The Key Technology Of Transmitting System For The Pre-polarization Surface Nuclear Magnetic Resonance

Surface nuclear magnetic resonance(SNMR)technology is widely used for hydrological investigations.When using a meter-level small coil detects shallow groundwater,utilizing a pre-polarization field can enhance the amplitude of the SNMR signal in a high ambient noise level,such as cities,tunnels,and mining areas.The transmitting intensity and the shutdown manner of the pre-polarization field is extremely important for the enhancement of SNMR signal.The characteristics of pre-polarization current are high-power cyclic transmitting with long time,high intensity and many superposition times.This paper proposes the technology of the pre-polarization current-sharing with paralleled IGBT,and realize the high-power DC pulse transmitting.According to the spatial distribution characteristics and shutdown conditions of the pre-polarization field,we propose the technology of the controllable adiabatic shutdown,realizing the controllability and consistency of the shutdown process of the pre-polarization field.At the same time,the AC field transmitting circuit and control system are designed,combined with the overall structure of transmitting system,and we finally completed the the design of transmitting system.The main research contents are as follows.(1)We study the basic principle of pre-polarization field surface nuclear magnetic resonance.Through the calculation and simulation,the spatial distribution characteristics of the pre-polarization field are analyzed.It is concluded that increasing the transmitting intensity of the pre-polarization field can enhance the amplitude of SNMR signal by the calculation of magnetization and sensitivity kernel function.We analyse the precession process of magnetization during the pre-polarization field shutdown,and study the shutdown conditions of the pre-polarization field.(2)The pre-polarization field DC pulse transmitting circuit is designed.The influence factors of paralleled IGBT unbalanced pre-polarization current are analyzed by calculation and simulation.We propose the technique of the pre-polarization current-sharing.The transmitting principle and scheme are introduced,and the simulation verification is executed.We design the pre-polarization field transmitting circuit and conduct experimental tests.When the main power switch is designed by two IGBTs in parallel,current-sharing transmitting of the pre-polarization can be realized.(3)The controllable adiabatic shutdown circuit is designed.We analyse the shutdown circuit characteristics and the traditional shutdown circuit.Aiming at the shortcomings of traditional shutdown circuits,we propose the controllable adiabatic shutdown technique and design a shutdown circuit based on half-controlled full bridge circuit and capacitor bank.Then we study the working sequence of shutdown circuit and calculate the shutdown process numerically.The control circuit and shutdown capacitor voltage sampling circuit are designed.And controllability and consistency of the shutdown process are verified by the simulation and experimrnt results.When the initial voltage of the capacitor is between 65 V and 300 V,the initial amplitude of the exponential decay of the pre-polarization current increases from 15 A to 48 A,and the shutdown time decreases from 1.4 ms to 1 ms.Which is,the shutdown ramp and time can be optimized.In addition,when the parameters of the pre-polarization current transmitting are inconsistent,the pre-polarization current can have the same exponential decay ramps and the total shutdown time is the same.(4)We have completed the development of the pre-polarization field surface nuclear magnetic resonance transmitting system and conducted field experiments.The AC field transmitting circuit is designed,including full bridge inverter circuit and current sampling circuit.We design the transmitting control system,and the structure of control system,communication unit and transmitting control flow are introduced in detail.Finally,the transmitting system was tested and SNMR experiment was carried out.And the effective SNMR signal was obtained through the measurement of the aquifer under the ice surface in the urban high noise environment,which verified the performance and effectiveness of the system.we have completed the development of the transmitting system,and carried out laboratory tests and magnetic resonance detection experiments in urban environment,which verified the effectiveness of the system.And the research of this paper will promote the application and development of pre-polarization technology in high noise environment such as city,tunnel and mining area.