All PGS seismic vessels are equipped with steerable source sub-arrays. These minimize lateral deviations in source position and create better-conditioned 4D baseline and monitor surveys.
- More uniform separation of individual sub-arrays improves the consistency of the emitted source wavefield from each shot
- Each source sub-array can be independently steered with unmatched accuracy, for 4D reservoir monitoring and CCS projects
- Assists source-position control during wide-tow multi-source acquisition
- Controls up to six sources with <450 m overall lateral separation
- Enables highly efficient acquisition and high-resolution near-surface imaging
Highly Repeatable 4D Monitoring Survey Geometry
Steerable source systems improve survey efficiency and increase 4D repeatability by minimizing the effects of variable currents on the acquisition geometry. Each sub-array can adjust to maintain survey pre-plot lines while the vessel optimizes streamer spread locations. The maximum lateral source correction is up to ±4 m from the natural towing position, and the mean lateral source position error from the target trajectory is less than ±2 m.
Computer-controlled adjustable wings on each source sub-array work together to minimize the lateral position error, while also maintaining sub-array separation. Positional data from the GPS and acoustic network sensors deployed on each sub-array are compared with a predefined source pre-plot and a lateral positioning error is computed by the onboard navigation system. This positional error is communicated to the source-steering system, which then computes the required wing angles to steer the source into the correct position. The graphs below are an illustration of position performance for a pre-plot scenario with the source steering system disabled and enabled.
An illustration of the lateral source positioning error with active source steering (dark blue curve) and without source steering (light blue curve).
The display below shows a real 4D monitor survey vessel pre-plot, where the color scale represents the average lateral-source-position error controlled by a prototype source steering system used in 2014 (now improved). The inline source position error is influenced by the source-firing-time error at each baseline shot position and is usually small. For modern PGS 4D surveys, more than 90% of radial shot-position errors are typically less than 3 m.
4D monitor survey shot positions color-coded by the crossline source position error from the 4D baseline shot positions.
The figure below depicts a schematic representation of three sub-arrays with the PGS source steering solution on the front of each float. Two relative GPS (RGPS) towers are mounted on each float. A near-field hydrophone mounted above each mounting location along the source keel is complemented by seven depth sensors along each keel. When combined with the PGS eBird solution for the lateral steering and depth control of each multisensor GeoStreamer, this offers unbeatable precision and repeatability of all source and receiver positioning.
A standard PGS source array with three independently steerable sub-arrays.
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