Seismic signal coupling device and method

Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type

Reexamination Certificate

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Details

C367S020000

Reexamination Certificate

active

06262945

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the field of takeout structures which connect a seismic sensor string to a cable, and particularly to a structure and a method for a coupling a seismic signal to a geophone in an ocean-bottom cable.
BACKGROUND OF THE INVENTION
To conduct an ocean bottom seismic survey, a cable having a plurality of pairs of electrical conductors is laid out along a survey line and connected at one end to a recording device. At spaced locations along the cable, a “takeout” is provided that enables the leader wire of a string of geophones to be electrically connected to one of the pairs of conductors inside the cable. Each pair of conductors and the phones connected to it form a data channel so that acoustic waves that are reflected upward from underground geologic structures are recorded on that channel.
Once a set of data has been captured and recorded along the survey line, the cable is commonly dragged to the next survey line in preparation for another set of data. The cable may also be retrieved and redeployed in a new location to acquire another set of data.
In dragging or redeploying the cable from one survey line to the next, the seismic sensor package is commonly subjected to a number of potentially damaging obstacles on the ocean floor. In the past, the sensor, takeout, and the region of the cable in close proximity to them have been wrapped in heavy tape, and sometimes enclosed in a shrink-wrap plastic to minimize this damage. Unfortunately, this wrapping has often proved less than satisfactory, resulting in damage to the cable, the sensor, and often leading to catastrophic sea water in-leakage at the cable penetrations.
Aside from sealing the takeout, the tape and wrap applied to the cable are intended to anchor the sensor package along with its connecting leader cable. As the wrapping is damaged, the sensor package with the connecting leader cable is allowed to move freely, causing a further mechanical damage to the sensors and connecting cable.
In U.S. patent application Ser. No. 09/255,452, filed Feb. 19, 1999 and assigned to the same assignee as the present application, a combination takeout anchor and protective cover is disclosed. This application is incorporated herein by reference. This structure fills this long-felt need of protecting the sensor components on an ocean-bottom seismic cable.
The structure disclosed in that application very satisfactorily fulfills its intended purpose. However, the overall performance of the entire seismic cable could be improved if the anchor and protective cover were to more effectively couple the sensor components to the ocean bottom. More effective coupling of the sensor and ocean bottom leads to improved imaging of the geologic structures which may bear the desired hydrocarbons which the seismic survey is intended to find.
Thus, there remains a need for a coupling structure to firmly embed the geophones of an ocean-bottom seismic cable to the sea floor. Such a structure should also resist the bending force on the sensor elements as the cable is reeled aboard a host vessel, and should also help to protect the sensor package as the cable is dragged along the ocean floor.
SUMMARY OF THE INVENTION
This invention solves these and other drawbacks of the prior art by providing an open-structure coupling device which is used in conjunction with or separate from the enclosure system of U.S. patent application Ser. No. 09/255,452 to firmly couple a seismic sensor package to the sea floor. The structure is open for the free flow of sea water and mud through the structure so that it moves freely through the sea water and solidly embeds itself in the ocean bottom. It is preferably made of a corrosion resistant metal, such as stainless steel, which also resists bending forces on the cable take-up reel, and provides further mechanical protection for sensor components enclosed within.
The preferred embodiment of the coupling device generally comprises a segmented ring at each end to clamp around the protective enclosure which encloses the sensor package, a further pair of segmented rings inside the ends and around the enclosure, and a plurality of axially oriented longitudinal bars coupled to the rings to form a squirrel cage around the enclosure. The bars may be inserted through holes in the rings, but the preferred rings are gear-like mounts with open lands to receive the bars. The bars are then welded or otherwise joined to the rings. The segments of the rings are also joined, preferably by bolts or screws so that the coupling device can be easily removed from the cable for easy access to the sensor package within for maintenance.


REFERENCES:
patent: 3939464 (1976-02-01), Swenson
patent: 4013990 (1977-03-01), Devine
patent: 5883857 (1999-03-01), Pearce

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