Method and Apparatus for Optical Sensing

Abstract

An optical fiber distributed acoustic sensor system makes use of a specially designed optical fiber to improve overall sensitivity of the system by a factor in excess of 10. This is achieved by inserting into the fiber weak broadband reflectors periodically along the fiber. The reflectors reflect a small proportion of the light from the DAS incident thereon back along the fiber, typically in the region of 0.001% to 0.1%. To allow for temperate compensation to ensure that the same reflectivity is obtained if the temperature changes, the reflection bandwidth is relatively broadband. The reflectors are formed from a series of fiber Bragg gratings, each with a different center reflecting frequency, the reflecting frequencies and bandwidths of the gratings being selected to provide the broadband reflection. The reflectors are spaced at the desired spatial resolution of the optical fiber DAS.

What is claimed is: 
     
         1 . An optical fiber distributed sensor system, comprising:
 an optical source arranged in use to produce optical signal pulses;   an optical fiber deployable in use in an environment to be sensed and arranged in use to receive the optical signal pulses; and   sensing apparatus arranged in use to detect light from the optical signal pulses reflected back along the optical fiber and to determine any one or more of an acoustic, vibration, or temperature parameter that perturbs the path length of the optical fiber in dependence on the reflected light;   the system being characterized in that the optical fiber comprises:   at least one first sensing region having a plurality of reflector portions distributed along its length; and   at least one pulse transmission portion being a section of optical fiber in which no reflector portions are provided, the at least one pulse transmission portion being located between the optical source and the at least one first sensing region, and configured in use to transport the optical signal pulses from the optical source to the at least one first sensing region having the reflector portions,   wherein the at least one pulse transmission portion has a length greater than a distance between the reflector portions of the at least one first sensing region, and   the system is an optical fiber distributed acoustic sensor system arranged to sense acoustic signals incident upon the optical fiber.   
     
     
         2 . A system according to  claim 1 , wherein the sensing apparatus determines the any one or more of an acoustic, vibration, or temperature parameter that perturbs the path length of the optical fiber in dependence on the reflected light from the reflector portions for the at least one first sensing region, and determines the any one or more of an acoustic, vibration, or temperature parameter that perturbs the path length of the optical fiber in dependence on backscatter from the at least one pulse transmission portion. 
     
     
         3 . A system according to  claim 1 , wherein a plurality of separate first sensing regions are provided each having reflector portions formed therein, connected in series by transmission portions of fiber where no reflector portions are formed. 
     
     
         4 . A system according to  claim 3 , wherein the plurality of separate first sensing regions have respective sets of reflector portions that are arranged to reflect different wavelengths of light, wherein the optical fiber distributed sensor system is able to provide spatial selectivity in terms of which set of reflector portions at which spatial location it wants to receive reflections from and thereby enable sensing at that location, by varying the wavelengths of the transmitted pulses to match the reflector wavelengths of the set of reflector portions that are to be selected. 
     
     
         5 . A system according to  claim 1 , wherein the optical fiber is deployed in a U shape having an outward leg of optical fiber and a return leg of optical fiber, where the reflector portions in the at least one first sensing region are positioned at the far end of the outward leg of the optical fibre and continue to the top of the return leg of optical fiber. 
     
     
         6 . A system according to  claim 5 , wherein the optical fiber U-shape is deployed into an oil well, wherein the first reflector portion encountered in the at least one first sensing region is at the bottom of the well. 
     
     
         7 . A method of operating an optical fiber distributed sensor system, comprising:
 using an optical source arranged in use to produce optical signal pulses;   using an optical fiber deployed in an environment to be sensed to receive the optical signal pulses; and   using a sensing apparatus to detect light from the optical signal pulses reflected back along the optical fiber and to determine any one or more of an acoustic, vibration, or temperature parameter that perturbs the path length of the optical fiber in dependence on the reflected light;   the method being characterized in that the optical fiber comprises at least one first sensing region having a plurality of reflector portions distributed along its length and at least one pulse transmission portion being a section of optical fiber in which no reflector portions are provided, the pulse transmission portion being located between the optical source and the at least one first sensing region, and configured to transport the optical signal pulses from the optical source to the at least one first sensing region having the reflector portions,   wherein the at least one pulse transmission portion has a length greater than a distance between the reflector portions of the at least one first sensing region, and   the system is an optical fiber distributed acoustic sensor system arranged to sense acoustic signals incident upon the optical fiber.   
     
