Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1990-01-30
1991-02-12
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
G01V 138
Patent
active
049929909
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for determining the position of at least two seismic streamers in a reflection seismic measuring system, in connection with marine seismic surveys.
Lately there have come into use three-dimensional exploration methods for marine seismic surveys. Such exploration methods place heavy demands on navigation and positioning and one is dependent on knowing the relative positions of seismic sources and the hydrophones of the seismic streamer with great precision. In modern seismic exploration methods several sources and several seismic streamers are usually employed, and the mutual distances between these must also be determined with relatively high precision. The seismic sources and the seismic streamers are usually towed by one or more exploration vessels, and the exploration vessels' absolute position at any time is determined by means of surface navigation systems aboard the vessel or the vessels, these navigation systems preferably being land-based or satellite-based radio navigational systems which give a resolution below 0.5 m and a repeatable positioning accuracy of a few meters. It is then necessary to determine the position of the seismic streamers or in reality the position of the hydrophones of the seismic streamers with as high accuracy as possible, the basis for this position determination being the absolute position found by the navigational system of the vessel.
A seismic streamer comprises a plurality of active streamer sections with known length and equipped with hydrophones or hydrophone groups with known location in the active sections. The seismic streamer is arranged between stretch sections, a fore stretch section being connected with a towing means on the towing vessel, whereas a tailbuoy with means for determining the position is provided at the end of the aft strech section. The means may for instance be an active navigation system of the same type that is employed aboard the towing vessel or a microwave system, possibly combined with a goniometer. The position of the tailbuoy may also be determined by passive distance measurements between the towing vessel and the buoy, for instance by means of radar or laser reflectors. Now knowing the positions of the vessel and the tailbuoy, the position of the hydrophones in the seismic streamer is determined on the basis of the known length of the streamer, the known location of the hydrophones in the streamer and the orientation of the separate sections of the vessel and the tailbuoy. This orientation may in principle be provided by taking a bearing between the vessel and the tailbuoy using compass devices aboard the vessel. However, due to swing of the seismic streamer caused by sea currents, an angular deviation between the streamer and the ship's bearing is generated. In practice the seismic streamer will hence have the shape of a plane or a spatial curve, but the said angular deviation may be determined by providing a plurality of magnetic compasses in the cable, typically for instance twelve compasses in a streamer of three kilometers length, and normally with a compass close to each end of the streamer. Compasses are also provided in the stretch section. The curve of the streamer may then be determined for instance by means of mathematical estimation in connection with compass bearings, known distances and section lengths. As a rule, however, the error in the actual distance to a streamer section will lie within the errors of the position determining system, referred to the navigation accuracy.
At present a number of methods are employed or proposed to be employed, for determining the streamer's position, based on distance measurements in connection with compass bearings. One method is to determine supposed distances to the single streamer sections based on how much towing cable is being handed out from the towing vessel, at the same time as bearings are taken with the magnetic compasses in the streamer. The method, however, is encumbered with substantial errors, since the stretc
REFERENCES:
patent: 4532637 (1985-07-01), Baeckel et al.
patent: 4660185 (1987-04-01), French
patent: 4669067 (1987-05-01), Roberts
patent: 4845686 (1989-07-01), Brac
Sonerdyne Limited Publication, "A Hydro-Acoustic System for Precision Tracking of Twin Seismic Hydrophone Streamers," 7/87.
"Improving the Accuracy of Merile 3D Seismic Surveys," Ocean Industry, Jan. 1987.
Langeland Jan-.ANG.ge
Nordmoen Bjorn
Sheim Stein
Vigen Erik
Geco A.S.
Lobo Ian J.
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