Research Of The Seismic Sensors Based On Fiber Bragg Gating

Along with the development of seismic exploration technology, especially the high-resolution depth exploration, the higher requirement about accuracy and quality has been proposed. In order to get better identification of lithology, fluid, fractured reservoir as well as improved reservoir location, reservoir characteristics, reservoir connectivity and enhanced oil recovery and so on, high-resolution capability, high SNR, high-fidelity, high-definition, high accuracy and high confidence will be required. On the thesis, a new type of double-diaphragm structure of fiber-optic seismic sensors has been presented. The seismic exploration for the "six high" requirement as an ideal technical means is provided.The thesis includes four sections:the developmental background of geophone, the basic principle and structure of fiber Bragg gratings and DFB fiber laser, the technology program of fiber optic seismic sensing systems, double-diaphragm structure of FBG geophone and DFB fiber laser geophone.Based on the theory of the coupled wave and the transfer matrix method, the spectral characteristics and sensor principles of the fiber Bragg gratings and DFB fiber laser have been analyzed. The system solution and its network technology which is based on the seismic sensors of fiber Bragg gratings and the seismic sensors of DFB fiber laser is introduced. In the sensor system, the key component—the packaging technology of unbalanced Michelson fiber-optic interferometer is introduced in detail. The optical fiber interferometer is vulnerable to outside temperature, sound and vibration, so a much higher phase noise than the resolution of the demodulation system is caused. Base on the theory of noise control and the theory of vibration control, the package program of sound isolation and vibration isolation in the fiber-optic interferometer is given, and an experiment about sound isolation and vibration isolation has been done. The result of the experiment shows that the effect of the sound isolation and vibration isolation in the interferometer with package structure is obvious. The average volume of noise is 30dB. The effect of vibration isolation is 20dB. The anti-environmental interference capability in the optical fiber interferometer has been improved. A double membrane structure of fiber-optic seismic sensors is proposed, whose structure can effectively restrict lateral vibration sensors. Base on the principle of elasticity, the mechanical model of fiber-optic seismic sensors with double-diaphragm structure has been established. The external acceleration response under vibration signal in the fiber-optic seismic sensors with the double membrane structure has been analyzed. The factors that the sensitivity is effected has been presented. From the experiment, It was found that the longitudinal acceleration sensitivity in the fiber grating seismic sensors is 24.5pm/g, the natural frequency is 890Hz, and the lateral acceleration sensitivity is about 10%.