Study On Water Content Inversion Process Of Multi-scale Vadose Zone Based On Ground Penetrating Radar

The aeration zone,a complex system of soil particles,water and air,is the bridge for water exchange that connects surface water and ground water,which plays an important role in the hydrologic cycle.In addition,the aeration zone also plays an important role in groundwater pollution.Pollutants are stored in the aeration zone after infiltration from the surface,and coupled with physical and chemical reaction.Therefore,the study of distribution of water content in the aeration zone is of great significance for hydrologic cycle and pollution control.Traditional measurement of water content has some limitations,so it is of great practical significance to explore a non-destructive,rapid and accurate method for the determination of moisture content of continuous sections in the aeration zone.Ground penetrating radar has the characteristics of fast measurement speed,high resolution and non-destructive detection,thus it is widely used.This study uses the methods of indoor physical simulation experiments and in-situ monitoring and field scale experiments.of the attenuation process of ground penetrating radar electromagnetic wave in the shallow aeration zone with moisture content and depth are analyzed.The attenuation characteristics of electromagnetic wave is studied.In addition,the mathematical model of ground penetrating radar electromagnetic wave attenuation and the moisture content was developed.This study can be the theories for inversing the moisture content using ground penetrating radar in the basin scale.Based on the physical simulation experiments and theoretical analysis,the following conclusions are obtained:(1)The dielectric constant is an important data in parameter setting of Ground penetrating radar.In this paper,the following conclusions are obtained from the indoor physical simulation tests:(1)under conditions of the same particle gradation,the most important factor affecting the dielectric constant of the medium is the moisture content.The dielectric constant increased with the moisture content.(2)Under the condition of the same water content,the smaller the particle size of the medium,the larger the dielectric constant.The dielectric constant and particle size is negatively correlated.The detailed dielectric constant test results are:under the condition of a certain water content,clay>powder>sand.According to the method of inverse distance weighted average,the calculation software of soil dielectric constant was compiled,and the dielectric constant of the medium was determined quickly by setting the proportion of medium particles and water content.(2)Based on the indoor physical simulation tests and in-situ monitoring tests,the transportation rules of electromagnetic wave propagation under conditions of different media,different water content and depth were revealed,and the following conclusions were obtained:(1)In the sand media,the values the represents the water content changes best are root mean square amplitude,average absolute value amplitude,average amplitude and total amplitude;In silty soil,the amplitude of total absolute value and the amplitude of average wave peak can best characterize the changes of water content.In silty clay,the amplitude of total absolute value is the best indication of moisture content change.The amplitude of root mean square,the amplitude of total absolute value and the amplitude of average absolute value can best characterize the change of water content in aeolian sand.(2)The influence of the lithological characteristics of the medium on the property of sensitive amplitude:when the soil composition of 0.5?0.25 mm particle size in the sandy medium is relatively large(such as medium sand),the average amplitude and the total amplitude can be used to characterize the change of the moisture content of the medium.With the increase of the proportion of fine particles(such as fine sand),the amplitude of root mean square and the amplitude of mean absolute value can also characterize the change of water content of medium.When soil with a particle size of 0.25?0.075mm occupies a large proportion in sandy media,the amplitude of root mean square,the amplitude of average absolute value and the amplitude of total absolute value can characterize the change of water content in the medium.When the content of particles in the medium is high(such as silt),the average absolute value amplitude and the total absolute value amplitude can characterize the change of water content in the medium.When there are few sand particles in the medium,and the proportion of powder and clay particles is relatively high,only the amplitude of total absolute value can characterize the change of water content in the medium.(3)Based on the electromagnetic wave propagation theory,the concept model of electromagnetic wave was developed with the moisture content as a variable.This study build four different medium sensitive attributes to the measured electromagnetic wave amplitude value of the independent variable,the moisture content of the dependent variable characteristic function model,on the basis of correlation to extract the each medium can characterize its moisture content changes of the amplitude attribute.Fitting characteristic functions of sensitive amplitude and water content of various media by nonlinear least square method.In medium sand medium,the correlation of the characteristic function model with the average amplitude as the independent variable is 0.95.In silty medium,the correlation of the characteristic function model with the amplitude of the total absolute value as the independent variable was 0.94.In silty clay medium,the correlation of the characteristic function model with the amplitude of the total absolute value as the independent variable was0.94.In the aeolian sediment medium,the correlation of the characteristic function model with the amplitude value of root mean square as the independent variable is0.94.(4)The feasibility of reverting the moisture content of aeration zone through the amplitude attribute of electromagnetic wave of ground penetrating radar on the field scale was verified.The predicted and measured moisture content distributions of the three profiles were extracted and analyzed.Using root-mean-square error(Re),correlation coefficient(R~2)and Nash efficiency coefficient(Ens),the water content of ground penetrating radar electromagnetic wave inversion aeration characteristic function model to evaluate the precision,drawing a conclusion:the characteristic function in section three model inversion prediction of the moisture content,the root-mean-square error is within 1%,the overall correlation coefficient is above 0.95,the Nash efficiency coefficient is above 0.9,was highly correlated,which indicates the result is ideal.It is proved that it is feasible to invert the moisture content distribution of aeration zone through the amplitude attribute of electromagnetic wave of ground penetrating radar.