| China is facing the dilemma shortage of fresh water.Using surface nuclear magnetic resonance technology to detect groundwater scientifically and rationally can solve water shortage effectively.The early single-channel surface nuclear magnetic resonance detector is only suitable for one-dimensional detection of groundwater,and the detection efficiency is low and the resolution is poor.After years of development,China has successfully developed a multi-channel surface nuclear magnetic resonance detector,which has greatly improved the lateral resolution of the detection area and the shallow longitudinal resolution,and realized the multidimensional detection of groundwater.When multi-channel surface nuclear magnetic resonance detector is used for multi-dimensional groundwater detection,magnetic resonance signals in the array receiving coils are coupled due to the coupling,it will change the intensity of the original signal in the receiving coils and then affect the water content of signal inversion interpretation.In this paper,Type ? decoupling circuit which is consists of capacitance and inductance is proposed to solve the problem of signal coupling between array receiving coils and improve the accuracy of groundwater multi-dimensional detection.The main research work of this paper contains:(1)The basic principle of surface nuclear magnetic resonance detection is studied,the change process of macro magnetization of hydrogen proton in groundwater under magnetic resonance detection is analyzed,the calculation formula of macro magnetization of hydrogen proton is derived,and the sensitivity kernel function of magnetic resonance signal is constructed.The multi-dimensional surface magnetic resonance detection technology is introduced,and the advantages of array receiving coil measurement for groundwater detection are expounded.The reasons of signal coupling between array receiving coils are studied,several decoupling methods commonly used in array coils are listed,and the advantages and disadvantages of these decoupling methods are analyzed through principle and realization process.(2)Type ? decoupling circuit is designed.Equivalent circuit model is constructed according to the receiving coil receive coil impedance matrix equation,add between receiving coil after decoupling circuit to obtain new impedance matrix equation.The parameters of decoupling inductance and decoupling capacitance are obtained by solving the impedance matrix equation.By circuit simulation and experimental test,verify the practicability of decoupling circuit.Using the LCR meter to measure the decoupling experiment using the receive coil impedance parameters,at the same time,the coupling between the receiving coils in the decoupling experiment is measured by the forward and reverse series method.Using MATLAB software to write a simulation program to calculate the mutual inductance and the relative position between coils,mutual inductance simulation result is consistent with the measured results.(3)The receiving system circuit of surface nuclear magnetic resonance detector is designed.To meet the requirements of n V level weak signal detection,the low noise coil sensor,signal conditioning circuit,signal acquisition circuit,control circuit and power supply circuit are designed.The background noise of the preamplifier is1 n V/?Hz,the bandwidth of the bandpass filter is 1.8k Hz,the gain of the whole receiving system circuit is 79 d B,the accuracy of the acquisition card is 24 bit,and the highest sampling frequency is 156 k Hz.The receiving system adopts STM32+FPGA control architecture to complete the data interaction between the receiving system and the host computer,and realize the precise timing control of the data acquisition card.The receiving system has been tested in the laboratory and tested in the field,and the effective signals have been obtained,which verifies the reliability of the receiving system.(4)Array receiving coil decoupling experiment was carried out respectively in the electromagnetic shielding room and field,the indoor and outdoor experiments are achieved well decoupling effect.The field experiment results are as follows: When the receiving coils are laid side to side,the signal intensity in the coil is 79.16% of the original signal,after decoupling,the signal intensity in the coil is 97.25% of the original signal;When the receiving coil is half-covered and laid,the signal intensity in the coil is 48.08% of the original signal,and after decoupling,the signal intensity in the coil is 95.45% of the original signal.The results show that ? decoupling circuit can effectively suppress signal coupling between receiving coils.To sum up,This paper completes the decoupling study of the receiving coils of surface nuclear magnetic resonance and designed type ? decoupling circuit.In order to carry out the experiment of nuclear magnetic resonance signal decoupling,the receiving system circuit of surface nuclear magnetic resonance detector is designed.The effectiveness of type ? decoupling circuit is verified through experiments of electromagnetic shielding chamber and field environment.The research in this paper is of great significance to the application and development of surface nuclear magnetic resonance detection. |
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