Distributed Optical Seismic Wave Sensing System And Its Application In Exploration

The distributed optical seismic sensing system based on the phase-sensitive optical time domain reflection(?-OTDR)principle utilizes the principle of modulating vibration signals from Rayleigh scattered light in an optical fiber to detect vibration signals along the optical fiber.Because the optical fiber itself is both a transmission medium and a sensing medium of the ?-OTDR system,it can realize long-distance distributed vibration sensing,and is therefore widely used in border warning,building structure monitoring,high-speed rail safety monitoring,pipeline leakage detection,etc..At present,the research direction of the ?-OTDR system is mainly focused on the algorithms for digital signal acquisition and phase information demodulation and processing of Rayleigh scattered light,but the acquisition and processing of real-time digital signals requires high-performance acquisition cards and calculations.equipment.Therefore,this paper proposes a system structure based on double-pulse heterodyne demodulation to demodulate the phase information of the beat frequency signal of two ?-OTDR systems facing away from Rayleigh scattered light.The main work contents and innovations are as follows.Focus on the following points:First,a distributed fiber-optic seismic wave sensing system was designed and built according to ?-OTDR theory.And it focuses on the reasoning analysis of the working principle of this system.And in the process of building,the main parameters of the data acquisition,optical pulse modulation and other design optimization,so that it reaches the requirements of the detection performance.Then the effect of optical power,light source linewidth and frequency drift on system performance in the ?-OTDR system was analyzed and verified,and the signal-to-noise ratio of the system was optimized.In the experimental verification part,first of all,a functional test is performed for the entire system to realize the detection of a certain phase in the optical fiber link and realize the reduction of the interference signal.Secondly,the system is applied to the field of seismic wave exploration for systematic exploration and application research.The results show that the system can achieve good detection of different frequencies and amplitudes,especially for low frequency signals below 100 Hz can still maintain a high signal-to-noise ratio.The longest detection length of the system can be up to 10 km and the detection frequency can range from 10 Hz to 2 k Hz.For seismic engineering and other engineering fields that focus on low-frequency signals,this system has a very high application prospect.Second,by using dual-pulse coherent detection,the frequency of the beat frequency signal is lower than that of the original signal of the ?-OTDR system using the single-pulse scheme,further reducing the requirements of the capture card,eliminating the need for a high sampling rate capture card,and greatly reducing the system The cost further reduces the requirements on the processing performance of the computer.Therefore,the system has great engineering application and research value.