Method and apparatus for optical sensing

Patent metadata

Field	Value
Publication number	US-11079269-B2
Application number	US-201916508522-A
Country	US
Kind code	B2
Filing date	Jul 11, 2019
Priority date	May 27, 2009
Publication date	Aug 3, 2021
Grant date	Aug 3, 2021

Abstract

The present invention provides novel apparatus and methods for fast quantitative measurement of perturbation of optical fields transmitted, reflected and/or scattered along a length of an optical fibre. The present invention can be used for point sensors as well as distributed sensors or the combination of both. In particular this technique can be applied to distributed sensors while extending dramatically the speed and sensitivity to allow the detection of acoustic perturbations anywhere along a length of an optical fibre while achieving fine spatial resolution. The present invention offers unique advantages in a broad range of acoustic sensing and imaging applications. Typical uses are for monitoring oil and gas wells such as for distributed flow metering and/or imaging, seismic imaging, monitoring long cables and pipelines, imaging within large vessel as well as for security applications.

First claim

The invention claimed is:

1. An apparatus for monitoring seepage along a dam or a dyke, the apparatus comprising:
an optical fiber distributed acoustic sensing system, the optical fiber distributed acoustic sensing system comprising an optical sensing fiber deployed in use along the dam or dyke and arranged to receive an input optical signal and propagate the received optical signal along its length; and
an acoustic source located in use within a body of fluid contained by the dam or dyke, or within the dam or dyke itself;
wherein the optical fiber distributed acoustic sensing system is arranged in use to process light backscattered and/or reflected from along the optical sensing fiber as the optical signal travels therealong, the light being backscattered and/or reflected in dependence on acoustic signals incident along the length of the optical sensing fiber, the backscattered and/or reflected light being processed to determine a relative optical phase modulation along at least one section of the optical sensing fiber to thereby measure a strength of an acoustic signal generated by the acoustic source, and to determine areas of seepage in the dam or dyke based thereon;
wherein the optical fiber distributed acoustic sensing system is arranged in use to detect a louder acoustic signal in areas of seepage, wherein areas of seepage provide low acoustic impedance paths for acoustic wave transmission.

2. An apparatus according to claim 1 , wherein the optical fiber distributed acoustic sensing system includes an interferometer arranged to receive backscattered light from along the optical sensing fiber, the interferometer comprising at least two optical paths with a path length difference therebetween, the backscattered light interfering in the interferometer to produce interference components, the DAS system further comprising plural photodetectors to measure the interference components, and a processor arranged to determine optical phase angle data therefrom.

3. An apparatus of claim 2 , wherein the interferometer further comprises an optical coupler arranged to introduce a relative phase shift between the interference components.

4. An apparatus of claim 2 , wherein the interferometer further comprises an optical amplifier arranged to amplify the received backscattered light to produce an amplified light signal.

5. An apparatus of claim 4 , wherein the interferometer further comprises an optical filter to filter out the out of band Amplified Spontaneous Emission (ASE) noise generated by the optical amplifier from the amplified light signal.

6. An apparatus according to claim 1 , wherein the optical fiber distributed acoustic sensing system includes an interferometer arranged to receive reflected light from along the optical sensing fiber, the interferometer comprising at least two optical paths with a path length difference therebetween, the reflected light interfering in the interferometer to produce interference components, the DAS system further comprising plural photodetectors to measure the interference components, and a processor arranged to determine optical phase angle data therefrom.

7. An apparatus of claim 6 , wherein the interferometer further comprises an optical coupler arranged to introduce a relative phase shift between the interference components.

8. An apparatus of claim 6 , wherein the interferometer further comprises an optical amplifier arranged to amplify the received reflected light to produce an amplified light signal.

9. An apparatus of claim 8 , wherein the interferometer further comprises an optical filter to filter out the out of band Amplified Spontaneous Emission (ASE) noise generated by the optical amplifier from the amplified light signal.

10. An apparatus of claim 6 , wherein the interferometer is further arranged to receive backscattered light from along the optical sensing fiber.

11. An apparatus according to claim 1 , wherein the optical fiber distributed acoustic sensing system comprises an optical source configured to output a pulsed optical signal.

12. An apparatus according to claim 1 , wherein the optical sensing fiber is arranged in use to receive a pulsed optical signal, the pulsed optical signal being constrained by the optical sensing fiber such that it propagates therealong in a first direction, the pulsed optical signal being backscattered and/or reflected along the length of the optical sensing fiber, the backscattered and/or reflected light being constrained by the optical sensing fiber such that it propagates therealong in a second direction opposite the first direction, wherein acoustic perturbations generated by the acoustic signal of the acoustic source incident along the length of the optical sensing fiber cause the optical sensing fiber to expand and contract such that the backscattered and/or reflected light is modulated.

13. An apparatus according to claim 12 , wherein the optical fiber distributed acoustic sensing system comprises a means for receiving the backscattered and/or reflected light from along the length of the optical sensing fiber.

14. An apparatus according to claim 13 , wherein the means for receiving comprises optical componentry.

15. An apparatus according to claim 13 , wherein the optical fiber distributed acoustic sensing system comprises a means for processing the received backscattered and/or reflected light to measure the phase, frequency and amplitude data of the received backscattered and/or reflected light to provide quantitative measurements of the acoustic perturbations incident along the length of the fiber to thereby measure a strength of the acoustic signal generated by the acoustic source, wherein the processing comprises measuring the backscattered and/or reflected light received from each contiguous section of optical sensing fiber along its length based on the time taken for the pulsed optical signal to propagate along the length of the optical fiber in the first direction, and the time taken for the backscattered and/or reflected light to propagate back in the second direction, to thereby map the received backscattered and/or reflected light to a respective section of optical fiber.

16. An apparatus according to claim 15 , wherein the means for processing comprises a plurality of photodetectors and a processor.

17. A method of monitoring seepage along a dam or dyke using an optical fiber distributed acoustic sensing system, the method comprising:
measuring, using the optical fiber distributed acoustic sensing system, an acoustic signal generated by an acoustic source located within the body of fluid contained by the dam or dyke, or within the dam or dyke itself, wherein the optical fiber distributed acoustic sensing system:
transmits an optical signal into an optical sensing fiber deployed along the dam or dyke, the optical signal being backscattered and/or reflected in dependence on acoustic signals incident along the length of the optical sensing fiber; and
processes the backscattered and/or reflected light to determine a relative optical phase modulation along at least one section of the optical sensing fiber to thereby measure a strength of the acoustic signal generated by the acoustic source; and

determining areas of seepage in the dam or dyke based on the measured acoustic signal, wherein the optical fiber distributed acoustic sensing system detects a louder acoustic signal in areas of seepage, wherein areas of seepage provide low acoustic impedance paths for acoustic wave transmission.

18. A method according to claim 17 , wherein the method further comprises:
transmitti

Assignees

• Silixa Ltd

Inventors

• Farhadiroushan Mahmoud
• Parker Tom Richard
• Shatalin Sergey

Classifications

• G01M11/085

  the optical fiber being on or near the surface of the DUT · CPC title

• G01M5/0091

  by using electromagnetic excitation or detection · CPC title

• G01M5/0058

  of elongated objects, e.g. pipes, masts, towers or railways · CPC title

• G01D5/35358

  using backscattering to detect the measured quantity · CPC title

• G01V1/40

  specially adapted for well-logging · CPC title