Research On Key Technology Of Borehole Geomagnetic Observation

Since the invention of the magnetometer by German physicists Carl Friedrich Gauss and Wilhelm Eduard Weber in 1832,geomagnetic field observations have played a fundamental and important role in geophysics and planetary physics.The research of geomagnetic observation system,down to the model of magnetohydrodynamic generators of the Earth’s outer core and up to the reconnection of magnetic fields and flux transport events in the magnetotail magnetosphere,geomagnetism and its applications have flourished and developed rapidly.In recent years,borehole geomagnetic observation,seafloor geomagnetic observation and satellite magnetometry have gradually become hot spots for domestic and international research,and joint observation with landbased geomagnetic stations to achieve refined model descriptions of the Earth’s main magnetic field,crustal magnetic anomalies and the Earth’s interplanetary magnetic field in order to study and explore important geophysical and planetary physics research such as subsurface structure inversion,pre-seismic electromagnetic anomalies and space weather forecasting.Borehole geomagnetic observation system is considered as one of the development directions of the next generation of ground-based geomagnetic stations,and its main advantage is that it can effectively shield the magnetic field interference noise generated by human activities on the ground,such as vehicle flow and farming.The borehole geomagnetic observation system,which realizes the relative observation of geomagnetic field variation and absolute measurement of geomagnetic vector,can also effectively reduce the station construction cost and provide technical support for the gradual realization of the encryption of geomagnetic basic stations nationwide in the "China Geomagnetic Station Network Plan(2020-2030)".The main work and research innovations of this paper are as follows.(1)Development of borehole three-component fluxgate sensor adapted to absolute geomagnetic field observation modeA three-component fluxgate sensor adapted to the absolute observation mode of the borehole geomagnetic field has been developed.In contrast to the±2,500 nT measurement range of the conventional ground station fluxgate magnetometer sensor,a precision digital phase shift/excitation circuit and a large range negative feedback circuit were designed to ensure the measurement resolution while widening the measurement range.Laboratory test results show that the developed borehole three-component fluxgate sensor achieves a measurement range of±60,000 nT while maintaining a noise level comparable to that of a ground station fluxgate magnetometer of 0.1 nT,and achieves the capability of a basic geomagnetic station.(2)Research on magnetic interference avoidance technology for borehole geomagnetic observationThe magnetic interference between the component sensors of the borehole fluxgate magnetometer and the magnetic interference between the simulation and acquisition circuit boards in the same non-magnetic housing of the borehole magnetometer to the adjacent sensors are studied.The magnetic interference between the component sensors is modeled by simplified physics and mathematics,and the additional magnetic field outside the finite length DC solenoid is simulated and experimentally verified by station observation experiments on the variation of the magnitude of the two static magnetic disturbances with distance.Based on the magnetic disturbance study,the optimal distance between sensors is 56 cm,and a three-component fluxgate sensor structure with vertical orthogonal rows borehole is designed accordingly;the optimal distance between the analog acquisition board and the adjacent sensors is determined to be 44 cm.The experimental verification shows that the designed sensor has good magnetic avoidance effect,which is less than 0.1 nT.(3)Research on vector geomagnetic data attitude correction technologyThe attitude correction technique for converting borehole geomagnetic observation data from the carrier coordinate system with unknown attitude of the borehole magnetometer to the surface geomagnetic coordinate system is studied.The relative attitude angle of the carrier system with respect to the ground geomagnetic coordinate system is calculated by a global optimization search of the Eulerian rotation angle through genetic algorithm,and the coordinate transformation and attitude correction are realized using the rotation matrix method.Using Bland-Altman plots,a statistical determination method commonly used in the biomedical field,the data consistency of the vector geomagnetic comparison observations is qualitatively and quantitatively evaluated and analyzed to check the attitude correction effect.In the B-A plot,the scattered distribution of the data with attitude angle deviation is of "connected domain" type and the confidence interval is large,and the daily comparison data of ground stations generally exceed 1.5 nT.After attitude correction,the distribution of points in the B-A diagram has no typical pattern and the length of confidence interval is greatly reduced,and the daily specific measurement data of ground stations are generally less than 0.4 nT.The feasibility and rationality of the absolute observation of the synthetic total intensity of the geomagnetic field are investigated and verified by the station attitude rotation experiment.The borehole three-component fluxgate magnetometer,which is applicable to the absolute observation mode,measures the uncompensated vector geomagnetic data with unknown attitude,and its synthetic total intensity can be used for absolute observation of the total geomagnetic field.This conclusion is verified by the operational test experiments of the borehole geomagnetic observation system in Baijiatong,i.e.,the systematic difference between the synthetic total geomagnetic field intensity of the high-precision uncompensated fluxgate magnetometer three-component data and the total geomagnetic field intensity obtained from the absolute observation instrument of the station FHDZ-M15 is basically maintained at a smooth level.The station attitude correction experiments show that the systematic difference between the synthesized total intensity of the geomagnetic field and the measured total intensity of the geomagnetic field basically remains smooth and its standard deviation changes little when the attitude changes,and it is possible to use the synthesized total intensity for the observation of the total intensity of the geomagnetic field when the deviation of the borehole magnetometer attitude from the standard magnetometer attitude is not too large.