Research On Key Technology Of Surface Nuclear Magnetic Resonance Based On Cooled Coil Sensor

After years of continuous development,surface nuclear magnetic resonance technology has become an indispensable and important method in geophysical exploration.Its main applications are water prospecting in arid areas,dynamic monitoring of landslides,detection of dam leakage and detection of hidden water hazards in underground projects.In particular,with the development of magnetic resonance sounding from surface to underground,the detection instruments have also changed,mainly reflected in the change of detection coil,from 100 meters to meters and sub-meters.How to develop coil sensors with higher sensitivity and lower noise in limited size is a good problem of the techniques of surface nuclear magnetic resonance.At present,the problems existing in surface magnetic resonance coils mainly include:The coil used in underground is made smaller,resulting in shallow detection depth,low sensitivity,and difficult to reduce noise level.The ringing caused by the high-impedance of small coil leads a distortion of magnetic resonance signal,and mutual inductance of array coil causes the acquisition signal to be abnormal.When peak and harmonics noise are existing at the same time,the harmonic model based method results in repeated peaks.In view of the above problems,this paper proposeds a key technology of surface nuclear magntic resonance based on a cooled coil,which uses liquid nitrogen refrigeration to reduce the noise level,improve sensitivity and improve detection accuracy and depth.At the same time,according to the forward and inversion of nuclear magnetic resonance,the signal derivation process is combined with the characteristics of the coil sensor for the first time.The design method of the cooled coil sensor is proposed,and the effectiveness of the cooled coil is verified by field experiments.The noise ratio is superior to conventional coils.On this basis,a nuclear magnetic resonance instrument with cooled coil system and capable of suppressing ringing was developed,and a data processing algorithm suitable of suppressing spikes and harmonic noise was developed.The main research and achievements of the thesis are as follows:1.Based on the modeling and analysis of the cooled coil sensor,the forward model of the nuclear magnetic resonance groundwater detection with kernel function is introduced,and the definition method of the new cooled coil sensor signal to noise ratio is proposed,which is the ratio of the magnetic resonance signal size to the cooled coil sensor noise.The sensitivity analysis results of the cooled coil sensor obtained the number of cooled coil and the type of preamplifier.2.Through the model of nuclear magnetic resonance,the bandwidth of the cooled coil sensor is obtained.Combined with the existing LC resonance and the 3rd-order Butterworth bandpass front-end form,the design of the coil-coupled passive bandpass filter is proposed.In this way,the coil parameters in the cooled coil sensor are used as part of the passive bandpass filter design,which simplifies the front-end circuit form and reduces the insertion loss.3.Using the low temperature condition,the subsequent active circuit of the cooled coil sensor is cooled to reduce its noise,and the FET type operational amplifier is also used instead of the bipolar operational amplifier to realize the short circuit noise reduction of the cooled coil sensors by 1 nV/(?).4.Aiming at the ringing phenomenon occurring in the measurement process of cooled coil sensor,a bipolar phase pulse method is proposed to suppress ringing.The ringing is suppressed from the mechanism layer that generates the magnetic resonance signal,and the two initial phases are excited by a pulse with a phase difference of 180°,and the obtained data is subjected to positive and negative superimposition,which can eliminate the fixed phase ringing caused by hardware.5.Assuming that the noise existing in cooled coil detection,a quadratic spike noise processing method based on autoregressive model(AR)is proposed.When spike noise and power frequency harmonics exist simultaneously,the AR algorithm can remove the spike to the maximum extent.The spike noise is not affected by harmonic modeling(HNC).The measured data proves the superiority of AR algorithm,step-by-step HNC and same-frequency HNC processing.6.Based on the above hardware implementation and integration,the cooled coil instrument system was developed.The system adopts a highly integrated design,and the JLMRS series instruments are miniaturized and improved,the volume is reduced to 1/4,and the weight is reduced to 1/7.Single-handed operation,performance indicators are better than current tunnel and mine detection systems.Through the research of the above content,this paper realizes the design of cooled coil sensor and the research and development of cooled coil nuclear magnetic resonance detecting instrument.The main innovative work is as follows:1.In the development process of underground nuclear magnetic resonance,the problem that the detection coil changes from 100 meters to sub-meter level,combined with the magnetic resonance forward method,this paper proposes a new method for obtaining ground-based magnetic resonance groundwater detection signals by using cold coil sensors.Low temperature conditions optimize the transfer function of the cooled coil sensor and reduce thermal noise,improving the detection capability of small-sized coils.2.Integrating the theory of magnetic resonance groundwater detection into the research and development of cooled coil measuring instruments,a coil-coupled passive bandpass filter is designed to ensure the bandwidth requirement of the short relaxation time of magnetic resonance signals,and can be used for two-dimensional and three-dimensional to solve the problem of coil mutual inductance.3.Aiming at the ringing phenomenon occurring during the magnetic resonance detection of cooled coils,the bipolar phase pulse method is proposed to suppress the ringing from the mechanism of magnetic resonance signal generation.The magnetic resonance data obtained by using two pulses with initial phase differences of 180° is positive.Inverted superposition eliminates the ringing of the fixed phase caused by mechanical hardware and controls the ringing process of the high-impedance cooled coil within 5 ms.4.Aiming at the spike noise and power-frequency harmonic noise collected during the cold-coil magnetic resonance detection process,a secondary spike processing method based on the autoregressive model is proposed to avoid the impact of harmonic modeling on the generation of small spikes and the spikes and effects of Harmonics on cooled coil sensors.