     
         8 . A method according to  claim 7 , and further comprising using the sensing apparatus to determine the any one or more of an acoustic, vibration, or temperature signal that perturbs the path length of the optical fiber in dependence on the reflected light from the reflector portions for the at least one first sensing region, and determines the any one or more of an acoustic, vibration, or temperature parameter that perturbs the path length of the optical fiber in dependence on backscatter from the at least one pulse transmission portion. 
     
     
         9 . An optical fiber distributed sensor system, comprising:
 a first optical source arranged in use to produce optical signal pulses of a first wavelength;   a second optical source arranged in use to produce optical signal pulses of a second wavelength;   an optical fiber deployable in use in an environment to be sensed and arranged in use to receive the optical signal pulses;   a first sensing apparatus arranged in use to detect light from the optical pulses of the first wavelength reflected back along the optical fiber and to determine acoustic signals incident on the optical fiber in dependence on the reflected light; and   a second sensing apparatus arranged in use to detect light from the optical pulses of the second wavelength reflected back along the optical fiber and to determine any one or more of an acoustic, vibration, temperature or other parameter that perturbs the path length of the optical fiber in dependence on the reflected light;   wherein the optical fiber is provided with first reflector portions arranged to reflect at least a portion of signals of the first wavelength in a first section of the optical fiber, and the with second reflector portions arranged to reflect at least a portion of signals of the second wavelength in a second section of the optical fiber.   
     
     
         10 . A system according to  claim 9 , wherein the first reflector portions are spaced apart from each other differently to the second reflector portions, whereby to provide different spatial sensing resolutions in the first and second sections of the optical fiber. 
     
     
         11 . An optical fiber distributed sensor system according to  claim 9 , wherein the system is an optical fiber distributed acoustic sensor system arranged to sense acoustic signals incident upon the optical fiber. 
     
     
         12 . An optical fiber distributed sensor system according to  claim 9 , wherein a product of the number of reflector portions and the average reflectivity of the reflector portions is 0.1 or less. 
     
     
         13 . An optical fiber distributed sensor system according to  claim 9 , wherein the first section of the optical fiber and the second section of the optical fiber are different portions of the optical fiber. 
     
     
         14 . An optical fiber distributed sensor system according to  claim 9 , wherein the first section of the optical fiber and the second section of the optical fiber are overlapping portions of the optical fiber. 
     
     
         15 . An optical fiber distributed sensing system, comprising:
 an optical fiber deployable in an environment to be sensed, the optical fiber having reflector portions regularly distributed in at least a first region thereof and having a first spacing therebetween;   an optical signal source arranged in use to input optical pulses into the optical fiber; and   sensing apparatus arranged in use to detect light from the optical pulses reflected back along the optical fiber and to determine any one or more of an acoustic, vibration, temperature or other parameter that perturbs the path length of the optical fiber in dependence on the reflected light;   wherein the reflectivity of the reflector portions alters in dependence on the position of the reflector portion along the fiber.   
     
     
         16 . A system according to  claim 15 , wherein the crosstalk response of the fiber is tuned by varying the reflectivity of the reflector portions. 
     
     
         17 . A system according to  claim 15 , wherein the reflectivity of the reflector portions increases in dependence on:
 a) the distance along the fiber from the optical signal source; or   b) an optical loss along the fiber from the optical signal source.   
     
     
         18 . A system according to  claim 15 , wherein the reflectivity of the reflector portions increases:
 a) in dependence on optical losses of connectors and feedthroughs within the system; or   b) deterministically in accordance with a mathematical function of the distance along the fiber, wherein the mathematical function is a monotonic function relating distance along the fiber to reflectivity of the reflector portions.   
     
     
         19 . A system according to  claim 15 , wherein the reflectivity of the reflector portions are tuned to compensate for optical losses in the fibre and so equalise, or otherwise tune, the signal to noise performance, as well as tune crosstalk, along the fibre. 
     
     
         20 . A system according to  claim 15 , wherein a crosstalk contribution from regions along the optical fiber with a large acoustic signal of low importance can be negated by providing low reflectivity reflectors or no reflectors along that section.