Development Of Surface Magnetic Resonance Receiving System Based On Prepolarization Field

China's freshwater resources are extremely scarce,efficient and accurate detection of underground available freshwater resources is an effective way to solve the scarcity of water resources.The method of detecting groundwater by magnetic resonance sounding is the only geophysical method for direct detection of groundwater,which directly obtains underground hydrogeological information through non-intrusive detection methods.However,the magnetic resonance signal is extremely weak and can only reach n V(10-9 V).It is difficult to adapt to the environment of high noise detection around the city,and the detection of signal to noise ratio and sensitivity is serious insufficient.The acquisition of reliable magnetic resonance signal is the difficult and bottleneck of the ground magnetic resonance technology.In this paper,in view of the low signal to noise ratio of ground magnetic resonance detection,a method of ground magnetic resonance detection based on prepolarization field is proposed for the first time.By artificially applying prepolarization field,the macroscopic magnetization of groundwater is greatly improved,and then the magnetic resonance signal is enhanced.At the same time,in order to acquire the high signal to noise ratio magnetic resonance signal,the magnetic resonance sensor based on prepolarization field(MSBP),which has low noise and high sensitivity,is developed can effectively improve the sensitivity of the system,reduce the noise of the system and suppress the ringing.Based on the above research,a highly sensitive surface magnetic resonance receiving system based on prepolarized field is builted.Through the experimental results and practical examples are used to prove the ability of the system to be measured in the area of high noise interference.This paper mainly completed the following research work:(1)The principle of magnetic resonance detection based on prepolarized field is expounded.The calculation formula of the macroscopic magnetization of groundwater under the prepolarization field is given,which theoretically shows that the prepolarization field can improve the feasibility of the intensity of the groundwater magnetic resonance signal.By comparing the prepolarization field magnetic resonance detection and the conventional magnetic resonance detection of one dimensional sensitivity forward core function,it is proved that the prepolarization field magnetic resonance detection method can improve the intensity of the magnetic resonance signal at the same water content.(2)In order to improve the sensitivity of the meter-level detection coil used by magnetic field detection method based on prepolarized field,the design idea of MSBP structure with high sensitivity is put forward.Based on Faraday's law of electromagnetic induction,the sensitivity of MSBP is optimized.Meanwhile,aiming at the ringing phenomenon generated on MSBP,a Q-switch circuit based on field effect transistor is designed to suppress.Through the above research work,we have completed the design of MSBP,and its sensitivity can reach 9.7(?).(3)In view of the high noise interference in the detection area,the overall design scheme of the surface magnetic resonance receiving system with low background noise,wide gain dynamic range and strong filtering ability is proposed.It includes preamplifier,two stage amplifier,program controlled amplifier,bandpass filter,power frequency filter and low pass filter.Meanwhile,the design of the power supply module of the receiving system is completed.Through the design of the above key circuits,we have completed the design of the prepolarized field surface magnetic resonance receiving system,whose background noise is as low as 0.87(?).(4)The shielding room and outdoor experiments were carried out by the independent development prepolarized field surface magnetic resonance receiving system.The effectiveness of the receiving system is verified by simulating the geomagnetic field in the low noise shielding room and obtaining effective magnetic resonance signals.At the same time,in the high noise environment of the city center(the average noise in the passband is above 5 ?V),the experiment is carried out by artificial water body,compare with the traditional alternating magnetic field emission,which further validates the effectiveness of the receiving system in the area of high noise interference.Comparing the theoretical model and the measured data inversion results,it is proved that the prepolarized field surface magnetic resonance receiving system has achieved the development requirements in the field of measurement.To sum up,we completed the development of surface magnetic resonance receiving system based on prepolarized field,and verified the effectiveness of the system through shielding room and outdoor measurements.The theoretical method and system development proposed in this paper provide the basis for further development of the application range of surface magnetic resonance detection technology and the enhancement of groundwater detection ability in high noise interference areas.