Precision measurements in a fiber optic distributed sensor system

• US 9,476,760 B2
• Filed: 06/25/2010
• Issued: 10/25/2016
• Est. Priority Date: 06/25/2010
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: Schlumberger Technology Corporation (SLB)
• Inventors: Brady, Dominic, Hartog, Arthur H.
• Primary Examiner(s): Kundu, Sujoy
• Assistant Examiner(s): Ngon, Ricky
• Application Number:
  US12/823,685
• Publication Number:
  US 20110320147A1
• Time in Patent Office: 2,314 Days
• Field of Search: 702/66, 702/57, 702/72, 385/12, 385/13, 385/76, 385/55, 356/447, 356/446, 356/445, 356/477
• US Class Current: 1/1
• CPC Class Codes: G01D 5/35361 using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01V 1/40 specially adapted for well-logging

Abstract

A fiber optic distributed vibration sensor provides a highly sensitive measurement of a measurand with a high degree of linearity. The distributed vibration sensor includes subsections configured to have a high sensitivity to a measurand of interest interspaced in an alternating manner with subsections having a low sensitivity to the measurand. The distributed vibration sensor is interrogated such that a phase difference between the backscattered signals generated in low sensitivity subsections surrounding a high sensitivity subsection can be determined. Characteristics of the measurand may then be determined based on the phase difference.

Claims

1. A method of measuring a measurand in a fiber optic distributed sensor system, comprising:

providing a fiber optic distributed sensor configured to provide a distributed response along its length to a measurand of interest, the fiber optic distributed sensor having first sections interspaced with second sections, wherein each first section is located between second sections of a pair of second sections, wherein the first sections have an enhanced sensitivity to the measurand relative to the second sections; and

launching optical pulses into the fiber optic distributes sensor;

determining phase differences between detected backscattered optical signals generated in the second sections of the pairs in response to launched optical pulses to determine the distributed response; and

determining discrete responses from backscatter optical signals generated in the first sections in response to launched optical pulses, wherein a characteristic of the measurand is determined based on the distributed response and the discrete responses.

2. The method as recited in claim 1, wherein the detected backscattered optical signals comprise coherent Rayleigh noise.

3. The method as recited in claim 2, further comprising:

detecting coherent Rayleigh noise generated in at least a subsection fo the first section; and

using the detected coherent Rayleigh noise to assist unwrapping of the phase difference between the detected backscattered optical signals generated in the second sections of the pair between which the first section is located.

4. The method as recited in claim 1, wherein the first sections comprise coiled lengths of optical fiber coupled to a transducer.

5. The method as recited in claim 4, wherein the transducer transduces the measurand to a strain on the optical fiber.

6. The method as recited in claim 1, wherein the second sections are shielded to reduce sensitivity to the measurand.

7. The method as recited in claim 1, wherein the launched optical pulses comprise a first set of optical pulses and a second set of optical pulses, wherein the first set of optical pulses has at least one of a wavelength, frequency, and/or polarization that is different than the second set of optical pulses.

8. The method as recited in claim 1, wherein the launched optical pulses comprise a first set of optical pulses and a second set of optical pulses, wherein the first set of optical pulses has at least one of a wavelength, frequency, and/or polarization that is different than the second set of optical pulses.

9. A system comprising:

a fiber optic distributed sensor having sections of contrasting sensitivity to a measurand; and

an interrogation and acquisition system to launch optical pulses into the fiber optic distributed sensor and to detect coherent Rayleigh noise (CRN) generated in the sections of contrasting sensitivity in response to the launched optical pulses, the interrogation and acquisition system configured to determine a distributed response along the length of the fiber optic distributed sensor based on a phase difference between the CRN generated by sections of a pair of sections of lower sensitivity, where a section of high sensitivity is located between the section of the pair of sections of lower sensitivity, the interrogation and acquisition system further configured to determine individual responses along the length of the fiber optic distributed sensor based on detected CRN generated in the sections of higher sensitivity,

wherein characteristics of the measurand are determined using the individual responses to adjust the distributed response.

10. The system as recited in claim 9, wherein the sections of higher sensitivity to the measurand alternate with the sections of lower sensitivity to the measurand.

