Research On A High-precision Magnetometer Based On The DNP Optimization

As an effective method to detecting material properties and unknown world,weak magnetic field measurement has significant application in many areas such as geophysics magnetic measurement,marine magnetic measurement,space weakness magnetic field Measurement,etc.The structure and properties of all weak magnetic field measurement instruments are different with each other because of the different measurement principle,hence,their applications have a difference to some extent.The Overhauser magnetometer based on dynamic nuclear polarization(DNP)effect is a high-precision device for weak magnetic field measurement.The Larmor precession of excited protons around the geomagnetic field can generate a free induction decay(FID)signal.The strength of the magnetic field can be obtained by measuring the frequency of this signal.However,with the advances in the exploration technology,there is a need for the Overhauser magnetometer to have higher accuracy and higher sensitivity.The traditional DNP which is adopted in Overhauser magnetometer can improve the factor of the hydrogen nucleus polarization through the coupling of electron and hydrogen nucleus.However,it depends on the natural coupling between nucleon and molecule.Therefore,the factor of polarization transfer is not maximized.In recent years,considerable attention has been given to the optimization techniques of DNP.One such class of techniques is Hartmann-Hahn nuclear effect,which has many attractive theoretic properties,specifically,the ability to detect non-predefined energy transitional relations such as nitrogen and proton.Many researchers have analyzed this theory.However,there is almost no report on an Overhauser magnetometer with DNP optimization,especially the Hartmann-Hahn based approach is still a concept.The main research work and contributions of this thesis are as follows:1.Starting with a brief introduction to the proton precession magnetometer and its applications in different areas.A complete discussion and overview of the related work in domestic and foreign are implemented.The basic concepts of Overhauser nuclear effect and Hartmann-Hahn nuclear effect are introduced,respectively.A polarization model of double magnetic field based on Bloch equation is built and the steady state solutions are obtained,focused mainly on a conceptual understanding of the whole optimized DNP procedure and the corresponding derivation process.The total magnetization of optimized DNP based on Hartmann-Hahn shows a significant improvement over the original polarization method.The theoretical derivations further demonstrate that the Hartmann-Hahn based approach can enhance the strength of FID signal.2.Different types of noise models for the FID signal are established,which are further applied to evaluate the influence of noise on the FID signal quality.Simulation is first conducted to identify how the measurement accuracy affected by different SNRs.Moreover,the error for the frequency measurement of the FID signal has been analyzed and the measurement accuracy for the SNR of the amplified FID signal is determined.The relationship between the total noise model of FID signal and the error of frequency measurement is formulated in a quantitative way.The trend of the simulation and the field test results are approximately consistent.3.A secondary tuning algorithm based on singular value decomposition(SVD)and short time Fourier transform(STFT)is proposed,which cannot only improve the accuracy of the sensor's tuning but also shorten the time of tuning process.The proposed tuning algorithm has higher accuracy and higher speed than the current commonly used methods,peak detection,and auto-correlation,even in the interferential environment.It makes up for the insufficiency of the existing tuning methods by improving the magnetometer's ability to adapt to the environment,thereby solving the detuning phenomenon in the process of measurement.4.A multichannel frequency measurement algorithm is developed to improve the accuracy of the measured magnetic fields in a broader dynamic range and in a noisier environment.The absolute error measured by the proposed frequency measurement algorithm is smaller than that measured by the original method with a single channel.5.The design and development of the improved Overhauser magnetometer are discussed briefly.An alternative design of a geomagnetic sensor with differential dual-coil structure is proposed.In addition,a test apparatus for measuring the critical parameters of this sensor is developed.The signal acquisition and processing system including the polarization unit and signal conditioning unit are presented.Furthermore,the theoretical calculation of the sensor and preamplifier noise is implemented.6.Numerous laboratory and field tests on the proposed prototype including noise level,frequency measurement accuracy,magnetic field measurement accuracy,geomagnetic observation in 24 hours and ferromagnetic target localization compared with one of the commercially available Overhauser magnetometers in the world market are conducted.Moreover,a specific discussion and expectation about the Overhauser magnetometer based on the optimization of DNP are implemented.Multiple sources and references have been used,this provides readers with different possible approaches which makes the comparisons more sensible.For instance,in the case of frequency measurement algorithm,several existing and proposed methods for FID signal are discussed,or in the case of tuning algorithm,both philosophical-historical background and practical point of view are considered.