Research On Continuous High-Precision Measurement Technology Of The Overhauser Magnetometer

The geomagnetic field is the fundamental physical field of the Earth system,which reflects various physical changes in the Earth’s interior such as mineral deposits,underground fluid changes,and geomagnetic field anomalies caused by magnetic substances on the ground.Measuring weak geomagnetic anomalies is an important tool for conducting earth science,mineral exploration and military exploration,of which Overhauser magnetometer is the main equipment for measuring high-precision geomagnetic absolute information.As the demand for range,distance,and information increases in deep space,deep sea,deep earth,and extraterritorial exploration,higher requirements are placed on the performance of magnetometers such as accuracy and bandwidth.The traditional Overhauser magnetometer has low accuracy and bandwidth due to its weak electronic resonance energy,short signal duration,and complex coupling measurement noise,etc.In this thesis,we carry out research on the high-precision continuous measurement technology of Overhauser magnetometer and develop a continuous measurement type Overhauser magnetometer prototype with 10 Hz high-frequency sampling rate.Innovation points include:1.To address the problems of weak dynamic nuclear polarization transfer energy and slow static magnetic field magnetization in the traditional Overhauser magnetometer,which leads to the weak amplitude and short duration of the output FID signal,we study the continuous measurement method of geomagnetic field based on the continuous strong spin excitation Overhauser effect and microsecond pulse repolarization technology.First,the study determines the free radical hyperfine coupling structure based on the theory of strong spin excitation Overhauser effect,and uses the15 N external magnetic field to realize the zero-field splitting of free electrons and enhance the electron spin energy,combined with the dynamic nuclear polarization electron-proton spin energy forced transfer mechanism to greatly enhance the macroscopic magnetization intensity of the proton system.Then,using the NMR pulse wave excitation method,the macroscopic magnetization intensity of the proton system is aggregated in the magnetization direction in microsecond time,which greatly reduces the signal dead time and realizes the near-continuous output of the FID signal.Finally,based on the above method,a high-precision geomagnetic field continuous measurement mode based on the strong spin excitation Overhauser effect is proposed.2.To address the problems of traditional sensors such as low excitation efficiency of Overhauser effect,large interference in the FID signal acquisition process and direction-sensitive resulting in weak sensor output signal,Overhauser magnetic sensors based on high-efficiency flap-like resonant cavities are studied.First,the effect of ohmic loss and eddy current effect on the excitation efficiency of the dynamic nuclear polarization microwave excitation cavity is studied,and a four-flap multi-slot capacitance-loaded coaxial resonant cavity is proposed to achieve the suppression of transverse eddy current electric field and ohmic loss between the waveguide walls and improve the excitation efficiency of the strong spin excitation Overhauser effect on free radical matter.Then,the double-section short-end pitch symmetric conjugate solenoid coil is studied to eliminate the common-mode electromagnetic interference by using the symmetric conjugate structure,and the short-end pitch structure is adopted to increase the non-uniformity of the polarized magnetic field and make the spin-in direction of the proton non-isotropic,so as to suppress the directional sensitivity.Finally,combining the two,the design of continuous measurement type high-precision Overhauser magnetic sensor is proposed.3.To address the problem of conflicting accuracy and resolution of the sensor output FID signal in the measurement process,which leads to the low accuracy of magnetic field measurement of Overhauser magnetometer,the FID signal adaptive extraction and frequency high-precision intelligent measurement techniques are studied.Firstly,the research on the mechanism of the influence of multi-source interference noise on the frequency measurement accuracy of FID signal is carried out,and the closed-loop feedback high signal-to-noise ratio FID signal extraction method is proposed to improve the signal-to-noise ratio of the signal entering the frequency measurement device.Then,for the demand of high precision measurement of low SNR FID signal,the relative error and quantization error caused by noise in the signal to the frequency measurement device are analyzed,and the multi-channel structure and multi-phase reference clock structure are correspondingly proposed to achieve the suppression and elimination of error sources and improve the measurement accuracy.Finally,for the high signal-to-noise ratio FID signal high precision and high resolution measurement needs,the frequency measurement algorithm based on time series quantization regression analysis is proposed to make each FID signal frequency information involved in the measurement process,which greatly improves the measurement resolution,and at the same time with intelligent noise rejection technology to ensure the final measurement accuracy.4.On the basis of the above theoretical and technical research,the impact of the non-stationary characteristics of the magnetic field in the complex environment on the measurement process of the magnetometer is studied for the problem of the existence of time-varying,non-uniform and noisy factors in the background environment magnetic field in the magnetometer engineering application,which is difficult to assess the impact on the measurement accuracy of the Overhauser magnetometer.Firstly,the FID signal model in an arbitrary magnetic field background is established by using the integration method to determine the mechanism of the effect of background magnetic field nonuniformity on the relaxation of the plasmonic system,to simplify the model of the FID signal in a nonuniform magnetic field by using the idea of equivalent relaxation effect,and to propose the magnetic field nonuniformity measurement method.Then,the factors interfering with the accuracy of the instrument in the engineering environment and the corresponding degree of influence are studied,and the degree of influence of the plasmonic relaxation process,signal measurement error and the time-varying characteristics of the background magnetic field on the reliability of the measurement results are characterized,and the confidence evaluation method of the instrument measurement data is proposed.On the basis of the above,a prototype Overhauser magnetometer with continuous measurement was developed,and performance characterization and application experiments were carried out.First,the practical development of the prototype was carried out based on the above theory and technology.The high precision continuous measurement Overhauser magnetic sensor was fabricated,and the excitation timing parameters such as the saturation excitation power of the continuous wave strong spin Overhauser effect,the shortest saturation excitation time,and the optimal time of short-time pulse polarization were investigated,and the improvement of the continuous wave strong spin excitation Overhauser effect in terms of signal amplitude and signal dead time compared with the conventional Overhauser effect was experimentally verified at the FID signal level.The improvement in signal amplitude and signal dead time of the continuous-wave strong spin excitation Overhauser effect compared with the conventional Overhauser effect is verified.Then,the performance of the developed prototype is tested and characterized.The tests of various indexes such as instrument accuracy,frequency band,directional sensitivity,and magnetic field gradient tolerance were carried out in a standard magnetic laboratory environment under a natural geomagnetic field.Through theoretical research,sensor design,signal processing research,data analysis,and prototype design and testing,the developed prototype maintains the same level of measurement accuracy compared with the leading international commercial instruments,but can achieve high precision continuous measurement of the geomagnetic field in the high frequency band of 10 Hz,which is useful for geoscientific research,"three deep" exploration projects,national defense projects,etc.It provides equipment and technical support for high-frequency and high-precision weak magnetic field measurement needs in earth science research,"three deep" exploration projects,national defense projects,etc.