Research On Seismic Physical Model Imaging Technology Of Diaphragm Optical Fiber Fabry-Perot Interferometric Ultrasonic Sensor

The seismic physical model imaging technology can simulate and predict the propagation laws and signal characteristics of the geophysical wave field of resource storage in near-ideal conditions.And it can also provide reliable experimental data for the geological storage calculation simulation.It is a bridge connecting seismic field with theoretical calculations.Seismic physical model imaging is generally achieved by ultrasonic scanning.Traditional ultrasonic measurement usually uses the piezoelectric ceramic transducers.In view of its disadvantages of large volume,low sensitivity,susceptible to electromagnetic interference,not suitable for harsh environments of high-temperature and strong corrosion,etc.In this paper,we propose the improved technology of the optical fiber Fabry-Perot interferometric ultrasonic sensor,which greatly reduces the sensor's volume and enhances the ultrasonic sensitivity with good reliability.Through the different diaphragm optical fiber structures,three optical fiber Fabry-Perot interferometric ultrasonic sensors are researched.And they successfully achieve ultrasonic measurements and seismic physical model imaging.The use of a functionalized material 353 ND which has high-temperature resistant and the end face of a standard single-mode fiber to form a Fabry-Perot interferometer,not only significantly reduces the sensor's volume,but also improves the performance of high-temperature resistant.Ultrasound will lead to the vibration of 353 ND diaphragm and the change of diaphragm thickness,which changes the cavity length of Fabry-Perot interferometer and causes a changed phase difference between the interfering beams.Eventually the interference spectrum is changed.Then the signal is demodulated by the spectral sideband filtering technology.And the reflected signals at different interfaces of the seismic physical model are displayed in time domain.The imaging result shows that the different layers of the model structure can be intuitively identified.By changing the basic diaphragm optical fiber structure,we propose two improved diaphragm optical fiber Fabry-Perot interferometric ultrasonic sensors.The first one is started from the basic principle of edge filtering demodulated method.A novel type of fringe visibility enhanced optical fiber Fabry-Perot interferometric ultrasonic sensor is fabricated by adding the design of fiber collimator and using high-performance material poly tetra fluoroethylene(PTFE).The fiber collimator can convert exit light into parallel light,which greatly reduces the optical energy loss in interference cavity.This making the interference spectrum to have a larger extinction ratio while obtaining a smaller free spectral range.Therefore,the slope of the spectrum sideband is greatly improved.And the ultrasonic sensitivity of the sensor is improved significantly when the spectral sideband filtering technology is used to demodulate the ultrasonic signal.It achieves inversion imaging of multiple models in seismic physical model imaging.Another is based on the mechanical structural characteristic of diaphragm.Modulation punching in polydimethylsiloxane(PDMS)diaphragm by femtosecond lasers,which changes the mechanical distribution near the center of the circular diaphragm structure.Then the deformation of the diaphragm's center is increased under ultrasonic sound pressure.Therefore,a higher ultrasonic sensitivity is acquired.Experiments show that the proposed sensor successfully achieves ultrasonic measurement in the air.