Distributed fibre optic sensing
Techniques for determining lateral offset of the source of an acoustic disturbance in a distributed acoustic fiber optic sensor are described. The sensor comprises an optical source ( 112 ) for interrogating an optical fiber ( 104 ) and a detector ( 116 ) and processor ( 108 ) arranged to detect any backscattered radiation and determine a measurement signal for a plurality of discrete longitudinal sensing portions of the optical fiber. The processor is also arranged to analyse the measurement signals to identify signals corresponding to the same acoustic wave arriving at different parts of the fiber and determine from the time of arrival of said acoustic wave the direction and/or distance of the origin of said acoustic wave from the optical fiber. The geometry of the fiber may be arranged to ensure that any positional ambiguity can be resolved and the use of multiple fibers ( 501, 502 ) is disclosed.
1. A distributed acoustic sensor comprising:
a first length of optical fibre, a source of electromagnetic radiation, a detector apparatus and a processor;
a second length of optical fibre running alongside the first length of optical fibre but separated therefrom in a first direction;
wherein the source of electromagnetic radiation is configured to launch electromagnetic radiation into said first length optical fibre and said second length of optical fibre;
the detector apparatus is for detecting electromagnetic radiation back-scattered from said first length of optical fibre and said second length of optical fibre; and
the processor is configured to:
process data corresponding to the detected back-scattered radiation to determine a measurement signal for each a plurality of discrete longitudinal sensing portions of each the first length of optical fibre and the second length of optical fibre;
analyse the measurement signal from said longitudinal sensing portions to identify signals corresponding to an acoustic wave arriving at one or more longitudinal sensing portions of the first length of optical fibre and at one or more longitudinal sensing portions of the second length of optical fibre; and
determine from the time of arrival of said acoustic wave at said longitudinal sensing portions the direction and/or distance of the origin of said acoustic wave from the optical fibre.
2. A distributed acoustic sensor as claimed in claim 1 wherein the first and second lengths of optical fibre comprise separate optical fibres.
3. A distributed acoustic sensor as claimed in claim 1 wherein the first and second lengths of optical fibre comprise different parts of the same optical fibre.
4. A distributed acoustic sensor as claimed in claim 1 wherein the processor determines the direction of the acoustic wave in the first direction based on which length of optical fibre detects the wave first.
5. A distributed acoustic sensor as claimed in claim 1 wherein the processor is configured to use the time of arrival of the acoustic wave at a plurality of longitudinal sensing portions of the first and/or second lengths of optical fibre to determine the distance to the origin of the acoustic wave.
6. A distributed acoustic sensor as claimed in claim 1 wherein the processor uses the time of arrival and a value representing a propagation speed of an acoustic wave to determine a lateral offset.
7. A distributed acoustic sensor as claimed in claim 6 wherein the value representing the propagation speed of the acoustic wave is a value determined previously through test or calibration and stored by the processor.
8. A distributed acoustic sensor as claimed in claim 1 comprising a third length of optical fibre running alongside the first and second lengths of optical fibre and offset therefrom in a second direction, wherein the second direction is perpendicular to the first direction.
9. A distributed acoustic sensor as claimed in claim 8 wherein the processor is configured to use the returns from all three lengths of optical fibre to determine the location of the origin of the acoustic wave in three dimensions.
10. A distributed acoustic sensor as claimed in claim 1 wherein the processor is further configured to perform frequency analysis on the measurement signals to determine the distance of the origin of said acoustic wave.
11. A method of determining the origin of an acoustic wave in distributed acoustic sensing comprising the steps of:
detecting with an electromagnetic radiation detector, electromagnetic radiation back-scattered from a first length of optical fibre and back-scattered from a second length of optical fibre running alongside the first length of optical fibre and separated therefrom in a first direction;
directing data corresponding to the detected electromagnetic radiation which has been back-scattered from the first length of optical fibre and detected electromagnetic radiation which has been back-scattered from the second length of optical fibre to a processor;
processing said data to provide a measurement signal for each of a plurality of longitudinal sensing portions of each the first length of optical fibre and the second length of optical fibre;
analysing the measurement signal from said longitudinal sensing portions to identify signals corresponding to a same acoustic wave arriving at one or more longitudinal sensing portions of the first length of optical fibre and one or more longitudinal sensing portions of the second length of optical fibre and
determining from the time of arrival of said acoustic wave at said longitudinal sensing portions the direction and/or distance of the origin of said acoustic wave from the optical fibre.