| As an important part of freshwater resources available to human beings,groundwater plays an irreplaceable and important role in guaranteeing the life of residents,maintaining industrial and agricultural production,supporting social and economic development and maintaining ecological balance.However,the groundwater supply in China is seriously insufficient,and the problems of over-exploitation and pollution of groundwater resources are very serious,which have seriously affected the economic development and people's life,and even threatened the national security.Therefore,there is an urgent need to study the technical means for groundwater detection to achieve the scientific objectives of accurate detection of groundwater content,scientific assessment of groundwater reservoir characteristics and effective monitoring of groundwater contamination.Hydrogeological magnetic resonance logging technology is an emerging international geophysical method for shallow surface hydrogeological investigations,which has the outstanding advantages of direct,quantitative and efficient detection of groundwater resources compared with other geophysical methods.However,limited by the size of hydrogeological monitoring well apertures,the raw signals detected by such instruments are very weak.In order to enhance the signal strength,foreign researchers have used a reduced vertical resolution to increase the number of hydrogen protons involved in resonance,at the expense of the instrument's ability to identify thin aquifers.At the same time,it is also an important challenge to design such instruments by setting up a reasonable magnet layout in a limited space,optimizing the matching between RF magnetic field and static magnetic field,and improving the weak signal detection capability of the receiving system.In this thesis,the key technologies and design solutions involved in the shallow surface hydrogeological magnetic resonance logging system are carefully studied.In order to improve the measurement resolution and enhance the measurement signal strength,a high-resolution magnetic resonance logging probe design scheme based on compensating magnets is proposed;in order to realize the accurate matching of RF magnetic field and static magnetic field,so that the hydrogen protons in the resonance region can be accurately excited to reduce the inversion interpretation error,a fade-free emission technique based on a dynamic duty cycle adjustment scheme is proposed;in order to suppress the multiple noise interferences in the receiving system to improve the detection capability of the weak signal,the technique is proposed.In order to improve the detection capability of weak signals,the key technology of noise suppression in the MRI logging receiving system is proposed.The main research contents of the paper are as follows:(1)Starting from the basic principles of hydrogeological magnetic resonance logging,the advantages of magnetic resonance technology in measuring parameters such as the number of hydrogen nuclei contained in the sample,longitudinal relaxation time and transverse relaxation time are illustrated,and the intrinsic connection between magnetic resonance echo signals and parameters such as water content,porosity and permeability of the sample is revealed.Equations for calculating the response signal of magnetic resonance logging are derived,and the way in which the two key elements,static magnetic field and RF magnetic field,affect the resonance signal frequency,longitudinal resolution and response signal intensity is analyzed.The influence of noise on the logging signal is explained,and it is pointed out that the system internal background noise and external environmental noise are the main noise interference sources faced by the system,and the corresponding solution strategies are given to provide ideas for the design of shallow surface hydrogeological magnetic resonance logging systems.(2)To address the problem of insufficient longitudinal resolution of the current hydrogeological magnetic resonance logging system,we start from analyzing the design elements of the logging probe and point out that the essential cause of the problem is the high longitudinal thickness of the effective resonance region.Then the permanent magnet material is analyzed and the equations for calculating the magnetic field distribution are derived based on the equivalent current model.Based on the traditional Jackson logging probe design scheme,an innovative design scheme is proposed to extend the radial range of the effective resonance region by introducing compensation magnets,and the optimal solution of the compensation magnet layout is realized by using the Uniform Design method.The measurement and evaluation analysis of the external magnetic field of the new probe show that the new scheme retains the advantage of high longitudinal resolution(about 1 cm)and theoretically improves the signal-to-noise ratio by a factor of 6,which provides technical support for the design of high-resolution logging probes.(3)The problem of energy depletion of the storage capacitor leading to the attenuation of the emitted current waveform is investigated.Starting from the analysis of the logging probe construction scheme,the inevitability of integrating the core electronic circuit short section and the related auxiliary function short section into the probe is pointed out,and the necessity of using the energy storage capacitor as the transmitting power source is also illustrated.Based on the equation of motion of the macroscopic magnetization intensity of hydrogen protons,the influence of the pulse decay phenomenon on the accuracy of the hydrogen proton toggle angle is indicated.The attenuation-free emission technique based on the duty cycle dynamic adjustment strategy is introduced,and the feasibility of the technique is verified by simulation.Then the test work of the attenuation-free CPMG emission technique in practical application is carried out,and the overall fluctuation range of the emission current waveform amplitude is controlled within 1.4%,and the accurate control of the hydrogen proton toggle angle is realized.(4)To address the problem of weak signal amplitude and simultaneous noise interference in the MRI logging system,a series of key technologies and design solutions for noise suppression in the MRI logging receiving system are proposed.For external noise,a differential coil is used to suppress common mode noise interference,the fast-switching characteristics of MOSFETs are used to isolate strong transmit pulses and absorb oscillation noise,and the bandwidth of the signal conditioning circuit is limited by the high Q value of the series LCR circuit.The most suitable power supply scheme for the signal conditioning circuit in the magnetic resonance logging system is determined.The test results show that the signal conditioning circuit voltage noise level is about 4.3 n V/Hz1/2,and the overall background noise level of the receiving system is about 46 n V/Hz1/2,which can meet the requirements of?V or even n V-level signal acquisition.Finally,the principle prototype of the shallow surface hydrogeological magnetic resonance logging system was developed based on the above design scheme and key technologies,and an upper computer based on Lab VIEW software was developed to realize the functions of measurement parameter configuration and echo data acquisition.The results show that the new system designed in this paper can accurately measure the water content and pore space distribution of water-bearing samples,and the comparison with published literature shows that the technical index of the newly developed shallow surface hydrogeological magnetic resonance logging system reaches the advanced level of similar instruments in the world.The research in this paper provides a strong technical support to fill the gap in the field of shallow surface hydrogeological magnetic resonance logging technology in China. |
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