Development Of Surface Nuclear Magnetic Resonance Detection Principle Prototype Based On Pre-polarization Field

China is facing the dilemma of shortage of fresh water.How to use scientific and rational geophysical methods to detect groundwater is the key to solve the problem of water shortage effectively.Traditional Surface Nuclear Magnetic Resonance(SNMR)as a groundwater exploration method developed in recent years,which can realize non-invasive and quantitative detection of groundwater.But the magnetic resonance signal is only nanovolts,therefore,it is often overwhelmed by noise when detecting groundwater in an urban environment.To detect groundwater effectively in urban high-noise environment,the pre-polarization field is used in SNMR.This paper analyzes the principle of pre-polarization field in SNMR detection and develops the pre-polarization detection prototype prototype and introduces the design process of the receiving system and the transmitting system in detail.The prototype prototype can fundamentally improve the macroscopic magnetization of the water body and improve the signal intensity,which can also accurately explain the water content and position information.This method will become an effective means to detect groundwater sources in urban high-noise environments.This paper mainly completed the following research work:(1)The basic principle of applying the pre-polarization field to SNMR is discussed.The detailed variation process of hydrogen proton macroscopic magnetization and the formula for calculating the macroscopic magnetization are given.In addition,the numerical calculation of the SNMR detection model based on pre-polarization field is carried out,which including the division of the underground space structure,the calculation of the magnetic field distribution,the solution of the sensitivity kernel function,and the simulation of the magnetic resonance response curves of different aquifer depths.At the same time,the overall structure,controlstructure and communication mode of the prototype of the pre-polarization field ground magnetic resonance detection principle are designed.(2)The overall transmitting scheme and key hardware circuits of SNMR detection principle prototype based on pre-polarization field are designed.Aiming at the problem of current independence between pre-polarized current and alternating current in the common coil,a isolation technique is proposed.According to the requirement of adiabatic shutdown of pre-polarization field,a fast and stable shutdown technology of pre-polarization current is proposed.The measured results show that the designed transmitting system can realize the independent output of pre-polarized current and alternating current.Besides,when pre-polarization current is 91 A,it can be turned off within 3 ms,so that the polarized hydrogen proton remains complete magnetization.(3)The receiving structure of SNMR detection principle prototype based on pre-polarization field is designed,and the LC resonance sensor and signal conditioning circuit are developed.In actual detection,it is further found that the background noise of the LC resonance sensor is different under different detection environments,and this phenomenon will affect the quality of the signal.This finding was analyzed and finally verified that the ringing effect is the cause of this phenomenon.Aiming at the problem that the ringing effect affects the signal quality,the sensitivity optimization technique of LC resonance sensor is proposed.The test results show that the signal-to-noise ratio of the optimized LC resonance sensor is improved to-4.2 dB.(4)The tests are carried out to verify the ability of the SNMR detection prototype based on the pre-polarization field to find groundwater.Based on the introduction of the test procedure and the required equipment,the prototype test in the electromagnetic shielding room and urban space are carried out.The experimental results show that the prototype can measure the effective magnetic resonance signals in the low noise environment of the electromagnetic shielding room and the high noise environment in the urban space,and the interpretation results of free water below 0.6 m on the ice surface are consistent with the actualconditions.Therefore,the prototype achieved the expected design goals,which further verified the feasibility of applying the pre-polarization field to SNMR.In summary,this paper completed the development of a prototype of ground magnetic resonance detection based on pre-polarization field.The effectiveness of the prototype was verified in electromagnetic shielding chamber and urban space environment.The theoretical methods and detection systems proposed in this paper provide a research basis for further expanding the application range of terrestrial magnetic resonance detection technology and enhancing the groundwater detection capability in high noise interference areas.