| Magnetic Resonance Sounding(MRS)is a geophysical method capable of directly measuring the distribution of water content in the subsurface.MRS has emerged as an effective means for qualitative and quantitative detection of groundwater,as well as for supporting the development,management,and utilization of groundwater resources.At present,commercial instruments abroad are equipped with various pulse sequences including single pulse,Carr-Purcell-Meiboom-Gill(CPMG)pulse,steady-state pulse,and amplitude-frequency synchronized modulation pulse.These detection modes offer direct advantages in obtaining formation permeability,enhancing detection efficiency,and improving the amplitude of excitation signals,which enable precise characterization of subsurface aquifers.However,domestically-produced instruments generally available are limited to single pulse detection mode,which results in issues such as low excitation signal amplitude and vulnerability to environmental noise interference.These instruments are only suitable for estimating water content information,making it challenging to reliably detect groundwater in complex geological conditions and strong noise interference environments.Consequently,they lack competitive advantages compared to international counterparts in the field of MRS.The development of MRS pulse emission control technology and detection systems for complex groundwater exploration in our country is important for improving groundwater resource exploration capabilities and promoting domestically-produced instrument equipment.This thesis addresses the demands for refined and multi-parameter groundwater detection in China.With the support of the National Natural Science Foundation of China(NSFC)Key Program “Research on Magnetic Resonance Detection Method of Groundwater Organic Pollution Plumes Based on Adiabatic Pulse Sequences” and the NSFC Joint Fund project “Research and Development of In-situ Detection Technology for Spatiotemporal Evolution Mechanism of Magnetic Resonance Signals of Petroleum Hydrocarbon Contaminants in Seasonal Freeze-thaw Zone Groundwater,” a pulsed transmission system with amplitude-frequency synchronized modulation for MRS has been developed.An overall architecture for pulse emission of the MRS system has been designed following the research approach of “System Overall Architecture Design,Amplitude Modulation Control Scheme Design,Amplitude-Frequency Synchronized Modulation Scheme Design.” Additionally,an amplitude modulation pulse emission technology based on carrier phase-shift control and an amplitude-frequency modulation pulse emission technology based on synchronous phase-shift frequency control has been developed.The detection performance of amplitude-frequency synchronized modulation pulses has been validated through simulation and outdoor experiments.The main research content of the thesis includes:(1)Study the principles of MRS and analyze the influencing factors of pulse shaping to clarify the effects of different technical factors on pulse shaping,which provides a research framework for the development of amplitude-frequency synchronized modulation pulse emission technology.By analyzing the principles of magnetic resonance and the relaxation mechanism of hydrogen atoms in groundwater,this thesis elucidates the unique role of magnetic resonance characteristic parameters in qualitative and quantitative detection of groundwater.This study investigates the role of excitation dynamics processes caused by different pulse parameters in generating magnetic resonance signals by analyzing the types and characteristics of ground magnetic resonance excitation pulses.Furthermore,it explores the influence of power supply types,load forms,and modulation schemes on emitted pulses,providing theoretical support for the development of amplitude-frequency synchronized modulation pulse emission technology.(2)Study on the overall architectural design scheme of MRS pulse emission systems,addressing the systematic design issue of “power supply form-topology structure-control scheme.” Designing a hybrid single-bus independent controllable indirect energy storage system involves determining suitable energy storage devices and their parameters using power-to-energy-weight ratio and energy balance equations.Designing a singlephase inverter-based pulse emission system architecture and comparing the current response and residual current tailing time of different loads to design an appropriate load circuit.This study investigates methods for controlling pulse energy transformation and determines the timing of control signals based on the principle of carrier comparison pulse width modulation,providing a research framework for implementing pulse emission modulation technology.(3)Study on amplitude modulation pulse emission technology based on carrier phase-shift control to address the amplitude control issue of resonance pulse sequences Using differential equations to establish a mathematical model of the transmission system’s topology,constructing expressions with pulse current as the computational objective,and solving for the envelope of the pulse current to achieve amplitude control.Analyzing the relationship between the current amplitude within a single switching cycle and the phase shift angle of the pulse voltage,and integrating dynamic phase-shift control technology into the system equations to achieve steady-state and amplitude modulation control for both single-pulse and multi-pulse scenarios.Considering the operating mode of the transmission loop,calculating the boundary values of the phase shift angle,and the parameter value range of the rapid shutdown circuit for residual current,and validating the performance of the phase-shift amplitude control scheme through simulation and practical testing lay the technical foundation for achieving amplitude-frequency synchronized modulation pulse.(4)Study on amplitude-frequency modulation pulse emission technology based on synchronous phase-shift frequency control to address the amplitude-frequency synchronized modulation issue under the conditions of amplitude attenuation of energy storage power supplies and nonlinear loads.Analyzing the response characteristics of the transmission loop,deriving the system’s amplitude-frequency and phase-frequency response functions,and analyzing the effects of system group delay,phase delay,and phase offset on pulse amplitude modulation and frequency modulation.By studying methods to compensate for pulse waveform distortion and mitigate the adverse effects caused by amplitude and phase deviations,while simultaneously introducing two degrees of freedom-control signal phase shift angle and switching cycle-to design a phase-shift frequency control scheme,deriving amplitude-frequency synchronized modulation control equations,and investigating initial phase shift angle estimation methods.Finally,validating the performance of the synchronous phase-shift frequency control scheme through simulation and practical testing.(5)Developing a pulsed transmission system with amplitude-frequency synchronized modulation for magnetic resonance sounding,and constructing a prototype of the detection system by integrating pulsed transmission system with the communication unit and the reception unit.The performance indicators of this prototype should reach international advanced levels.Based on the detection system,conduct field experiments in the water-rich environment of Nanhu in Changchun.By comparing the results of single pulses and adiabatic pulses,verify the detection performance and technical advantages of the amplitude-frequency synchronous pulses.The aforementioned research results have shattered the technical monopoly of foreign instrument manufacturers by achieving,for the first time,amplitude-frequency synchronized modulation pulses in domestically-produced MRS instruments.This breakthrough can offer technical support and reference solutions for advancing independent research and development of domestically-produced instruments. |
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