Application Of Distributed Optical Fiber Sensing System In Seismic Wave Detection

The distributed optical fiber sensing system based on the ?-OTDR system utilizes the phase change of the backward Rasyleigh scattered light in the optical fiber to sense the vibration signal along the optical fiber.As a transmission medium and as a sensing medium,optical fiber can realize long-distance distributed vibration sensing.It has been widely used in perimeter security,pipeline inspection,building structure monitoring,seismic exploration and other fields.Research on the ?-OTDR system has focused on optical systems and signal processing.However,in fact,the sensing fiber is also an important factor in determining the ?-OTDR system.This paper focuses on the application research of ?-OTDR system in seismic wave exploration.The main work contents and innovations are concentrated in the following points:Firstly,the changes of the optical field inside the fiber under vibration are analyzed and deduced,including axial vibration and radial vibration,and the relationship between multiple parameters of the fiber and the final phase change.Theoretically,the influence of material properties and fiber-optic packaging on the sensitivity of the cable,including material properties and internal winding angle,was analyzed.Aiming at the shortcomings of ordinary optical cables that are not sensitive to ground vibration and downhole lateral vibration,a variety of optical cables with special structures have been designed.These types of fiber optic cables have different materials,characteristics,and optical fibers.These cables were then buried on the ground and tested for their response to the same vibrational excitation source using basketball free fall.The response results are analyzed,and the influence of material properties and winding characteristics on the ground vibration response of the cable is obtained by means of controlled variables.Based on the analysis results,an optical cable suitable for ground vibration sensing and downhole lateral sensing is pointed out.Next,in the fourth chapter,a structure with direction selectivity is proposed.In ground vibration and downhole applications,conventional fiber optic cables cannot produce three-component sensing data due to cable characteristics.In order to enrich the application scenarios of distributed optical fiber sensing systems in seismic wave detection,a fiber-optic seismic wave sensor with directional selectivity in a distributed optical fiber sensing system is proposed.The structure consists of a variable cylindrical vibrator and an external metal structure.The axial deformation of the variable cylinder is converted into a radial direction by the incompressibility of the variable cylinder,and the fiber is pulled to cause deformation to achieve its directional selectivity.The design and comparison with the electronic detector are used to verify the effect,and the direction selection ability and response consistency are proved.