Design And Experimental Research Of Optical Fiber Vibration Sensor

Oil and natural gas are the main energy sources for human life and production.With the rapid growth of demand for oil and gas resources,the development of unconventional oil and gas reservoirs has become the main way to increase oil and gas production.Microseismic monitoring technology has become a reliable basis for judging the formation and development trend of fracturing fractures.The monitoring results provide an important guarantee for improving the level of shale gas exploitation and the recovery of unconventional oil and gas,and are of great significance for monitoring the stability of mines,engineering structures and their geological structures.Vibration sensor is a key device in this technology,and its performance is very important.Compared with electrical sensors,optical fiber vibration sensors have the advantages of high sensitivity,large bandwidth,anti-electromagnetic interference,and easy large-scale networking.For this reason,the basic research of fiber-optic vibration sensor used in microseismic monitoring technology is carried out using fiber bragg grating(FBG)and fiber Mach-Zehnder(M-Z)interferometer as sensing elements.The main work of the paper is as follows:(1)The geophone types in oil and gas seismic exploration are briefly introduced,and the working principle,development and research status of fiber-optic vibration sensors are summarized.The FBG coupled mode theory and the FBG strain and temperature sensing principle are expounded,the mechanical model of the FBG vibration sensor is established,and its response characteristics are analyzed.(2)A FBG vibration sensor based on elastic hinge structure is proposed,the working principle of the sensor is expounded,the calculation formula of the rotational stiffness of the elastic hinge is deduced,the strain model and vibration theoretical model of the sensor are established,and the effect of the sensor parameters on the natural frequency and sensitivity is theoretically studied.Based on the establishment of the theoretical model,the performance of the sensor is optimized.The finite element model of the sensor is established using the COMSOL finite element simulation software,the characteristic frequency and stress modal distribution of the sensor are solved,and the amplitude-frequency characteristic curve of the sensor is given.The finite element simulation results are consistent with the theoretical numerical simulation results.(3)According to numerical analysis and finite element simulation,the structural parameters of the FBG vibration sensor based on the elastic hinge structure are optimized,and the sensor sample is fabricated.Further experimental results show that the resonant frequency of the sensor is 630 Hz,and the vibration response of the sensor is tested when the excitation frequency is 50 Hz,100 Hz,200 Hz and 300 Hz,and the sensitivities are 59.97 pm/g,53.58 pm/g,54.45 pm/g and 63.8 pm/g with linearities of 0.9998,1,0.9982 and 1,respectively.The sensor dynamic range is up to 58 d B.The anti-transverse interference experiment of the sensor shows that the lateral response amplitude does not exceed 9% of the working direction,indicating that the sensor has a good ability to resist lateral interference.The free tapping response experiment shows that the peak of the sensor’s spatial spectrum is 621.67 Hz,which is close to the resonant frequency in the amplitude-frequency characteristic experiment.The temperature response experiment shows that the temperature sensitivity of the sensor is 15.53 pm/℃,and the crosssensitivity between acceleration and temperature reaches 0.268 g/℃.The decoupling temperature method is discussed.Finally,the sensor performance is compared,and the further optimization of the sensor is discussed.(4)A fiber M-Z vibration sensor based on fiber cylinder structure(FCS)is proposed and designed.The working principle of the sensor is expounded,the mode field distribution and dispersion characteristics of seven core fiber(SCF)are studied by means of finite element software,and a preliminary experimental study of the sensor is carried out.The strain sensitivity is 1.26 pm/με and the maximum tensile force it can withstand is 5 N.Finally,the demodulation method of the vibration sensor based on fiber optic interferometer is discussed.