to repeat the original baseline survey in order to measure very small production-related changes in the reservoir. Repeatability is crucial and source locations must be established as accurately as possible.
Without a source steering system, the sole means of controlling the lateral source positions is by moving the vessel that is towing the source array. The source is normally towed a few hundred meters behind the vessel, and its position is affected by currents and local sea conditions. As a result, the vessel alone is not able to accurately control the lateral position of the source.
The PGS source steering system dramatically reduces positional error. Computer-controlled adjustable wings are installed on each source sub-array. In a dual-source configuration with six sub-arrays there are typically six wings. These work together to minimize the lateral position error, while also maintaining the desired array separation.
Positional data from the GPS and acoustics network are compared against a predefined source pre-plot, then a lateral positioning error is computed by the onboard navigation system. This position error is communicated to the Source Steering System which then computes the required wing angles to steer the source into the correct position.
The system has benefits for regular 2D and 3D surveys as well, particularly when the source is towed deeper. The controlled source arrays are more stable and towed closer to the pre-plot. This produces a more robust source signature output. Deployment and recovery are more efficient, since each array can be steered individually, and the sub-array separations can be adjusted online to compensate for current changes.