Fibre optic acoustic sensing
First Claim
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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, andanalysing 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, anddetermining 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.
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Abstract
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.
35 Citations
7 Claims
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1. A method of detecting P and S waves in a solid medium comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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Specification