Research On Fiber Optic Acceleration Sensors For Microseismic Monitoring

With the increase in the depth of energy extraction such as coal mines and the digging of deep buried tunnels,the phenomenon of high gas pressure and high geostress becomes more and more obvious.Dynamic disasters dominated by coal and gas outburst and tunnel impact ground pressure often occur,causing major casualties and economic losses.The breeding and occurrence of these disasters involve the whole process of coal and rock mass fracture from micro scale,small scale,mesoscale to large scale,and accompanied by large numbers of microseismic?MS?activities,the effective monitoring of these MS activities can realize the prediction and early warning of dynamic disasters.Research and practice show that MS monitoring technology has become one of the most effective methods for MS activity monitoring.At present,traditional electrical MS monitoring equipment has been widely used,but there are still problems such as front-end power supply and susceptibility to electromagnetic interference,which limits its use in flammable and explosive environments such as coal mines.The introduction of interferometric fiber-optic sensors into MS monitoring has unique advantages such as intrinsic safety,high sensitivity,wide frequency band,and large dynamics,leading to a good application prospect.Based on the principle of fiber-optic interferometric sensing,this paper has developed diaphragm structure and compliant cylinder fiber optic acceleration sensors for MS monitoring,and designed an 8-channel highly sensitivity fiber optic MS monitoring system,and a field test of this system was carried out in Huainan Mining Group Coal Company.The main research of this work as follows:1.The MEMS?Micro-Electro-Mechanical System?micro-nano processing technology is used to prepare the aluminum-polyimide-aluminum composite film,and the intermediate silicon-based part is reserved as an inertial mass form a composite film-mass integrated structure in the etching process,and the thickness of the film can be precisely controlled on the order of nanometers.Combined the composite film with Fabry-Perot interferometer,three 10-mm diameter acceleration sensors with the same parameters were produced,and had comprehensive performances such as acceleration resolution,frequency response characteristics,and high temperature resistance,and perform stress analysis and experimental verification.The experimental results are consistent with the theoretical analysis and have good consistency.It can provide theoretical guidance for the mass production of fiber optic acceleration sensors.2.Dynamic spectrum demodulation technology was put forward to be applied in the FOFPA system,studing the relationship between dynamic spectrum and acceleration frequency and amplitude,which realize the demodulation of vibration signal amplitude with sensitivity of17 mW/g.Vibration time-domain signal is recovered from the dynamic spectrum self-fitting difference.The feasibility of this algorithm is verified by MATLAB numerical simulation and experimental analysis.Finally,the frequency detection limit of dynamic spectrum demodulation technology is analyzed.The dynamic spectrum demodulation technology of the self-fitting difference method solves the problem of difficult detection of low-frequency signals caused by external environmental interference.3.An asymmetrical compliant cylinder fiber-optic acceleration sensor with adjustable damping is proposed.The influence of geometric dimensions such as the base thickness,wall thickness,and height of the acceleration sensor package on its performance is analyzed for the first time,and ANSYS finite element modal analysis and experimental verification are performed.Optimize the sensing unit parameters and prestress analysis,and fiber-optic acceleration sensors are developped for coal and gas outburst microseismic monitoring,with a sensitivity of 350 rad/g and a frequency bandwidth of 20 Hz-1.9 kHz.The impact of dampers of different materials such as metal bellows,springs,polymers,etc.on system damping is mainly studied.The sensitivity and resonance frequency of the acceleration sensor can be adjusted through the design of the damping mechanism,and a wide-frequency response probe with a bandwidth of 10 kHz is developed,which provide effective solutions for the application of optical fiber microseismic monitoring system in hard rock environments such as metal mines.4.An 8-channel high-sensitivity fiber-optic MS monitoring system was built based on space division multiplexing technology.The system has an acceleration resolution of 0.15?g/Hz^1/2,a dynamic range of 116 dB,and an operating frequency band of 20 Hz to 1.9 kHz,and each channel has good consistency.The MS field test was carried out at Paner Mine of Huainan Mining Group Coal Company,and the effective signals such as knock,blast and MS were obtained and analyzed,which verified the feasibility of the optical fiber MS monitoring system.The research in this thesis provides key support for the prediction and prevention of dynamic disasters such as coal and gas outbursts in deep high gas mines,and rock bursts in tunnels,and it will play a positive role in promoting the development of mining,rock mechanics and engineering disciplines.