Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
1998-10-09
2001-02-27
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S480000, C250S227190
Reexamination Certificate
active
06195162
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to interferometric sensing apparatus. In particular it relates to the configuration particularly suited to the simultaneous interrogation of a large number of hydrophones.
BACKGROUND OF THE INVENTION
There is a demand in the oil and gas industry to improve the hit rate of locating recoverable reserves, and for increasing the percentage of oil and gas recovered from reservoirs. This has resulted in the demand for improvements in the quality of seismic surveys and in a demand for in-reservoir fluid-imaging techniques. Both these requirements demand large numbers of sensors networked together.
Similar requirements in defense applications have been met using time-division multiplexing techniques, involving interrogating a number of hydrophone elements using a single pulse of light. The technique relies on the fact that for each hydrophone along the path part of the pulse energy will be modified by the hydrophone and reflected. This results in a series of reflected light pulses returning to a detector at different times from the separate hydrophone elements. The problem with this approach is that bandwidth is limited because of aliasing effects, which also restricts dynamic range. A further problem is that the number of elements addressable by a single source is relatively limited leading to a fairly large number of expensive electro-optic sources required in the total system.
A particular demand is for large arrays of optical hydrophones which can be interrogated simultaneously over single fiber leads, in real-time with high-dynamic range, and relatively wide bandwidth response. Such hydrophones are attractive for pumping through narrow bore conduits into oil reservoirs. Conventional coiled hydrophones are unsuited for this application because their diameter is not small enough. The hydrophone needs to have a diameter of no more than around 2 mm. The implication here is that the length of fiber which can be used in each hydrophone can be no longer than around 1 m to 10 m, which is significantly shorter than the 30 m to 300 m used in conventional optical fiber hydrophone systems which are coiled. These short lengths pose significant problems for the time-division multiplexing systems currently employed. In particular, the bandwidth of the resulting system is restricted owing to aliasing effects, and the pulse length (which when coherent light from a laser is used, conventionally corresponds to around the length of each coil) becomes excessively short making the electronic instrumentation difficult to implement.
Arrays exceeding 10,000 hydrophone elements can be envisaged in thin arrays which are extremely attractive for seismic streamers. These hydrophone elements would also need to be relatively short (around 1 m to 10 m).
Apparatus suitable for the simultaneous acquisition of high-bandwidth information in very long arrays was disclosed in a previous patent application GB2284256A. Wavelength division multiplexing was used in this apparatus such that hydrophone arrays could be interrogated with broadband light, and the information from each hydrophone returned at unique wavelengths. These wavelengths were separated and routed to different detectors. This apparatus has the drawback in that it utilizes a very large number of detectors—one per hydrophone element. Nevertheless, it is probably the only way to achieve very high bandwidth (500 kHz) interrogation of very short (1 m) hydrophones. The apparatus is probably not cost-effective for very large hydrophone arrays where the bandwidth requirement is relatively modest (100 Hz to 6 kHz).
Arrays using fiber Bragg grating pairs are particularly attractive—particularly if ways can be found to eliminate, or dramatically reduce, cross-talk between hydrophones. Such cross-talk is inherent in many architectures.
Conventional electrical seismic streamers contain hydrophones which are grouped together to reduce tow noise. Such groups are typically 12.5 m long and may contain 24 hydrophones. Optical hydrophone arrays can be constructed in a similar fashion, combining the outputs of groups of hydrophones in signal processing electronics. A more cost-effective solution is to replace each hydrophone group with a single hydrophone constructed in a linear fashion. However, this approach will not have the sensitivity of the coiled-hydrophone approach.
A problem with hydrophone arrays which has been published widely in the literature is that of polarization fading. Polarization fading is particularly problematic in linear hydrophones utilizing pairs of Bragg gratings. Many solutions to polarization fading have been published but none are truly satisfactory. The most robust solutions utilize either polarization maintaining optical fiber or polarizing optical fiber throughout the apparatus. However there are cost penalties associated with such solutions.
Similar polarization-fading problems exist in other sensing interferometers for the measurement of other parameters.
SUMMARY OF THE INVENTION
An aim of the present invention is to reduce polarization fading in interferometric sensing apparatus.
Accordingly in one non-limiting embodiment of the present invention, there is provided apparatus for interferometric sensing, which apparatus comprises a broadband optical source, a depolarizer for depolarizing optical radiation emitted by the broadband optical source, a matched interferometer, a sensing interferometer, and a detector, the matched interferometer being such that it contains a phase modulator and the apparatus being such that the optical path length difference in the sensing interferometer is approximately equal to the optical path length difference in the matched interferometer.
The broadband optical source may be a light emitting diode, or a superfluorescent fiber source, or a super-luminescent diode.
The depolarizer may be a Lyott depolarizer, for example a Lyott depolarizer fabricated out of polarization maintaining optical fiber.
The phase modulator may be a frequency shifter.
In a first aspect of the invention, the sensing interferometer is a fiber optic sensing interferometer constructed using optical fiber Bragg gratings at either end of the fiber optic sensing interferometer. The optical fiber Bragg gratings at either end of the fiber-optic sensing interferometer may be chirped in opposite directions. As used herein, the term “chirped grating” refers to the fact that the periodicity in the refractive index perterbation increases along the length of the grating.
The sensing interferometer may be an optical fiber hydrophone which may be constructed from an optical fiber twisted around a compliant member and bonded. The apparatus may be such that there is a reduced number of separate hydrophones within each group of a seismic streamer may be achieved. Cross talk between hydrophones within the array may be dramatically reduced.
In a second aspect of the invention, the apparatus contains a plurality of sensing interferometers positioned along an optical fiber, and means for pulsing the optical radiation transmitted along the optical fiber such that the sensing interferometers positioned along an optical fiber, and each sensing interferometer reflects optical radiation at a substantially different wavelength.
The wavelength of the optical radiation from the broadband source may be scanned such that each sensing interferometer is interrogated separately.
The light returning from each sensing interferometer may be directed to a different detector by means of a wavelength division demultiplexer.
The optical radiation at the substantially different wavelengths may be derived from at least one optical source, and the optical radiation at each substantially different wavelength may be propagated through different matched interferometers prior to being propagated through the optical fiber containing the plurality of sensing interferometers.
The optical path length differences in each sensing interferometer may be different, and the optical path length differences in the sensing interferometer and the match
Bull Phillip Sam
Kluth Erhard Lothar Edgar
Luscombe John
Varnham Malcolm Paul
Felsman Bradley Vaden Gunter & Dillon, LLP
Geosensor Corporation
Turner Samuel A.
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