11. The system as recited in claim 9, wherein the interrogation and acquisition system is further configured to use the detected CRN generated within the sections of higher sensitivity to assist unwrapping of the phase difference.

12. The system as recited in claim 9, wherein the sections of higher sensitivity comprise discrete sensors configured to respond to the mesaurand.

13. The system as recited in claim 12, wherein the discrete sensors comprise coils of optical fiber coupled to transducers, and the sections of lower sensitivity comprise lengths of optical fiber that are not coupled to a transducer.

14. The system as recited in claim 12, wherein the sections of lower sensitivity are shielded from the measurand.

15. The system as recited in claim 9, wherein the interrogation and acquisition system comprises an optical source for launching first optical pulses to measure a distributed response along the length of the fiber optic distributed sensor and second optical pulses to measure individual responses from the sections of higher sensitivity.

16. The system as recited in claim 15, wherein the fiber optic distributed sensor further comprises wavelength selective elements at opposing ends of the sections of higher sensitivity.

17. The system as recited in claim 16, wherein the first optical pulses have a first wavelength and the second optical pulses have a second wavelength that is different than the first wavelength.

18. The system as recited in claim 16, wherein the wavelength selective elements are fiber Bragg gratings.

19. The system as recited in claim 9, wherein the interrogation and acquisition system comprises an optical source for launching a plurality of optical pulses into the fiber optic distributed sensor, wherein the optical pulses comprise a first set of pulses having a first characteristic and a second set of pulses having a second characteristic that is different than the first characteristic.

20. A method for making precision measurements in a distributed fiber optic sensor system, comprising:

launching a plurality of optical pulses into the distributed fiber optic sensor system;

acquiring a distributed measurement of a measurand from an optical fiber sensor configured to respond along its length to the measurand, wherein the optical fiber sensor has first sections having an enhanced sensitivity to the measurand interspaced with second sections having a reduced sensitivity to the measurand,

wherein acquiring the distributed measurement comprises;

detecting coherent Ralyleigh noice (CRN) generated by the optical fiber sensor in response to the plurality of optical pulses;

comparing phases of the detected CRN generated in second sections between which a first section is located; and

adjusting the compared phases based on detected CRN generated in the first section to determine characteristics of the measurand.

21. The method as recited in claim 20, further comprising using the detected CRN generated within the first section to guide unwrapping of a phase difference in the CRN generated between the second sections between which the first section is located.

22. The method as recited in claim 20, wherein the optical pulses comprise a first set of optical pulses having a first characteristic and a second set of optical pulses having a second characteristic that is different than the first characteristic.

23. The method as recited in claim 20 wherein the optical pulses include a first pulse having a first wavelength and a second pulse having a second wavelength, the method further comprising:

determining a distributed response along the length of the optical fiber sensor from the CRN generated in response of the first pulse; and

determining individual responses from the first sections having the enhanced sensitivity from the CRN generated in response to the second pulse.

24. The method as recited in claim 20, wherein each first section includes a length of optical fiber coupled to a transducer configured to response to the measurand, and each second section includes a length of optical fiber shielded from the measurand.

25. A system to determine characteristics of a measurand detected by a distributed fiber optic sensor, comprising:

an interrogation and data acquisition system comprising configured to launch optical pulses into the distributed fiber optic sensor;

at least one processor; and

at least one storage medium to store executable code that, when executed by the at least one processor, causes the interrogation and data acquisition system to;

acquire a distributed measurement of a measurand detected by an optical fiber sensor that responds along its length to the measurand, wherein the optical fiber sensor has first regions alternating with second regions, the first regions having an enhanced sensitivity to the measurand relative to the second regions;

detect coherent Rayleigh noise (CRN) generated by the optical fiber sensor;

compare phases of the detected CRN generated in second regions between which a first region is located; and

adjust the compared phases based on detected CRN generated in the first region to determine characteristics of the measurand.

26. The system as recited in claim 25, wherein the executable code, when executed by the at least one processor, further causes the interrogation and data acquisition system to use detected CRN generated with the first region to guide unwrapping of a phase difference in the CRN generated between the second regions between which the first region is located.