Research Of High-performance Fiber-optic Distributed Acoustic Sensor Based On Time-gated Digital Optical Frequency Domain Reflectometry

Compared with electrical sensor,fiber-optic sensor(FOS)has many advantages,such as having good resistance against corrosion,high temperature,high pressure,and electromagnetic interference.Fiber-optic distributed acoustic sensor(DAS)is one of the most attractive and promising FOS.In addition to advantages mentioned above,DAS has an unique capability,that is,every section of optical fiber can independently and simultaneously measure acoustic vibration.However,the most popular DAS based on phase-sensitive optical time domain reflectometry(?-OTDR)suffers from many drawbacks,such as the trade-off between spatial resolution and sensing distance,and fading noise.In 2014,a novel technique called time-gated digital optical frequency domain reflectometry(TGD-OFDR)was proposed.In 2015,a primary DAS based on it was realized,which can obtain high spatial resolution and long sensing distance at the same time due to pulse compression technique.Comprehensive performances of DAS based on TGD-OFDR are further enhanced in this thesis,as concluded below.1.Fading noise seriously deteriorates strain resolution of DAS and affects its reliability.The thesis proposes inner-pulse frequency-division method and vector-rotated-sum method,which improve the strain resolution and solve fading problem.In experiments,DAS has 5-8)spatial resolution,and 35-6)8)sensing distance.There is no serious fading noise occurring along the whole sensing fiber.The vibration can be linearly measured,and the bandwidth is 1.2 6) .2.Downhole applications require the spatial resolution is better than1 8),while sensing distance is km-scale,strain resolution is 9)-scale,the vibration can be linearly measured and the bandwidth is 6) -scale.The thesis proposes DAS based on TGD-OFDR with intensity modulator and I/Q receiver,and the fading-free methods above are adopted.In experiments,DAS has 0.8-8)spatial resolution,and 10-6)8)sensing distance,and the strain resolution is 245.6 /? .The vibration can be linearly measured,and the bandwidth is 5 6) .3.Pipeline,railway and cable monitoring need extremely long sensing distance.The thesis combines DAS based on TGD-OFDR and bi-directional first-order distributed Raman amplifier,and the fading-free methods above are adopted here.In experiments,DAS has 108-6)8)sensing distance.Compared with previous DAS that have sensing distance over 100 6)8),this DAS has best spatial resolution of 5 8)and strain resolution of 220 /? .Besides,the vibration can be linearly measured,and the bandwidth is 455 .4.Longer the sensing distance,narrower the response bandwidth of DAS.The thesis uses frequency division multiplexing(FDM)technique to extend the response bandwidth of DAS based on TGD-OFDR.Since there are multiple optical pulses with different frequencies existing in fiber,matched filter can realize frequency division demultiplexing,and improve the spatial resolution and strain resolution at the same time.In experiments,DAS has25-6)8)sensing distance.The response bandwidth is improved from 2 6) to 10 6) due to FDM,and the vibration can be linearly measured.5.The thesis proposes non-matched filter algorithm,and realizes DAS based on TGD-OFDR and rapid frequency demodulation method,which has flexible spatial resolution,is immune to fading noise,and can measure dynamic and static signal.In experiments,DAS has 1.6-8)spatial resolution,and 10-6)8)sensing distance.There is no fading noise,and the strain resolution is 10.2 9),or 144 /? .The vibration can be linearly measured,and the bandwidth is 5 6) .6.The thesis introduces a DAS prototype based on TGD-OFDR.The DAS prototype is applied in monitoring weak-pressure pipeline,and successfully detects the leaks of pipeline with 0.05- (6 pressure.The prototype has been set at Shengli Oilfield,Dongying City,Shandong Province.In conclusion,the key issues of DAS are discussed in this thesis.The configurations are modified,and the novel algorithms are proposed,in order to enhance the performances of DAS based on TGD-OFDR.The DAS is promising and has been applied in monitoring pipeline.