Research On Key Technology Of High-Sensitivity Borehole Helium Optically Pumped Magnetometer

Earthquake disasters have caused serious impact on socio-economic development and people’s production and life in highly developed economy and urbanized society,and early warning and prevention of earthquake is an urgent problem to be solved.Therefore,it is urgent to develop corresponding monitoring equipment to obtain earthquake precursor information effectively,which is of great significance to social development.Due to the"piezomagnetic effect",the abnormal geomagnetic field has a good correlation and regularity with the occurrence of earthquakes,and the abnormal change can be used to provide supporting information for early warning of natural earthquakes.Borehole magnetometry can effectively exclude the influence of ground magnetic noise and improve the measurement accuracy,so it is important to develop a magnetometer for long-term monitoring of the geomagnetic field in the harsh borehole environment for earthquake monitoring and early warning.In addition,borehole magnetometry can also be used to explore the deep buried mineral and petroleum resources.The current magnetic measurement instruments that can be applied borehole are mainly fluxgate and proton magnetometer,but the measurement accuracy is mostly in the n T range due to high temperature,which cannot meet the requirements of high precision measurement.The helium optically pumped magnetometer is an ideal instrument for long-term borehole magnetic field monitoring due to its high sensitivity and stable performance.However,the instrument faces harsh environments such as high temperature,high pressure and confined space in borehole applications,which can affect the long-term stability,limiting the monitoring of seismic-associated magnetic anomalies.Therefore,a magnetometer for borehole special application background needs to be developed to address the effects of the harsh environment on the magnetometer,and the following research work is carried out in this paper.（1）Principle analysis of borehole helium optically pumped magnetometer and overall system design.The physical effects,magnetic measurement principle and the polarization state conversion of light of the magnetometer were analyzed,and the mathematical model of magnetic resonance signal was established based on Bloch equation and Beer-Lambert law,and the calculation formula of sensitivity was given.Then the overall design scheme of the borehole magnetometer and the key technologies to be studied are determined based on the measurement principle.（2）Research on microscopic model of sensitivity and electrical excitation parameters optimization based on weakly excited helium cell atoms.To address the problem that the magnetometer magnetosensitive unit faces the effects of high temperature and space constraints in borehole applications,the motion and polarization state of atoms can change,which in turn degrades the magnetometer performance.Starting from the excitation circuit parameters that excite the sub-stable state of the magnetically sensitive material,the microscopic motion state of atoms within the magnetically sensitive unit is studied in depth based on a the power dissipation mechanism,and a microscopic theoretical model of the magnetometer is established,which provides a theoretical research basis for the performance enhancement of the magnetometer.And the transient high voltage and continuous high frequency excitation schemes are determined according to the characteristics of the helium cell,the impedance matching network from the excitation source to the cell is studied according to the characteristics of the electrical parameters of the cell,and finally the optimal value is determined by the effect of the output power of the excitation circuit on the sensitivity performance of the magnetometer through the actual measurement,and the measured results are in accordance with the tendency of the theoretical analysis,which verifies the correctness of the micro-theoretical model of the optically pumped magnetometer established.（3）Research on the negative feedback excitation technique of nonlinear pumping light source in high temperature confined environment.A negative feedback excitation technique based on the Extreme Learning Machine（ELM）-Hammerstein nonlinear model is proposed to address the problem of nonlinear drift of the light source output in the high temperature and confined environment of the borehole,which affects the stability of magnetic measurements.The factors affecting the stability of the light source and its impact on the accuracy of magnetic field measurement are analyzed,and the ELM-Hammerstein model is established to accurately describe the nonlinear system,with a relative error of model identification and verification results and system response is less than 1%.Based on the model,an integral separated PID negative feedback control system was designed to operate stably at temperatures up to 75°C.The closed-loop negative feedback excitation method was able to effectively suppress the magnetic field measurement drift of 1.3 n T compared to the open-loop form of the light source.In addition,experiments were conducted in a simulated environment of a kilometer borehole,and the actual output of the system was in good agreement with the theoretical analysis,and the proposed closed-loop negative feedback excitation technique was able to effectively suppress the nonlinear drift of the magnetic field by 0.8 n T within one hour.（4）Research on borehole magnetic resonance signal detection technology based on modulated absorption spectroscopy technique.In view of the characteristics of magnetic resonance signal and the shortcomings of its application in field environment such as underground,a modulation absorption spectroscopy technique based on RF modulation is proposed,according to the harmonic amplitude characteristics,the detection scheme combining the fundamental and the second harmonic wave is determined to lock the magnetic resonance frequency quickly and accurately.In order to effectively suppress the coupling interference and circuit temperature drift in borehole application,the differential photoelectric conversion preamplification technology is proposed,and the noise of each part of the photoelectric conversion circuit is analyzed.The results show that the difference photoelectric conversion circuit can increase the signal-to-noise ratio of the system by times theoretically.In addition,the digital frequency modulation technology based on FPGA is studied to generate the RF modulated field,and the digital model of the detection circuit is obtained according to the signal detection principle,based on which the control parameters of the tracking PID are determined to achieve the requirements of high accuracy and fast measurement of the magnetic field,and the sensitivity of the magnetometer is 20 p T/Hz^1/2.Finally,the system integration and kilometer borehole test application of borehole helium optically pumped magnetometer were completed.For the harsh application environment of high temperature and confined space in borehole,the integrated vibration-resistant magnetic sensor is developed,and a compact linear circuit was designed.Finally,the sensor and circuits were integrated by compact arrangement.And the titanium alloy cavity is chosen as the casing of the instrument to meet the requirements of high pressure and waterproof application in borehole.For the special application environment of kilometer borehole,the system output signal was tested at 45°magnetic declination,and the high temperature and high pressure tests were completed.The test results showed that the system could meet the borehole application.Then a comparison test was conducted in the field with the standard cesium optically pumped magnetometer,and the comparison results showed that the developed borehole helium light pump magnetometer has better practicality.Finally,the system was placed borehole for geomagnetic monitoring,and the magnetic field and temperature changes were successfully obtained during the borehole process,with the magnetic field gradient of 0.420 n T/m and the temperature gradient of 2.62°C/100 m.Long-term monitoring of the geomagnetic field is realized at the kilometer bottom of the well.The measured magnetic field fluctuations are mainly within 30 n T,which is consistent with the size of the magnetic diurnal variation.It indicates that the developed borehole helium optically pumped magnetometer can work normally and realize geomagnetic monitoring at the kilometer depth.