Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1996-12-10
1998-04-28
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367154, 1741015, G01V 138, G01V 120
Patent
active
057454361
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention concerns a semi-dry marine seismic streamer cable consisting of a number of streamer cable sections, wherein each streamer cable section comprises a plurality of seismic signal sensor means provided at the center of the cable and forming a detector array, signal transfer means for connecting the array to a signal processing device, a cylindrical mechanical jacket forming the outer portion of the cable and an inner hollow core for enclosing said signal sensor and transmission means.
A seismic streamer is a cable containing an array of hydrophones towed at about 5 knots behind a seismic vessel. The purpose of a streamer is to record acoustic signals usually generated by an air gun and reflected by layers of rock in the earth. The cable is often several thousands of meters long and is towed beneath the sea surface at a depth of about 10 meters. The cable is usually made up of sections with a length of 100 meters. So-called "wet" streamers are constructed with an outer plastic jacket filled with kerosene and containing hydrophone sensors, electronics and mechanical parts. The mechanical items include stress members kept in place by plastic spacers which are placed periodically along the length. The stress members are composed of steel or a man-made fiber like Kevlar. The hydrophone sensors are placed at intervals, typically of the order of one meter (not necessarily uniformly spaced), and the measurements communicated along the cable to the vessel through a bundle of wires. Sometimes signals are electronically digitized inside the streamer.
Other types of streamer have been invented, e.g. solid ("dry") streamer cables. In these the core of the streamer is not filled with kerosene, but with some light weight plastic or composite material.
Both wet and dry streamers need to be neutrally buoyant, something which is achieved by removing or adding kerosene, or by adding weights. This balancing must be repeated each time a new location is surveyed. This is because the density of the sea water changes in different parts of the world due to variations in its temperature and salinity.
There are several problems with wet streamers. The plastic skin may become punctured from shark bites or by smashing into submerged objects. The invasion of sea water short-circuits the electronics causing a failure of the system. If this happens the section is usually returned to the factory for repair.
Wet streamers are affected by bulge waves. These are waves which travel along the skin and induce radial pressure vibrations which propagate through the kerosene to the hydrophones. Bulge waves are often generated at connectors or at spacers both of which can act as pistons. Bulge waves appear as noise on the seismic signal.
The failure of any internal part of a wet streamer means it must be returned to the factory, the skin removed and the repair made. For both wet and dry streamers the electrical and mechanical parts are physically interleaved, i.e. for wet streamers the electronics can not be easily separated from the spacers and stress members.
Dry streamers also have drawbacks. One is the balancing the streamer. As discussed earlier, each new survey site requires a rebalancing of the cable. Hence the streamer, with, no weights attached, must be less dense than any sea water it might encounter during its life. Lead weights are often attached to the surface of the streamer using tape or are clipped on. This locally increases the diameter of the streamer. The turbulent flow over the surface is perturbed and eddies may be shed leading to an increase in the surface noise. This reduces the signal to noise ratio at the hydrophones reducing the equality of the recording.
Solid streamers are more difficult to repair. Generally the plastic body must be dissected, the repair made and new plastic molded into place. This again requires the return of the streamer to the maufacturing center.
In order to improve the mechanical and acoustic properties of seismic streamers, U.S. Pat. No. 4,809,243 pro
REFERENCES:
patent: 4491939 (1985-01-01), Carpenter
patent: 4809243 (1989-02-01), Bledsoe et al.
patent: 5089668 (1992-02-01), Havey
patent: 5272679 (1993-12-01), Carpenter
Geco A.S.
Lobo Ian J.
Smith Keith G.W.
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