DESCRIPTION OF THE RESEARCH INFRASTRUCTURE

The P-Cable 3D seismic system differs from conventional industrial 3D seismic systems by having a cable towed perpendicular (cross cable) to the ship's steaming direction, a so-called cross-cable, on to which up to 24 streamers are attached (Figure 1a). The streamers are only 25-m long, i.e. much shorter than in a conventional 3D seismic systems, which are usually several km long. The array of single-channel streamers acquire 24 seismic lines simultaneously, thus covering an approx. 240 m wide swath with close in-line spacing in a cost efficient way. The P-Cable system delivers a bin size of 6 x 6 m. In contrast, conventional 3D seismic data normally has a default spatial sampling (bin size) of 25 x 25 m. This results in an increase in spatial resolution for the P-Cable system of up to factor 16, providing unprecedented detail of subsurface structure. The P-Cable technology has proven data quality, surpassing conventional 3D and equal or better than HiRes 2D (Figure 1b & c).

Figure 1 B and C

Figure 1 b and c: Comparison between P-Cable 3D seismic data (bin size 10m) and high-resolution 3D seismic exploration data (bin size 12.5 m, interpolated) (Gay et al., 2007) of very similar fluid-escape structures. Horizontal and vertical scale are identical. Note the significantly higher resolution of the P-Cable data. Particularly, the spatial resolution shows much more details of the fluidescape structures than the conventional 3D exploration data, which looks rather blocky and doesn't allow visualizing the fine-scale nature of these features.

The major advantage of towed short streamers for 3D seismic data acquisition is that it can be run by University research vessels such as RV Jan Mayen and RV GO Sars. P-Cable test surveys during the last 8 years provided engineers and researchers at the University of Tromsø with outstanding experience and knowledge of high-resolution 3D seismic data acquisition and processing (Petersen et al., subm.). The only other experience of this kind exists at Lamont Doherty Earth Observatory in New York but there they concentrate on industry standard 3D seismic data acquisition and processing techniques that are not affordable for normal Universities.

Thus, we suggest to further develop our present design of the high-resolution P-Cable 3D seismic, and obtain a mobile and flexible system that can be used and shared by all geomarine institutions and Universities of Norway for research activities concentrating in Arctic and Subpolar regions. A small and light system can be deployed easily by a small crew in a short time and used from shallow to deep water.

Present design and geometry

The P-Cable seismic system used onboard R/V Jan Mayen consists of a perpendicular cross-cable, assembled from two 60-meter sections with four streamers each, giving a total of 8 streamers on a 120-meter long cross cable (Figure 3). Each streamer contains 8 digital channels with a group spacing of 3.125 m (Figure 4). A pair of submerged deflectors, paravane doors, is used to stretch the cross cable perpendicular to the ship's steaming direction. These doors are constructed such that they move in a direction transverse to the vessel's direction of travel spreading the streamers behind the vessel. The paravane doors and the GI guns are equipped with DGPS, which have an accuracy of better than 1 meter. The achieved accuracy allows to accurately position the receiver groups on the streamers. The streamer spacing along the cross cable is 15 meters but the cross cable curvature makes the distance between streamers smaller. The curvature of the cross-cable can be derived using physical models for catenary curves, i.e. a hanging flexible cable supported at its ends. This physical model provided the base for exact calculations of midpoint positions for all source-receiver pairs (Figure 4).

Figure 3

Figure 3: Top view of a 16-streamer P-Cable setup.


Figure 4

Figure 4. Side view of the P-Cable setup. A channel spacing of 3.125 m gives a theoretical CMP spacing of half that distance, 1.5625 m.

Each streamer is attached to the cross-cable via a 6 m-long drop-lead and an A/D Digitizer bottle. The A/D bottle converts the analog signal from the hydrophones in the streamer to a digital Ethernet signal which is sent to the fiber-optical (FO) switch at the end of the integrated tow cable. The FO switch converts the signal into a fiber-optic signal, which is sent on deck the ship to the FO-switch at the other end of the integrated tow cable. This FO switch converts the signal back into an Ethernet signal. The signal then reaches the Streamer Power Supply Unit (SPSU). The SPSU communicates with the recording PC, the FO switch and the gun trigger. A PC is recording the incoming data.

P-Cable applications / areas of use

The P-Cable 3D seismic system is designed and developed as a tool for marine geological/geophysical research and petroleum industry applications. It can be used in both frontier and mature regions in an intelligent, versatile way to acquire successive small-size surveys (25 to 250 km2) in target areas of special interest. Fast deployment and recovery of the P-Cable and short turns between adjacent sailing lines are an advantage.

The P-Cable system is already being considered as the premier tool for site survey investigations for scientific drilling operations. The detailed knowledge of subsurface structure will allow IODP drilling in frontier areas that hitherto have been classified as to risky. The flexibility and easy operation of the P-Cable system allows academic institutions to acquire high-resolution 3D seismic data in new frontier areas such as seasonally ice-free Polar Regions. A better understanding of this sensitive environment is important with respect to the ongoing global warming. In particular, the behavior of gas hydrate reservoirs during global warming, potential greenhouse gas (methane) releases, and geohazards in Arctic regions and elsewhere demands the use of such advanced high-resolution imaging technology.

The main areas of application for academia envisioned are research areas of:

The main areas of application for industry envisioned are:

P-Cable as an operational, mobile and flexible high-resolution 3D-seismic system

The development of the digital P-Cable system through the last 3 years has significantly improved the stability and endurance of the system. Several industrial partners (StatoilHydro, SEABED industry consortium) have worked closely with us to improve system design following our experiences during sea trials. The current system configuration delivers very good performance through 1-2 weeks. With an improved performance, there will be a much higher demand for such a system. Such P-Cable 3D seismic system should be available to all Norwegian geo-marine institutions and Universities. This will strengthen the role of geo-marine research in Norway in a global perspective. It will also strengthen Norway's role as a leader in innovative marine technology, as patent and cable technology have been developed by Norwegian companies.

We therefore, suggest to integrate the P-Cable 3D seismic technology to:

With the engineering infrastructure already in place at the University of Tromsø and Bergen, and the access to research vessels, we will be able to obtain an independent digital P-Cable 3D seismic system with increased stability that will deliver cutting-edge seismic data for a better understanding of subsurface structure and associate processes.

Existing P-Cable infrastructure at the University of Tromsø

Through the collaboration with VBPR and through two NFR PETROMAKS and two EU funded projects, the University of Tromsø already possesses components of the P-Cable system. These components are to be used or upgraded to the new mobile system, which allowed reducing our requested financial budget.

The University of Tromsø owns two large paravane doors that spread the cross cable. The size of the doors corresponds approximately to trawl doors used on fishing vessel with large trawl winches and accommodation space at the stern of the ship. The University of Tromsø also owns 8 streamers and 8 digitizers that are attached to the cross cable. Furthermore, we own a navigational Seatrack system including 4 transponders from Kongsberg for positioning of the paravane doors and the GI gun system. A large winch exists for either the cross cable, the tow cable, or the streamer sections during deployment.

The existing paravane doors, the eight streamers and digitizers including the compressor, the two GI guns, the navigation track and four transponders can be incorporated into the new infrastructure without major changes because components are compatible. The navigation track needs upgrading and new software, and investments are to be covered by an industry project. Though the new crosscable and streamer configuration cannot be handled with the existing winch (it has a too small diameter < 100 cm), it is still useful for the tow cable. To conclude, the existing components can be incorporated in the new infrastructure.

Back to top