PHASE SENSITIVE COHERENT OTDR WITH MULTI-FREQUENCY INTERROGATION

• US 20130113629A1
• Filed: 10/19/2012
• Published: 05/09/2013
• Est. Priority Date: 11/04/2011
• Status: Abandoned Application

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: Schlumberger Technology Corporation (SLB)
• Inventors: Hartog, Arthur H., Liokumovich, Leonid Borisovich
• Application Number:
  US13/656,499
• Publication Number:
  US 20130113629A1
• Time in Patent Office: Days
• US Class Current: 340/853.2
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• 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 1/226 Optoseismic systems

Abstract

A fiber optic sensor system includes a coherent-detection optical time domain reflectometry system to extract phase information from optical signals returned from a fiber optic sensor arrangement in response to a plurality of interrogating pulses. The system includes a frequency-shifting circuit to repeatedly translate the frequency of an optical pulse generated by a narrowband source to generate a train of interrogating pulses of multiple frequencies. The optical signals returned from the sensor arrangement in response to the pulse train is mixed on a photodetector with light from the narrowband source that has not been shifted to generate mixed output signals. The mixed output signals are filtered into frequency bands, and the phase for each frequency band is extracted.

Claims

1. An apparatus, comprising:

a narrowband optical source to generate a first optical signal having a first optical frequency;

a frequency-shifting circuit to generate a probe signal from the first optical signal to launch into a fiber optic sensor, the probe signal having a plurality of optical frequencies shifted from the first optical frequency;

a coherent detection system to mix backscatter signals generated by the fiber optic sensor in response to the probe signal with a local oscillator optical signal provided by the narrowband optical source to generate mixed output signals; and

a phase detection and acquisition system to filter the mixed output signals into frequency bands corresponding to the shifted frequencies, and to extract at least the phase of the mixed output signal for at least one of the frequency bands.

2. The apparatus as recited in claim 1, wherein the probe signal is a composite pulse composed of multiple optical frequencies, each optical frequency of the composite pulse being shifted from the first optical frequency.

3. The apparatus as recited in claim 2, wherein the local oscillator signal has an optical frequency shifted from the first optical frequency by a different amount than the multiple optical frequencies of the probe signal.

4. The apparatus as recited in claim 3, wherein the frequency-shifting circuit shifts the first optical frequency to generate the local oscillator signal.

5. The apparatus as recited in claim 4, wherein the frequency-shifting circuit generates a plurality of frequency sidebands to provide a corresponding plurality of probe signals and local oscillator signals, and wherein each local oscillator signal and backscatter signal generated in response to the probe signal derived from the same frequency sideband are mixed on a separate coherent detection system.

6. The apparatus as recited in claim 1, further comprising a first modulator to generate a first optical pulse from the first optical signal, and wherein the frequency-shifting circuit repeatedly shifts the first optical frequency of the first optical pulse to generate the probe signal, wherein the probe signal comprises a plurality of interrogating pulses having shifted frequencies.

7. The apparatus as recited in claim 6, where the local oscillator signal has an optical frequency that is not shifted from the first optical frequency.

8. The apparatus as recited in claim 6, further comprising a second modulator to select from the plurality of interrogating pulses selected interrogating pulses to launch into the fiber optic sensor.

9. The apparatus as recited in claim 6, further comprising a second narrowband optical source to generate a second optical signal, wherein the first modulator to generate a second optical pulse from the second optical signal, the optical pulse having a second frequency, and wherein the frequency-shifting circuit to repeatedly shift the second frequency of the second optical pulse to generate the plurality of interrogating pulses to launch into the fiber optic sensor, the plurality of interrogating pulses having a plurality of frequencies shifted from the first frequency and a plurality frequencies shifted from the second frequency.

10. The apparatus are recited in claim 6, wherein the frequency-shifting circuit repeatedly shifts the first frequency to generate a plurality of pulses of increasing frequencies.

11. The apparatus as recited in claim 6, wherein the frequency-shifting circuit repeatedly shifts the first frequency to generate a plurality of pulses of decreasing frequencies.

