Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1992-04-27
1995-08-15
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367 20, 367154, 367178, 367188, 181112, 181122, G01V 138
Patent
active
054425905
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a device and a method for performance of explorations of the kind which are indicated in the preambles to the main claims. Basically the invention concerns seismic exploration of the substratum offshore where pressure waves and shear waves from the substratum are measured in response to release of pressure and/or shear wave energy.
BACKGROUND ART
Marine seismic explorations are usually carried out by a seismic cable fitted with several hydrophones being towed at a certain depth. Pressure waves are released near the cable in several ways. Usually this is carried out by means of air guns. The pressure wave energy travels down through the substratum, but parts of the pressure waves are reflected from areas where there are acoustic impedance characteristics in the substratum. The hydrophones record the reflected pressure waves in the water and transform this information into electric signals which are detected and processed on the seismic ship which tows the cable. Using this method, only reflected and/or converted shear to pressure energy is recorded. However, it is known that down in the substratum both pressure waves and shear waves are reflected. The shear waves do not travel in water and cannot therefore be detected by a hydrophone cable. Moreover, it is not possible with today's conventional hydrophone technology to detect the directions of the measured signals and this severely complicates possibilities for three-dimensional data recording.
DISCLOSURE OF INVENTION
The objective of the invention is to provide a new and improved device and method for recording three-dimensional pressure and shear waves which travel in the substratum.
The new and inventive features of the invention are disclosed in the characterizing part of the two main claims. Further advantageous features are disclosed in the dependent claims.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be more closely described in the following, with reference to the accompanying drawings where:
FIG. 1 illustrates poles containing seismic equipment and connected to a main cable.
FIG. 2 shows a schematic outline of a pole.
FIG. 3 illustrates an underwater vessel after laying out of cable poles on the sea bed, during execution of seismic exploration.
FIG. 4 illustrates a surface-controlled performance of a seismic survey.
MODES FOR CARRYING OUT THE INVENTION
In FIG. 1 poles 1 which are installed into the sea bed 2 are shown. The poles are, via branch cables 3, connected to a main cable 4 which extends to a seismic ship, for example a submarine or a surface vessel. Both the branch cables 3 and the main cable 4 will in a practical embodiment comprise a data cable and one or several pulling cables. The actual installation of the poles in the sea bed will be more closely described later.
In FIG. 2 a schematic embodiment of a pole is shown in larger scale, the pole comprising a cylindrical portion 6, a lower conically formed or pointed end 7 and an upper portion 8. The upper portion 8 is made to accommodate equipment for inserting the pole down into the sea bed. In the shown embodiment, the upper portion is designed with a flange portion 9, but other types of embodiment could also be used. This design is advantageous in that the flange 9 can be utilized as a striking organ for insertion of the pole into the sea bed. As will be seen in FIG. 2, the upper portion 8 also comprises a centrally situated output terminal 10 for the branch cable 3. The upper portion 8 is connected to a flange 11 on the cylindrical portion 6 of the pole by means of a threaded connection 12. The passage of the branch cable will be arranged so that it is watertight.
The pointed end 7 of the pole is insulated from the rest of the pole 1 by a vibration insulating spacer 14. In a preferred embodiment the spacer 14 is made of an elastomer.
The actual point is shown in FIG. 2 with a purely conical shape which makes it easier for the pole to penetrate the sea bed. Other forms of embodiment can also be used, for example a point
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Berg Eivind
Svenning Bjornar
Den Norske Stats Oljeselskap A.S
Eldred J. Woodrow
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