Unilateral Nuclear Magnetic Resonance Sensor For Analyzing The Moisture In Superficial Soil

Superficial soil moisture is an important factor affecting crop growth.Soil moisture detection is helpful to assess the degree of drought and wetness and promotes the efficient use of water resources.Soil water storage capacity and movement characteristics affect the movement and transformation of chemical substances in soil.The detection of soil moisture is helpful to evaluate the absorption effect of chemical fertilizers effectively.It's significant to environmental protection.Soil moisture content and water molecular motion characteristics is essential for the seed germination,root growth and nutrient uptake.So how to measure the superficial soils to reflect the distribution of soil moisture is an important aspect of soil research.Detecting superficial soil moisture with unilateral nuclear magnetic resonance(UMR)sensor was proposed.It can not only reflect soil water content,but also size of soil pore.It can realize non-destructive detection of soil moisture on site.In this paper,a UMR sensor for superficial soil moisture detection was designed and tested for the performance.Application measurement experiment was carried out with it.The details are as follows:(1)The design of unilateral magnet structure.The object region was given in advance.Finite element simulation software(Ansoft Maxwell)was used to calculate the magnetic field distribution.The magnet structure with four symmetrically arranged magnet unit was designed which can produce horizontal static magnetic field.Every magnet unit consists of four magnets.An iron yoke was added under the magnets to reduce magnetic flux leakage and increase magnetic field of the object region.In order to reduce the influence of the eddy current above the aluminum shell,anti-eddy current module consists of silicon steel wad added over the sensor.Anti-eddy current effect was tested.The voltage was applied to the transmitting coil.The detected voltage of detection coil after adding anti-eddy current module was larger than the sensor without it.Anti-eddy current module had an effect of anti-eddy current.The target plane was in the range of 50mm×50mm at 75 mm above the sensor.The magnetic strength of the center point in the target plane was 71.1mT(frequency: f=3.027MHz).The magnetic field uniformity in the target plane was 0.95%.The gradient of the magnetic field in the range of 5 mm above and below the center point of the target plane was 1.03T/m.(2)The design of RF coil.Four flat spiral coils of different were simulated.B1/(?) was regarded as a design standard.B1/(?) of different height above the RF coils were compared.By contrast the smallest RF coil with the best B1/(?) was chosen.(3)The performance tests of the UMR sensor.(a)The verification of coil simulation results: CPMG Add experiment which was all the CPMG echo signals superimposed was carried out to measure the maximum detectable depth of coil.The experiment results showed that the smaller the coil size was,the better the signals were.The maximum detection depth of the smallest size of coil could be 45 mm whose measurement signals was the strongest.It was basically identical with the simulation results.The smallest size of coil was chosen.(b)The experiment of single excitation measurable thickness ?d: Analyzing the measured CPMG Add spectral we could know that ?d was 0.865 mm.(c)Longitudinal resolution experiment: The oil-immersed papers and coverslips were alternately stacked as a sample.Then we measured and obtained the spectral distribution of the signals.The smallest distance 441?m between different layers of oil-impregnated paper could be effectively distinguished by spectral distribution.(d)The measurement of temperature stability of magnetic field: The temperature coefficient of magnetic field at the center point of 75 mm above the sensor was-0.256%/? from 0? to 40?.(4)Application experiments of UMR sensor: The loam and sand samples of different moisture content were measured with the sensor.Single-exponential fitting and double-exponential fitting of the echo signal were carried out to obtain the equivalent transverse relaxation time T2 distribution.Invert the measured echo signals to obtain T2 spectral distribution curve.Comparing the T2 spectral distribution curve,it can be found that the main peak moved left and decreased as the sample dried.The integral area of the T2 spectral distribution curve decreased and were proportional to the moisture content obtained by the weighing method.A UMR sensor which can be used to measure superficial soil moisture was developed in this paper.It can also be used to measure moisture of wood,concrete and so on.Its scope of application will be more extensive as the detection depth increase.