Fibre optic acoustic sensing
This invention relates to the fiber optic distributed acoustic sensing to detect P and S waves in a solid medium. Distributed acoustic sensing can be achieved using an unmodified fiber optic by launching optical pulses into the fiber and detecting radiation which is Rayleigh backscattered there from. By analyzing the returns in analysis bins, acoustic disturbances can be detected in a plurality of discrete longitudinal sections of the fiber. The present invention extends such fiber distributed acoustic sensing to detection of S and P waves.
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,
analysing the evolution of a disturbance in the 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 to detect P and S waves, and
analysing the detected P and S waves to determine their origin using the differences in time of arrival of the P and S waves at two or more different discrete longitudinal sensing portions by the further steps of:
determining a first time difference between times of arrival of the P wave and the S wave at a first discrete sensing portion,
determining a second time difference between the times of arrival of the P wave and the S wave at a second discrete portion, and
determining a ratio of the first and second time differences, wherein said determined ratio is assumed to be the ratio between the distances of the first and second sensing portions from the origin.
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 comprising the step of determining the degree of curvature of one or both of the first wavefront and the second wavefront.
6. A method as claimed in claim 1 , 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.
7. A computer program which, when run on a suitable computer, performs the method as claimed in claim 1 .