Research On Technology Of Fiber Bragg Grating Acceleration Geophone In Crosswell Seismic

Optical fiber Bragg Grating (FBG) as a wavelength type device, it has many advantages,such as immunity from electromagnetic, high temperature survivability, corrosion resistance,small size and ability to multiplex, so it has a aroused a vast concert in the optic sensingdomain. Base on the sensing principle of temperature and strain, FBG can realizemeasurement of physical parameters, such as pressure, displacement, angle, acceleration etal.FBG sensors have unique advantages and important application in the structure monitoring,geodynamics, seismic exploration. On the backgrounds of crosswell seismic exploration, as isinnovative application, FBG acceleration geophone is researched in the crosswell seismic.This research is as follows: analysis on sensing mechanism and research on fabricationmethods of FBG, design and relazation of FBG acceleration geophone, research on randomerror model of FBG geophone system, design of FBG acceleration geophone system in thecross-well seismic, the main contents and achievement of this research are as follows:(1) By theoretical analysis of formation mechanis of FBG, the response characteristics ofreflective spectra power under influence of linear strain is researched and theoretical model ofFBG acceleration geophone is established. Firstly, sensing mechanism of FBG is researched,change law of reflective spectra power is analyzed when FBG is applied linear strain.Secondly, model of FBG acceleration geophone is established, and principle of FBGacceleration geophone is analyzed. Two kinds of fabration model and response characteristicsof FBG acceleration geophone are discussed.(2) FBG accelerometers base on the double cantilever and diplopore cantilever aredesigned and developed. The farication method of using short FBG for double cantilever isproposed, and FBG acceleration geophone base on the double cantilever is developed,experimental results indicate that the flat range of the sensor is5-30Hz, the sensitivity ofacceleration for20Hz is155.2pm/G, and cross-axis sensitivity is less than4.76%. For furtherbroadening frequency bandwidth, FBG acceleration geophone base on the diploporecantilever is proposed by utilizing double-point fabrication, the influence of natural vibrationis reduced by exerting prestress, experimental results indicate that the sensor has good flatrepose in the range20-400Hz, the sensitivity of acceleration for160Hz is17.6pm/G, the linearrelation between accleation and output peak wavelength is good, the linearity is99.8%. But itis compared with double cantilever, cross-axis sensitivity is little deterioration, is less than5.68%。(3) FBG geophone base on the elastic tube is designed and developed by using double point fabrication. The principle of the geophone is theoretically analyzed, and the response ofthe FBG geophone is experimentally researched. Experimental results indicate that the FBGacceleration geophone has a good flat response in the range of10-250Hz, the sensitivity ofacceleration for160Hz is63.0pm/G, in order to improve the ability of transversalanti-interference, the method of limiting transversal displacement is utilized, and thecross-axis sensitivity of FBG geophone is descend, is less than4.06%.(4) The model of FBG acceleration geophone base on the single diaphragm and doublediaphragms are proposed respectively, and which is respectively utilized full viscosity anddouble point fabrication. Researches show that the FBG accelerometer base on the singlediaphragm model has a good flat response in the range of10-200Hz, the sensitivity ofacceleration for160Hz is36.6pm/G, dynamic resolution is2mG/Hzin the flat range, thecross-axis sensitivity is less than1.3%, so it is has strong ability for transversalanti-interference. In order to expand the flat range and remain high main-axis sensitivity, theFBG geophone base on double diaphragms is proposed, experimental results indicate that theFBG geophone provides broad flat frequency range from50to800Hz, correspondingsensitivity range from23.8to45.9pm/G, while the flatness is less than3dB, dynamicresolution can reach0.385mG/Hz, dynamic range is80dB, and cross-axis sensitivity is lessthan2.1%of the main-axis sensitivity.(5) The random error model for evaluating FBG accelertation detection system isproposed and established by using Allan variance, and the error recognition for doublediaphragms FBG geophone is experimentally researched. The characteristic of random errorand error source for FBG acceletation detector system are analyzed, and the several technicalevaluation index of FBG accelerometer is analyzed by Allan variance, which provides basicargument for objective evaluation of the FBG acceletation detector system.(6) By analysis technical requirements and applied characteristic of the cross-wellseismic, the system scheme for cross-well sciemic is proposed. The system composition ofFBG acceletation detector system for crosswell sciemic is analyzed, the networkeddemulation scheme for crosswell seismic and wavelength distribution planning of FBGgeophone is discussed, and the downhole scheme of FBG geophone system is designed.