Fibre Optic Acoustic Sensing

• US 20120137781A1
• Filed: 08/21/2009
• Published: 06/07/2012
• Est. Priority Date: 08/21/2008
• Status: Active Grant

• Current Assignee: Optasense Holdings Limited (Luna Innovations, Inc.)
• Original Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Inventors: Hill, David John, McEwen-King, Magnus
• Granted Patent: US 9,625,348 B2
• Time in Patent Office: Days
• US Class Current: 73/655
• CPC Class Codes: F17D 5/06 using electric or acoustic means
• CPC Class Codes: G01D 5/48 using wave or particle radiation means G01D5/26 takes precedence
• 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: G01M 3/047 with photo-electrical detection means, e.g. using optical fibres
• CPC Class Codes: G01M 3/243 for pipes
• CPC Class Codes: G01P 3/14 by exciting one or more mechanical resonance systems

Abstract

This invention relates to the fibre optic distributed acoustic sensing to detect P and S waves in a solid medium. Distributed acoustic sensing can be achieved using an unmodified fibre optic by launching optical pulses into the fibre and detecting radiation which is Rayleigh backscattered there from. By analysing the returns in analysis bins, acoustic disturbances can be detected in a plurality of discrete longitudinal sections of the fibre. The present invention extends such fibre distributed acoustic sensing to detection of S and P waves.

Claims

1. A method of detecting P and S waves in a solid medium comprising:

repetitively launching at least a first optical pulse and a second optical pulse into a optic fibre located, at least partly, within said solid medium wherein the first and second pulses have a first optical frequency difference;

detecting light which is Raleigh backscattered from the optic fibre;

analysing the backscattered light to determine a measure of disturbance for each of a plurality of discrete longitudinal sensing portions of the optic fibre, and

analysing the evolution of a disturbance in the discrete longitudinal sensing portions to detect P and S waves.

2. A method as claimed in claim 1 wherein analysing the evolution of a disturbance in the discrete longitudinal sensing portions comprises identifying a first series of disturbances in the plurality of discrete longitudinal sensing portions followed by a second related series of disturbances.

3. A method as claimed in claim 2 wherein the second series of disturbances is related to the first series of disturbances by affecting substantially the same discrete longitudinal sensing portions in substantially the same order.

4. A method as claimed in claim 3 wherein the second series of disturbances has a slower propagation then the first series of disturbances.

5. A method as claimed in claim 1 wherein the method comprises analysing the evolution of the disturbance of discrete longitudinal sensing portions together with the location of each discrete longitudinal sensing portion to identify a first wavefront followed by a second, slower wavefront.

6. A method as claimed in claim 5 comprising the step of determining the degree of curvature of one or both of the first wavefront and the second wavefront.

7. A method as claimed in claim 1 comprising the further step of analysing the detected P and S waves to determine the origin of the P and S waves.

8. A method as claimed in claim 7 wherein the differences in time of arrival of the p and s waves at two or more different discrete longitudinal sensing portions are used to determine the origin of the P and S waves.

9. A method as claimed in claim 5, wherein the shape of at least one of the first and second wavefronts is used to determine the origin of the P and S waves.

10. A computer program which, when run on a suitable computer, performs the method as claimed in claim 1.

11. An apparatus for detecting P and S waves in a solid medium comprising:

an optic fibre located at least partly within said medium;

an optical source configured to launch at least a first optical pulse and a second optical pulse into the optic fibre wherein the first and second pulses having a first optical frequency difference, the optical power of the pulses being below a non-linear threshold;

an optical detector configured to detect Rayleigh backscattering from the optic fibre;

and a processor configured to;

analyse the backscattered light to determine a measure of disturbance for a plurality of discrete longitudinal sensing portions of the optic fibre; and

analyse the evolution of a disturbance in the discrete longitudinal sensing portions to detect P and S waves.

12. An apparatus as claimed in claim 11 wherein the optic fibre is a continuous, unmodified optic fibre.

13. An apparatus as claimed in claim 11 wherein the processor is arranged to determine the origin of the P and S waves relative to the optic fibre.

14. (canceled)