PGS Investigates Seismic Acquisition Without Sources
In a First Break paper this month PGS investigates the feasibility of acquiring seismic data without an active source and instead uses the acoustic wavefield generated by a vessel for imaging the subsurface.
The acoustic wavefield originating from a vessel has historically been treated as a source of noise and not considered as a possible signal for the imaging of marine seismic data. This new paper examines using the acoustic wavefield generated by a vessel to image the subsurface and tests its success with different acquisition configurations.
Using the Vessel as the Source
There are areas around the world where the use of active marine seismic sources is not permitted throughout the year, or only permitted during short time periods. In such areas, acquiring seismic data using the acoustic signals generated by a vessel as a source may be an alternative. Using a vessel as a seismic source may also offer a low cost and low impact 4D monitoring solution with an opportunity for much more frequent acquisition of time lapse data especially over permanent receiver installations.
Best Results from Vessel Sailing Over Towed Streamers
Three different acquisition configurations are considered; a towed-streamer configuration, a configuration with a vessel sailing over the top of a towed-streamer spread, and a configuration with a vessel sailing over a PRM system.
The most successful of the three tested was with a vessel sailing on top of a streamer spread and it shows excellent results. Good near-offset coverage and the recording of direct arrivals over a large range of emission angles means the acoustic wavefield generated by the vessel can be determined with higher precision over a wide bandwidth. This results in excellent broadband images (as shown below). Coherent signals have been demonstrated from the 2-4 Hz octave all the way up to 250 Hz in the shallow parts of the section.
Stack from data acquired without an active source (left) and with conventional seismic sources (right) from an acquisition configuration with a source vessel over the streamer spread. This comparison shows that the main features observed in the data acquired when using an active source can also be recognized in the data acquired when using the vessel as a source. Also, observe the resolution that can be achieved in the shallow section when using acoustic signals generated by a vessel, and with this specific configuration. The use of a continuous source wavefield, rather than discrete shot points, is likely a significant contributory factor to the high spatial resolution.
This work demonstrates that it is possible to obtain very high-resolution seismic images of the shallow subsurface by using acoustic signals generated by a vessel. This outcome is due to the broadband signals that the vessel generates combined with the fact that these signals are generated continuously while the vessel is moving, allowing for extremely dense source-side sampling along the vessel path.