Design And Calibration Of Low-frequency Geophones Based On Pole-zero Compensation

In recent years, microtremor survey and deep seismic exploration have becomeresearch hotpots in the field of low-frequency seismic exploration. Geophones thatcommonly used in low-frequency detection rely on imports. They are expensive, witha large size and have complex installation process, which is not conductive to thebuilding and construction of large seismic exploration projects. As a result of thelimitation of materials and processing technology, through changing the structure ofthe sensor itself can not effectively reduce the natural frequency of geophone.Therefore, in the precondition of not changing the mechanical structure, selectingreasonable low-frequency compensation scheme to design low-frequency geophonesfor large seismic exploration has far-reaching practical significance.Moving-coil velocity geophone has the following advantages: high outputimpedance and low signal-to-noise ratio, simple interface, user-friendly; flexible andthe ability to measure small amplitude of vibration signals because of no internalfriction in the movable element; output voltage is proportional to the velocity ofvibration signal, and having relatively low frequency bandwidth, which provides thepossibility of research on geophones with lower natural frequency. So research on lowfrequency characteristic compensation and parameter calibration in this paper arebased on the moving-coil geophone. On the basis of in-depth analysis of mechanicalstructure and working principle of geophones, build the dynamic model of and derivethe transfer function, so as to map the frequency characteristic curve of geophone.Finally some important characteristic parameters and the corresponding test methodsof geophone are introduced. Design and calibration of low-frequency geophone arebased on the properly built dynamic model and transfer function, so a correct model isthe premise of the whole research work in this paper.Commonly used methods in developing low-frequency geophone are mainly as following several kinds: changing the mechanical structure, detector and servocompensation, series compensation, based on force balance feedback technology andchange the transfer mode detector. Among them, Commonly used methods offrequency expansion are mainly as follows: changing the mechanical structure, servocompensation, pole-zero compensation, force-balance feedback and so on. Amongthem, pole-zero series compensation is an ideal solution to low-frequency expansion.It adjusts damping ratio and natural frequency according to design requirements,which can improve the dynamic quality, and keep the good performance of theoriginal system. By taking transfer function of moving-coil geophone and the targetindex into account, the transfer function of the compensation unit is achieved. Digitaland analog compensation network were designed according to the transfer function.Simulation result of the digital network and test result of the analog network showthat the frequency characteristics of compensation unit can meet the designrequirements and achieve the purpose of broadening the frequency bandwidth ofgeophone.As the most front-end equipment of acquisition and collection system ofvibration signal， the characteristics of geophone directly affects the accuracy ofseismic data. Therefore, it’s essential to calibrate geophones before seismicexploration. This ensures that all technical indexes are within the qualified scope，thusto ultimately guarantee reliability and accuracy of the whole seismic system. Severalcommon calibration methods were analyzed and compared in the paper. With60Hzand2Hz natural frequency geophones from Chongqing Geological Instrument Factoryas testing targets, several experiments were designed to conduct the test. The testresults prove that using electric sine signal as the excitation source to calibrategeophone is feasible and has high accuracy. The test results further prove thatdynamic model of geophone was correctly built. Aiming at calibrating thelow-frequency geophone designed in this paper，the impedance measurement andcalibration method was proposed. And calibration experiments were designed andconducted. New testing results show that the proposed impedance method is feasibleand has high accuracy. Also the results show that low-frequency geophone developed on the basis of pole-zero compensation method satisfied the design index.With sensitivity and damping ratio kept as the same with the intial system, naturalfrequency of the geophone fell down to0.2Hz from2Hz.