Study On Discrete-axes 4-Component Fiber Optic Geophone System Based On Heterodyne Detection Scheme

The geophone plays a significant role in many fields, such as earthquake early warning, oil exploration, and geological analysis. A fiber optic geophone system has the characteristics of large dynamic range, resistance to electromagnetic interference insulation, corrosion resistance, and long distance transmission, therefore it has attracted many attentions. The discrete structure geophone is easily manufactured and therefore suitable for time-division multilplexing(TDM) optical fiber sensing systems, which has great significance to promote the practical application of the geological system.In this subject, a discrete-axes 4-component fiber optic geophone system based on heterodyne detection scheme is studied. A one-dimensional asymmetric fiber optic acceleration geophone made of thin-walled shell is proposed and analyzed. Using optical sync reference signal, a heterodyne detection system is constructed and its stability is tested with the noise level reduced. Experiments are designed to test the discrete-axes 4-component fiber optic geophone system.The main research results and innovations are as follows:(1)A one-dimensional asymmetric fiber optical acceleration geophone made of thin-walled shell is proposed. The model of the asymmetric structure is proposed, and the influences of different parameters on its accelerator sensitivity and frequency response are analyzed. The numerical results show that, when both ends of the elastic cylinder radius are equal, and non-sensing side elastic column height is greater than the sensing side elastic cylinder, the accelerator sensitivity of the asymmetric structure is larger than the symmetric structure.(2)A heterodyne detection scheme for 4-component fiber optic geophone system is analyzed. The resource of the noise in low frequency is analyzed theoretically and tested in experiments. Methods are used to reduce the noise level to less than-90 dB/√Hz in the frequency range above 100 Hz. Experiments show the fluctuation of the heterodyne detection scheme is less than 0.1dB. The comparison system with phase generated carrier(PGC) system is built, showing that the difference between the experimental results of two systems is less than 2.5%.(3)With the tandem reflective structure, a discrete-axes 4-component fiber optic geophone is built. Experimental results show the acceleration response of its three axis from 20 Hz to 200 Hz is flat, and the acceleration sensitivity is 47.5d B,48.3dB and 49.5dB, respectively. The difference between three axis from 20 Hz to 200 Hz is less than 2.0dB. Quadrature crosstalk is less than-20.1dB. Its dynamic range is better than 137.4dB. At 200 Hz, the minimum detectable acceleration of the system is125.3 ng/ Hz.