Research On Fiber Optic Geophone In Cross-well Seismic

Due to complex geological conditions and low exploitation of unconventional oil fields,China’s petroleum resources have long been in short supply and heavily dependent on imports.In order to solve this problem,there is an urgent need to improve the exploration and development level of oil and gas resources.The cross-well seismic exploration technology can obtain high-resolution stratigraphic information and realize the deep development of unconventional oil and gas fields.Fiber optic sensors have many advantages over traditional electrical sensors,so the use of fiber optic geophones for cross-well seismic exploration has important academic significance and application value.In this paper,a series of studies on fiber optic geophones with different working frequency bands are conducted with the application of cross-well seismic exploration technology as the background,and the main work is as follows:(1)The common exploration and development technologies are investigated for the current situation of oil and gas resources in China,the principles,advantages and disadvantages of the traditional electrical geophones are explained,and the characteristics of fiber optic geophones are analyzed and compared with traditional electrical geophones.According to different structural classifications,the principle,advantages and disadvantages of Fiber Bragg Grating(FBG)acceleration geophones are discussed and the current status of research is investigated.The strain and temperature sensing mechanism of FBG is established and analyzed,and then combined with the acceleration geophone mechanics model,the FBG acceleration geophone model is established.(2)An FBG acceleration geophone based on equal-strength cantilever beam structure is designed.The working mechanism of the geophone is elucidated and its two important characteristic indexes are derived theoretically.The structure and performance of the geophone are optimized by numerical simulation and finite element simulation.The characteristics of the geophone are tested experimentally,and the results show that the resonant frequency of the geophone is 119 Hz,the sensitivity is 134.29 pm/g in the wavelength drift flat region of 2-60 Hz,the linear fit of the data in the repeated experiments is as high as 0.999,the interference of the signal in x and y directions to the main oscillation direction is 3.86% and 2.97%,respectively,and the dynamic range is 63.23 dB.This geophone can be applied to the detection of low frequency signals.(3)An FBG acceleration geophone based on series flexural hinge structure is designed.The sensing model of the geophone is established,and the key parameters such as sensitivity and resonant frequency of the geophone are analyzed theoretically.The influence of the thickness and long/short axis of the elliptical flexible hinge on the resonant frequency and sensitivity of the geophone is analyzed,and the main characteristics of the geophone are analyzed by finite element simulation software.The geophone sample is processed and encapsulated.The experimental results show that the resonant frequency of the geophone is 960 Hz,the operating band is 20-500 Hz,the sensitivity in the flat region is 93.45 pm/g,the interference in the two cross directions is 2.54% and 3.45% respectively,and the dynamic range is 66.53 dB.This geophone can be applied to the detection of medium and high frequency signals.