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.
The PGS Source Steering System (SSS) has been in use for over a decade and is regarded as best-in-class. It provides lateral steering control of up to ± 40 m, depending on vessel-source towing distance, with a calibrated accuracy of ± 2 m.
Benefits of Source Steering
- 4D repeatability for more accurate production monitoring
- More robust source signature for all surveys (2D and 3D) as the signal is closer to pre-plot design
- Increased operational efficiency during deployment and recovery
- Reduced infill as each sub-array can be steered individually and adjusted to compensate for current changes.
4D, or time-lapse, seismic surveys consist of a series of 3D monitor surveys that aim to repeat an 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 only means of controlling the lateral source positions is by moving the vessel that is towing the source array. As the source is normally towed a few hundred meters behind the vessel, and its position is affected by currents and local sea conditions, it is not possible to accurately control the lateral position of the source solely by steering the vessel.
The PGS source steering system dramatically increases positional accuracy. 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 deviation, while also maintaining the desired array separation.
Positional data from the GPS and acoustics network are compared against a predefined source pre-plot, to compute a lateral positioning error using the onboard navigation system. This position error is communicated to the Source Steering System which then calculates the 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.