Acoustics – Wave coupling – Earth
Patent
1995-08-24
1998-05-05
Moskowitz, Nelson
Acoustics
Wave coupling
Earth
367 17, 367165, 367177, 73644, G01V 1053, G01V 1047, G01V 120, G01V 138
Patent
active
057477540
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
The present invention relates to a method for performing marine seismic surveys by using a cable arranged on the sea bed and comprising geophone units and, the invention relates, moreover, to such a cable adapted for carrying out the stated method.
Use of such cables on the sea bed in the performance of seismic surveys in a marine environment is previously known. In this connection, reference is made to Norwegian printed specification No. 168,610 and U.S. Pat. No. 4,870,625.
From the above publications it is previously known to use a cable that at the time of survey rests on the ocean floor. Such a cable may be provided with geophones as well as hydrophones and, thus, can record both pressure and shear waves that originate from the seismic signals to be detected. A closer description of the survey conditions is given, inter alia, in the two above references.
In practice it has proved to be difficult to obtain a sufficiently good and stable mechanical connection between the geophones and the sea bed while the survey takes place. This may lead to somewhat indefinite detection, in particular of the shear waves that form part of the seismic signals.
The object of the present invention is to provide a method that leads to a more precise recording of the seismic signals than was previously the case, without increasing time consumption during the survey. Further, the object is to provide a simple and efficient cable for the performance of such a method and at the same time avoiding the drawbacks of previously known techniques.
This is obtained by using a method and a cable on the sea bed in accordance with the appended patent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to give a more clear understanding of the present invention, reference is made to the description of the working examples detailed below, as well to as the accompanying drawings, in which:
FIG. 1 is a perspective view of how a cable can be disposed on the seabed, for instance by means of a submarine vessel 1,
FIG. 2 shows a schematic view of a sensor that can be used in connection with a seabed cable according to FIG. 1,
FIG. 3 shows a sensor according to FIG. 2, but here shown in perspective view, and
FIG. 4 shows more in detail the construction of the geophone unit forming part of a sensor.
DETAILED DESCRIPTION
It is pointed out that the same reference numerals are used in all the figures of drawings that the figures have not been drawn to true scale and that the figures show simplified embodiments of the invention, the simplifications having been carried out in order that the principle of the invention is illustrated in as simple a manner as possible. Finally, it is pointed out that the drawings only show one possible embodiment of the invention, which means that many other embodiments may be contemplated without going beyond the scope of the invention.
FIG. 1 shows how a submarine vessel 1 can tow a cable on the seabed 3. It may be noted that the handling of the cable is, per se, previously known and lies outside the scope of the present invention. Details of such handling are therefore not mentioned here, except that the submarine vessel 1 can be equipped with a seismic source 9. However, the seismic signals can also be generated by separate sources. Likewise, a surface vessel can be used instead of a submarine vessel.
Along the cable 2 itself, which forms part of the cable, sensors 4 are provided that can comprise both geophones and hydrophones. The distribution of the sensors 4 can be regular or irregular along the cable 2, and the cable 2 normally has an extra weight at the end in the form of, for instance, an attached, drag chain 5, which helps to keep the entire length of the cable stretched out. It can also be noted that several such cables can be used in parallel on the sea bed and can then, for instance, be attached to a common transverse pull bar (not shown), which keeps the cables at a constant distance from each other during tansport. As a typical example, the cable can have a length of 1-2 km and be equippe
REFERENCES:
patent: 2654077 (1953-09-01), McLoad
patent: 3866162 (1975-02-01), Florian
patent: 3934218 (1976-01-01), Babb
patent: 4705137 (1987-11-01), Fair
patent: 5128900 (1992-07-01), Cole
Bugten Bjarne
Hals Tormod
Svenning Bj.o slashed.rnar
Den Norske Stats Oljeselskap A.S
Moskowitz Nelson
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