Research On Surface Nuclear Magnetic Resonance Technology With A High Signal-to-noise Ratio Based On Double-source Polarization

With the continuous deepening of the urbanization,the problem of water shortage and social conflicts on water caused by it are increasingly intensified.To alleviate the impact of water shortage on people's production and life,surface nuclear magnetic resonance(SNMR)technology is widely used in groundwater locations.However,the free induction decay(FID)signals obtained through the SNMR method are easily submerged in the complex noise,resulting in detection failure.Therefore,how to increase the signal amplitude and enhance the application capable of SNMR detection technology is very important.In recent years,the SNMR research group of Jilin University has proposed a method based on the combined field of the polarization magnetic field and the geomagnetic field,so as to replace the traditional geomagnetic field and enhance the FID signal amplitude.That is,a direct current(polarization current)is used to generate a polarization magnetic field in a coil laid on the surface,so as to participate in magnetic resonance detection on the basis of traditional SNMR.However,the signal amplitude is closely related to the distribution of polarization magnetic fields,the amplitude and the turn-off process of the polarization current.In addition,the noise suppression capability of the coil sensor is also one of the conditions determining whether the signals can be collected normally in the complex environment.Therefore,this paper is focused on the optimization of the polarization magnetic field,in which the distribution of the polarization magnetic field as well as the amplitude and shut-off process of the polarization current is studied.Combined with the characteristics of applications under complex-noise environments,the performance of the air-core coil sensor is optimized,and finally the development of a SNMR groundwater detection instrument with a high SNR based on double-source polarization is realized.The main research content and achievements of the paper are as follows:(1)Aiming at the problem of the uneven distribution and weak strength of polarization magnetic field in the central detection area caused by the traditional single-polarization-coil method,a double-source polarization magnetic resonance signal enhancement technology is proposed.Through this method,a single polarization coil is splitted into two polarization coils under the condition that the overall polarization power remains unchanged.Further,we discuss the influence of the polarization current direction,the side length and the relative distance of the polarization coils as well as other conditions in the polarization magnetic field of the detection area under the double-source scheme.We conclude that through the double-source scheme,the distribution of polarization magnetic field can be effectively optimized in central detection area and the amplitude of the FID signals can be enhanced.(2)It is known that the enhancement of signals requires a high-power polarization current based on the double-source polarization detection theory.Aiming at the characteristics of high-power output in polarization current and the thermal reliability of instruments in ultra-low frequencies as well as continuous operation,the research on key technologies of high-power polarization circuits based on voltage-controlled compensation is proposed.Through the design of parallel IGBT modules,we not only solve the problem of limited output power,but also optimize the thermal distribution and realize design for the thermal reliability of the detection instruments.To realize the current sharing control in the parallel connection of IGBTs,a polarization current sharing control strategy based on gate-level voltage compensation is proposed.Through the research,the performance of the polarization SNMR instruments has been greatly improved,ensuring the continuous and stable operation.(3)The double-source polarization theory indicates that the turn-off process of the polarization current also affects the signal amplitude.Aiming at the problem that the passive shutdown circuits cannot match with the load change,a key technology of the active controllable shutdown circuits based on the semi-controlled full bridge is proposed.A polarization current active shutdown circuit is designed through the capacitor,and the slope as well as time of the shutdown process is optimized by changing the initial voltage of the capacitor.In view of the possible inconsistent parameters of the polarization coils,the stable and controllable shutdown process of different polarization currents can be realized without changing the hardware circuit.Through this technology,the credibility of the original data of detection can be effectively guaranteed and the accuracy of the inversion results is improved.(4)A weak FID signal extraction under complex-noise environments is the research focus of dual-source polarization NMR detection,which determines the success or failure of detection.Aiming at problems that power frequency and low-frequency harmonic noise easily lead to the saturation of traditional air-core coil sensors and the slow response of high-voltage mechanical relays when the transceiver state is switched,a research on key technologies of low-frequency harmonic noise suppression based on broadband air-core coil sensors is done.By designing a MOSFET-based back-to-back fast switch after the differential coil,the switching process of the transceiver state is accelerated,thereby collecting early FID signals.Further,a passive matching network is designed in the post-stage of fast switching,so as to suppress the interference of power frequency and low-frequency harmonic noise.The designed air-core coil sensors can be used to effectively increase the gain of the signal conditioning circuits while ensuring that the signals are not distorted,which is beneficial to improving the signal-to-noise ratio of magnetic resonance data.(5)Based on the research of the above-mentioned key technologies,a SNMR instrument and host computer software based on double-source polarization have been developed.Under the complex noise environment in urban areas,an actual detection was carried out in a water-rich and real geological environment to verify the correctness of the above-mentioned key technologies as well as the effectiveness of the instrument.More importantly,the instrument has been used to realize the SNMR detection under a real geological structure in complex-noise environments for the first time,and the reliability of the detection results is verified by comparison with known drilling data.Through the above research content,the SNR of the detection data is improved from the aspect of signal enhancement and noise suppression respectively,and the problems faced by the SNMR method are solved based on polarization technology in practical application.This method can provide important technical support for groundwater detection in the complex-noise environments of urban areas.