Basic Research On Magnetic Resonance Measurement Methods Of Soil Moisture

In China,soil erosion is serious in sub-humid areas,semi-arid areas and arid areas,the unreasonable use of land accelerates the deterioration of soil environment.These problems seriously affect the sustainable development of ecology and agriculture in our country.Rapid and accurate measurement of soil moisture is of great significance for ensuring the sustainable development of agricultural production and ecological environment.The existing magnetic resonance measuring instrument can quantitatively detect the shallow soil moisture distribution,but because of the large volume and heavy weight of the instrument,it is inconvenient to realize rapid on-site measurement of soil moisture in agricultural applications.This project aims at miniaturization and weight reduction of sensors for soil moisture measurement,and solves two key problems in the field of soil moisture detection by studying corresponding magnetic resonance sensors and echo signal analysis methods.The two key scientific problems are:Electromagnetic design theory of magnetic resonance sensors;The characteristic index extraction method for the magnetic resonance signal of soil moisture.Regarding these two problems,the work done in this thesis includes:?1?Based on the idea of coordinate transformation,the design method of Halbach magnet structure was studied.According to the requirements of on-site soil sampling measurement,this thesis proposed a lightweight Halbach sensor.According to the periodic symmetrical structural characteristics of the Halbach magnet,the magnetic field distribution of a single magnet was calculated according to the equivalent surface current method,and then the magnetic field distribution of the remaining magnetic blocks was calculated by the coordinate transformation method.Finally,the Halbach magnet was optimized according to minimizing of the magnetic field inhomogeneity.The calculation theory based on coordinate transformation reduced the number of repeated calculations and improved the calculation and optimization efficiency of of Halbach magnet.?2?The designing of shimming coil for Halbach magnet was studied by equivalent magnetic dipole method.In order to improve the magnetic field uniformity of the Halbach magnet,an active shimming method was proposed on the basis of passive shimming to eliminate the high-order inhomogeneities related to the coordinates in the main magnetic field.The current density of the wiring area was equivalent to the form of a magnetic dipole,and the relationship between the stream function and the target magnetic field was established by using the equivalent magnetic dipole as an intermediate variable.Energy consumption minimization and construct unconstrained optimization problems were introduced to obtain stream function expressions.Finally,the winding structure of the shim coil was obtained by discretization of the stream function.Experiments showed that the magnetic field uniformity of Halbach magnets has increased from 114.2 ppm to 26.9 ppm with active shimming.?3?A fully open Inside-out sensor with constant gradient was designed to solve the problem that the existing magnetic resonance sensor was not light enough.According to the requirements of on-site soil plug-in measurement,this paper proposed an inside-out sensor.The sensor has the advantages in light weight,small volume,measurement not limited by sample size,and simultaneous measurement of the lateral relaxation time and diffusion coefficient of soil moisture.?4?An algorithm combining partial unit equivalent circuit and second-order vector bit analytic method was proposed,which can accurately calculate the AC impedance of RF coil in the inside-out magnetic resonance sensor.In order to evaluate the metal shielding layer influence on the impedance of the RF coil,the resistance of the RF coil in the free space was calculated by the partial unit equivalent circuit method,and then the resistance increment of the RF coil was calculated due to the metal shielding layer by the second-order vector potential analysis method.At last,AC impedance of the RF coil was obtained.?5?A broadband matching method for RF coils was proposed.For the inside-out magnet structure with constant gradient,parallel resistance was added to the traditional?-type matching network,thereby reducing the quality factor of the RF coil,increasing the RF bandwidth,and expanding the actual measurement area.Compared to the traditional?-shaped matching method,the improved broadband matching method increased the signal-to-noise ratio by 41.3%.?6?A characterization method for soil moisture with transverse relaxation time T₂and diffusion coefficient D is proposed.The integral state of the peak in the transverse relaxation time spectrum was used to characterize the existence and quantity of water,and the dynamic coefficient of water was characterized by the diffusion coefficient.The measurement results show that while the soil water content is smaller,the area of the T₂peak is smaller;the soil particle size is smaller,T₂ is smaller and the diffusion coefficient D is larger.According to the results,the diffusion coefficients of water in the three samples with particle radius of 0.5³mm are basically the same,and the diffusion coefficients of samples with particle radius of 0.2⁰.5mm are significantly larger than those of the first three samples.This is mainly because the moisture in the sample is basically saturated,and for the particle radius in the range of 0.5 to 3 mm,the moisture in the pores of the sample is basically in a free state,and the diffusion coefficient is substantially unchanged.When the pores are smaller,the water molecules is affected by pore walls,and the free travel of the water molecules is shorter,which accelerates the transverse relaxation process,and macroscopically exhibits a larger diffusion coefficient and a shorter transverse relaxation time.In this paper,a series of work were carried out in the electromagnetic theory optimization method of magnetic resonance sensor design,magnetic resonance signal analysis algorithm and soil experiment.A comprehensive study on magnetic resonance technology were carried out to promote magnetic resonance technology in situ soil moisture detection.