Nautilus Vessels Promise to Be Safer, Quieter and Better
CGGVeritas is confident that acquisition of the recently announced PEMEX Gulf of Mexico survey with the advanced Sercel Nautilus-Sentinel steered solid streamers will provide a new benchmark for safer operations, quieter data and better crew efficiency.
With one system already in production on the Symphony, CGGVeritas will make the next full-scale deployment of the advanced Sercel Nautilus-Sentinel steered solid streamers on the largest marine survey ever awarded.
The Alizé will shortly be starting the acquisition of a 75,000 sq km 3D survey for PEMEX in the Gulf of Mexico, using twelve Sentinel solid streamers equipped with Nautilus streamer control devices. Two of the Alizé's companion vessels, the Symphony and the Endeavour, have already been breaking industry production records, and the Alizé is now expected to surpass them. This will be partly due to the ease of deployment of solid streamers and the fact that, with a 12 x 8km x 100m streamer configuration, the Alizé will be towing the largest areal receiver array in the industry.
Sercel's Nautilus is the unique 3-in-1 automatic steering, positioning and depth control device, purpose-designed for Sentinel streamers. The combination of Sentinel and Nautilus provides the most advanced steered streamer available today.
The Alizé has always been a capable vessel and a full upgrade in 2008 extended her competitive advantage. With 16-streamer capacity, the latest vintage of solid Sentinel streamers and the addition of Nautilus, the Alizé is poised to deploy the world’s largest spread of steered streamers.
Combined with the reputation CGGVeritas has gained for service, data quality, safety and efficiency, this puts the vessel in an excellent position to set a new standard with the PEMEX project.
Nautilus-steered Sentinel streamers offer many HSE (Health, Safety and Environment) advantages. Nautilus is powered by the streamer and so requires no battery changes. This eliminates the need to handle and store lithium batteries and reduces workboat trips, making the system inherently safer.
The streamer has the largest tensile strength in the industry and is resilient to damage in-water and during deployment. Its solid fill means that it is preferred for use in sensitive environments such as the Arctic where, this year alone, Sentinel has seen service several times.
The Sentinel streamer, thanks to its solid design, is recognized in the industry as the quietest streamer at all frequencies and depths, particularly in marginal weather conditions. Recent advancements in the low-frequency response have extended this performance to the lowest usable frequencies, which are increasingly important for seismic inversion and the imaging of deep and sub-salt targets. Where high frequencies are important, Sentinel's excellent rough weather noise performance allows towing at shallow streamer depths, boosting high-frequency content without excessive noise.
Nautilus enhances the industry-leading low-noise performance of Sentinel, even under steering. Tests have shown that the overall noise levels do not vary with steering force, and are lower than those from conventional birds and acoustic devices.
Nautilus has an advanced fully-braced acoustic network which improves positioning accuracy compared with conventional tail-mid-head networks. Each advanced Nautilus unit communicates with the devices on adjacent streamers, decoding and interpreting ranges in real time, and adjusting their steering automatically to maintain separation and position, without waiting for feedback from the vessel. This provides automatic spread regularization and allows efficient 3D recording, as holes in the coverage caused by 'trousering' are eliminated, reducing dramatically the need for infill shooting. The depth of the streamers is also automatically controlled.
The large wings of Nautilus are designed to provide higher steering force than is currently available, allowing faster spread regularization and better feather control. This powerful steering mechanism can also position the cables in any predetermined feather pattern for maximum 4D repeatability.
The Nautilus steering fins are coupled to in-line nodes in the Sentinel streamer, so can be easily mounted and handled using existing streamer deployment systems. Streamer deployment and recovery is simplified using Nautilus, as the streamers can be fanned out to prevent tangles. Efficiency is also improved on line changes as the streamers can be steered out of turns faster.
Data quality is the most important aspect of acquisition. Sentinel streamers are proven to deliver low-noise high-quality data, especially in sub-salt areas. The PEMEX survey will include an area of wide-azimuth (WAZ) acquisition, a technique in which CGGVeritas has unrivalled experience and expertise. WAZ surveys have shown outstanding imaging results in this area, especially when processed using the true 3D algorithms and the unique TTI RTM of CGGVeritas. These are an important part of the new geovation seismic processing software.
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