Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-06-25
2001-04-03
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C385S012000, C250S227270, C356S478000
Reexamination Certificate
active
06211964
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of seismic exploration and more particularly, to a method and a structure for incorporating fiber optic acoustic sensors in a seismic array.
BACKGROUND OF THE INVENTION
Marine seismic exploration operations commonly include towing a seismic streamer behind a vessel. The seismic streamer includes data communications channels, power conductors, one or more strength members, and a number of sound-sensitive hydrophones or other sensors.
As the streamer is towed through the water during seismic operations, its primary function is to receive seismic signals at the sensors from subsurface geological structures, convert these seismic or acoustic signals to voltage signals, and transmit these voltage signals to a central receiver on board the vessel. Commonly, a number of hydrophones are coupled together as a group, and the group of hydrophones is coupled to a conductor within the streamer at a takeout.
The hydrophones so coupled together receive the seismic signals and provide a weighted average of the signal received by the hydrophones in the group. This weighted average is provided in analog form to a conductor within the streamer. This signal is digitized and recorded for later display of the subsurface geological structures.
The sensors are designed for extreme sensitivity because the seismic signals are well attenuated by the time they reach the sensors. Unfortunately, the sensors are equally sensitive to all kinds of noise, both from the cable itself and external to the cable. For example, as the cable is towed through the water, the cable is subject to sudden sharp movements and other mechanical perturbations which are conducted through strength members through the cable. Any such noise may be conducted to a sensor, and is thus detected as a signal or can mask the actual seismic signal.
Linear fiber optic hydrophones are based on the phenomenon that external measurands alter some optical characteristic of the optical fiber sensor, such as its index of refraction or the optical path length for a light signal in the fiber. For most known structures, any axial strain in the cable translates to a concomitant strain in the linear optical fiber, particularly if the fiber is axially oriented, and thus axial cable stress alters the optical path length of the fiber, and spoils any detection scheme.
Various means have been used in the past to reduced the noise conducted to the sensors. Discrete hydrophone elements have been mounted in a foam or a fluid volume to eliminate shear wave stresses, for example. However, when linear optical hydrophones are used, the geometry of the cable for encapsulating the optical hydrophone becomes problematic.
Aside from the mechanical geometry of the cable for retaining a linear optical sensor, a technique for interrogating the sensors must also be included in the streamer system. A structure and a technique for efficient multiplexing in an interferometric sensor array of a large number of sensors was disclosed in parent application U.S. patent application Ser. No. 09/169,252, filed Oct. 9, 1998, and incorporated herein by reference. In a preferred embodiment, the structure defines an apparatus for interferometric sensing comprising an optical source, a tunable filter, a depolarizer for depolarizing optical radiation emitted by the optical source, a matched interferometer, a sensing interferometer, and a detector. The matched interferometer contains a phase modulator and the optical path length difference in the sensing interferometer is approximately equal to the optical path length difference in the matched interferometer. The optical source may be a broad band source or light, or a narrow band source such as a laser.
Thus, the structure disclosed in U.S. patent application Ser. No. 09/169,252 provided wavelength-addressable interferometers containing optical fiber Bragg grating pairs as reflectors. This configuration also dramatically reduced cross-talk between hydrophones, which is inherent in many architectures.
However, there remains a need for a structure whereby the fiber optic acoustic sensors can be incorporated into a seismic array. Such a cable structure should preferably include a linear optical fiber sensor wound around a compliant core or suspended in a spiral tube filled with a fluid. Alternatively, the sensor may include point optical hydrophones. Also alternatively, the linear optical fiber sensor may comprise a spiral structure around a fluid-filled open-cell foam to eliminate common sources of noise.
SUMMARY OF THE INVENTION
The present invention addresses these and other challenges of the prior art in a seismic streamer including an optical fiber as the sensor element. The fiber is wound in a spiral about an axis in a manner to isolate the fiber from axial stresses in the cable. This may be accomplished by placing a fluid layer between the fiber and the source of the stress, or by winding the fiber around a compliant member in a manner to maintain a constant arc length of the fiber.
In a preferred embodiment, the streamer includes four tubes spiraling around a central strength member. One of the tubes encloses an optical fiber sensor which is maintained in an extended orientation by fluid flow through the tube. Another of the tubes serves as a return path for fluid through the sensor tube. The other tubes enclose high speed and low speed data conductors, respectively.
The sensor fiber penetrates the wall of the enclosing tube at a stuffing tube, which also serves as an anchor to retain the sensor fiber. The other end of the sensor fiber is not anchored, but, as previously described, the fiber is kept extended in the tube and substantially not in contact with the wall of the retaining tube by fluid flow through the tube. One sensor fiber is provided for each section of the streamer cable. Each sensor fiber has a plurality of optical fiber Bragg gratings, and each of the plurality of Bragg gratings is tuned to a different wavelength of light.
In another preferred embodiment, a linear optical fiber is wound around a buoyant rubber core, which is enclosed within a liquid filled open cell foam to reduce or eliminate shear stress. A load bearing Vectran stress member encloses the foam/core combination.
In yet another preferred embodiment, a compliant core is selected and then a lay length or pitch of the optical fiber wound around the core is determined. The structure will demonstrate a specific Poisson's Ratio, so that the radial and axial dimensions of the core change with strain in such a manner as to maintain a constant strain on the spiral wound optical fiber.
In still another preferred embodiment, the optical fiber wound around the core includes a plurality of point optical hydrophones.
These and other features of this invention will be apparent to those skilled in the art from a review of the following description along with the accompanying drawings.
REFERENCES:
patent: 4076382 (1978-02-01), Oestreich
patent: 2145237A (1985-03-01), None
patent: 2310280A (1997-08-01), None
patent: 2311131A (1997-09-01), None
Bull Sam
Kluth Erhard Lothar Edgar
Luscombe John
Maida, Jr. John L.
Varnham Malcolm Paul
Felsman, Bradley, Vaden, Gunter & Dillon L.L.P.
Geosensor Corporation
Turner Samuel A.
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