Vector Optical Fiber Geophone System Based On ?–OTDR Technology

The distributed optical fiber vibration sensing system based on ?-OTDR technology senses by detecting the phase information carried in the Rayleigh scattered light signal to achieve high-density,long-distance distributed vibration sensing.It has been widely used at present Petroleum geophysical prospecting,structural health testing,pipeline security and other fields.Although the ?-OTDR system can realize distributed vibration detection,its sensitivity is not high compared with point-type fiber geophones,and it cannot effectively detect weak vibration signals,which limits its application in seismic exploration.This paper mainly studies the vector optical fiber geophone based on the ?-OTDR system,and improves the sensitivity of the system at a single point by acoustic wave sensitization,so as to realize the detection of weak seismic signals.The main work of this paper is as follows:First,in response to the requirements of high detection sensitivity and broadband response of fiber optic geophones,by analyzing the structural mechanical characteristics of the elastic cylindrical fiber geophone,the acceleration sensitivity and resonance frequency calculation formulas are derived,and the main parameters of the geophone probe are analyzed for the performance of the geophone.And verify the consistency of simulation analysis and theoretical calculation through finite element software.Under the guidance of the simulation results,the structural design and production of the elastic cylindrical fiber geophone was completed,the optical modulation and demodulation system based on ?-OTDR technology was built,and the performance of the fiber geophone was tested.The test results It shows that the acceleration sensitivity of the fiber optic geophone is about 1356rad/g,the frequency response range is not less than300 Hz,the acceleration resolution at 20 Hz is about 116 ng,and it has excellent directivity.A comparison test with the electronic geophone is designed to verify the two Consistency of response waveform and frequency spectrum.In addition,this article introduces the application of fiber optic geophones in seismic wave detection.Firstly,a high signal-to-noise ratio vibration signal was collected by a fiber-optic geophone deployed in the field for surface velocity extraction;secondly,the difference in response of the fiber-optic geophone and the fiber-optic cable to the same percussion signal was compared to prove the fiber-optic geophone The advantage of higher sensitivity at a single point,and it is more suitable for real-time detection and early warning of key nodes in pipeline safety monitoring;finally,due to the high cost and difficulty of deployment in large-scale,high-density microseismic monitoring of fiber optic geophones,etc.For the problem,an alternative solution for the optical fiber geophone-the optical fiber ring was proposed,and the consistency of the signals collected by the two was verified.At the same time,the optical fiber ring deployed on the surface of the fractured well successfully collected microseismic events.