PGS acquired close to 20,000 sq km (7,700 sq miles) of dual-sensor towed streamer 4-D data on completion of the 25th GeoStreamer 4-D survey in May. The systems required to deliver superior 4-D results include identical steerable sources and steerable streamers, GeoStreamer recording technology, and onboard quality control and processing capabilities.
Each PGS vessel uses a dense acoustic network positioning system to link the precise location of the vessel and in-sea equipment with the locations of the sensors along each streamer for every shot. This acoustic positioning network is integrated with differential global positioning system units mounted on the vessel, each paravane and source sub-array (two GPS towers per sub-array) and the tail buoy at the rear of each streamer. Onboard the vessel the streamer positioning system is interfaced with a navigation command center that controls both the depth and lateral drift of each streamer and source sub-array.
The PGS source steering devices enable fully automated steering where each sub-array can move laterally up to about 40 m (130 ft) from the natural towing position. That is more than the width of a regulation soccer pitch. The solution is both powerful and precise. In calibrated testing, the mean lateral source position error from the target trajectory is less than 2 m (6.5 ft). The GeoStreamer steering solution for both depth and lateral steering control has been developed in partnership with Kongsberg. Attention was given to building a fully integrated low-diameter system that does not cause drag or turbulence effects at the bird location and is low-noise even under vessel line turning conditions. Streamer feathering can typically be controlled in an automated manner within about 3 degrees.
The use of dense GeoStreamer spreads that include overlap streamers maximize the available receiver traces for highly repeatable 4-D binning. A direct correlation is always observed between smaller combined source and receiver position error versus 4-D noise.
Ensuring highest detectability
These proprietary engineering innovations render the GeoStreamer seismic data acquired during any 4-D survey relatively insensitive to the many dynamic forces that traditionally mask subtle reservoir signals, thereby improving 4-D signal detectability.
Deep towing enables the sea state effects to be removed while maximizing signal bandwidth. Four-dimensional differencing of deghosted (“P-UP”) GeoStreamer baseline and monitor data contain no 4-D noise associated with dynamic sea surface effects, whereas 4-D noise will dominate the difference result if either/both the baseline or monitor surveys contain the ghosted (“P-DWN”) wavefield effects.
If the 4-D baseline survey has been acquired with hydrophone-only streamers, the ghost-free wavefield from a deep-towed 4-D GeoStreamer monitor survey can be accurately redatumed for backward compatibility to the hydrophone-only baseline data.
Variability in the emitted source wavefield is removed using shot-by-shot source signatures computed from nearfield hydrophones, standard on all PGS vessels. The use of flat GeoStreamer depth profiles ensures that the seismic wavefield has consistent spatial and temporal signal properties everywhere. This is important for any trace interpolation fidelity during data regularization and for the accuracy of trace-matching during 4-D binning in processing.
The 25 surveys have been carried out on four continents for all major oil and gas companies. Fast-track and full-integrity 4-D processing and quantitative interpretation are available on every PGS 4-D project.