| With the increasing difficulty of oil-gas exploration,the demand of oil-gas resources is much greater than the supply.It is urgent to study new oil-gas exploration technology.The seismic exploration technology has many advantages,such as simple,fast,low cost,higher layer resolution and fine imaging,so it is widely used in the field of oil-gas resources exploration.Fiber Bragg Grating(FBG),as a wavelength device,has the advantages of small volume,light weight,easy multiplexing and strong anti-electromagnetic interference ability.The FBG geophone has important potential value in the field of oil-gas resource exploration.In this paper,taking the FBG as the sensitive element,the theoretical mechanical models of FBG geophone in series and parallel are established,the sensitization scheme is proposed,the basis for the optimization design of sensitization is clarified,and two samples of high-sensitivity geophone are developed.The main work of this thesis are as follows:(1)This paper elaborates the methods commonly used in oil-gas resource exploration,summarizes the basic composition,working principle,advantages and disadvantages of various electrical geophones,compares and analyzes the technical advantages of the FBG geophone,and discusses the classification and technical characteristics of the FBG geophone.According to the research status of FBG geophone at home and abroad,the problems faced by geophone are given,and the research work of this paper is put forward.(2)The sensitization mechanism of the FBG acceleration geophone is studied theoretically.Firstly,based on the coupled mode theory,the theoretical formula of FBG temperature-strain sensing is deduced.According to the temperature-strain sensing mechanism,the theoretical mechanical model of the FBG geophone is established,and the analytical expressions of sensitivity and resonant frequency are deduced.Secondly,according to the encapsulated characteristics of FBG geophone,the theoretical models of series and parallel are established,the characteristics of the two models are compared and analyzed,the sensitization mechanism of geophone is studied,and the figure of merit is introduced to optimize and evaluate the sensitization effect.Finally,the main technical parameters of the geophone are analyzed and discussed.(3)The turbine-type’ acceleration geophone sample is designed and developed.Firstly,the parallel theoretical model of the FBG geophone is established,the working principle of the geophone is analyzed,and the analytical formula of sensitivity and resonant frequency are deduced.Combined with the figure of merit,the structural parameters of the geophone are optimized by the finite element analysis and numerical simulation,and the response characteristics are experimentally studied.The experimental results show that the flatness of the FBG geophone is less than 2.20 dB within 5-70 Hz,the sensitivity varies from 651.0 pm/G to 850.5 pm/G,the figure of merit is better than 87.9 nm-Hz/G,the cross-sensitivity is about-16.4 dB,the dynamic range is 68.6 dB,and the range of the noise level in the working frequency band is 0.149-0.659 mG/(?).(4)The’hollow-out’ acceleration geophone sample is designed and developed.The design idea is to improve the sensitivity of the principal-axis and reduce the cross-axis sensitivity,the’hollow-out’ model is established.Combined with the figure of merit,the structural parameters of the geophone are optimized to realize the sensitization design,and its response characteristics are studied.The experimental results show that the geophone has a good flat region within 5-60 Hz,the sensitivity is 763.2 pm/G,the flatness is less than 3 dB,the figure of merit is 78.6 nm-Hz/G,the cross-sensitivity is about-24.76 dB,the dynamic range is 71.1 dB,and the range of the noise level in the working frequency band is 0.064-0.285 mG/(?). |