12. The apparatus as recited in claim 6, wherein the frequency-shifting circuit repeatedly shifts the first frequency to generate a plurality of pulses of increasing and decreasing frequencies.

13. The apparatus as recited in claim 1, wherein the coherent detection system is a heterodyne detection system.

14. The apparatus as recited in claim 1, wherein the backscatter light is Rayleigh backscatter light generated in response to the interrogating pulses.

15. The apparatus as recited in claim 1, wherein the backscatter light comprises reflected light from a plurality of discrete sensors.

16. A method of detecting a parameter of interest using a fiber optic sensor, comprising:

frequency-shifting a frequency of an optical signal from an optical source to generate a probe signal of shifted frequencies;

launching the probe signal into a fiber optic sensor;

mixing returned optical signals generated by the fiber optic sensor in response to the interrogating pulses with a local oscillator signal from the optical source to generate mixed output signals;

filtering the mixed output signals into frequency bands, each frequency band corresponding to the shifted frequencies;

extracting phase of the mixed output signal from at least one of the frequency bands; and

determining the parameter of interest based on the extracted phase.

17. The method as recited in claim 16, wherein the probe signal is a composite pulse composed of the multiple shifted frequencies.

18. The method as recited in claim 17, further comprising frequency shifting the frequency of the optical signal by a different amount than the multiple shifted frequencies to generate a local oscillator signal having a shifted frequency.

19. The method as recited in claim 16, wherein the probe signal is a plurality of pulses, each pulse having one of the shifted frequencies.

20. The method as recited in claim 19, wherein the local oscillator signal has a frequency that is not shifted from the frequency of the optical signal from the optical source.

21. The method as recited in claim 19, further comprising launching only selected interrogating pulses from the plurality of pulses into the fiber optic sensor.

22. The method as recited in claim 19, wherein frequency-shifting the frequency comprises repeatedly increasing the frequency to generate a plurality of interrogating pulses of increasing frequencies.

23. The method as recited in claim 19, wherein frequency-shifting the frequency comprises selectively increasing and decreasing the frequency to generate a plurality of interrogating pulses of increasing and decreasing frequencies.

24. The method as recited in claim 16, wherein the parameter of interest is at least one of strain and temperature.

25. The method as recited in claim 16, further comprising deploying the fiber optic sensor in a wellbore.

26. A system to detect a parameter of interest in a wellbore, comprising:

a fiber optic sensor system deployed in a wellbore;

a narrowband optical source to generate a first optical signal having a first optical frequency;

a frequency-shifting circuit to generate a probe signal from the first optical signal to launch into the fiber optic sensor, the probe signal having a plurality of optical frequencies shifted from the first optical frequency;

a coherent detection system to mix backscatter signals generated by the fiber optic sensor in response to the probe signal with a local oscillator signal provided by the narrowband optical source to generate mixed output signals; and

a phase detection and acquisition system to filter the mixed output signals into frequency bands corresponding to the shifted frequencies, and to extract at least the phase of the mixed output signal for at least one of the frequency bands, wherein the phase is indicative of the parameter of interest.

27. The system as recited in claim 26, wherein the probe signal is a composite pulse composed of multiple frequencies, each frequency of the composite pulse being shifted from the first optical frequency.

28. The system as recited in claim 27, wherein the local oscillator signal has an optical frequency shifted from the first optical frequency by a different amount than the multiple frequencies of the composite pulse.

29. The system as recited in claim 28, wherein the frequency-shifting circuit shifts the first optical signal to generate the local oscillator signal.

30. The apparatus as recited in claim 26, further comprising a first modulator to generate a first optical pulse from the first optical signal, and wherein the frequency-shifting circuit repeatedly shifts the first optical frequency of the first optical pulse to generate the probe signal, wherein the probe signal comprises a plurality of interrogating pulses having shifted frequencies.

31. The apparatus as recited in claim 30, wherein the local oscillator signal has an optical frequency that is not shifted from the first optical frequency.