Research On Measuring Soil Moisture Content And Effective Sample Depth Based On GPR Ground Wave

In recent years,ground penetrating radar has been widely used in the detection of soil moisture content because of its ability to accurately,non-destructively,and continuously monitor the temporal and spatial distribution of surface soil moisture in a large range.The existing ground penetrating radar methods for detecting soil moisture content mainly include the reflected wave method,Transmitted wave method,borehole radar method and ground wave method.Among many methods,the ground wave method is considered to be the most potential method for detecting soil moisture content.However,because the uncertainty of the sample depth of this method affects the accuracy of the moisture content detection,it is necessary to study the water content detection within the effective sample depth range of the ground wave.Improve the accuracy of ground wave method to detect soil moisture content,and realize the application value of ground wave technology in soil moisture content detection in the future.In this paper,numerical simulation and physical experiment are used to study the effective sample depth of ground waves corresponding to the soil moisture content of different textures and different moisture conditions by the ground wave method,and the following conclusions are initially obtained:(1)First,the influence of soil physical parameters on the amplitude and velocity of the ground wave is studied.The research results show that low conductivity soil has almost no effect on the ground wave signal,the same moisture content and different clay content soil have different effects on the ground wave signal.The higher the soil clay content,the weaker the ground wave signal.When the clay content is greater than80%,there is basically no ground wave signal,so the ground wave method is not suitable for detection in heavy clay.(2)In the ground wave method of water content detection,the soil water content is detected based on the ground wave velocity analysis and amplitude analysis respectively.From the perspective of velocity analysis,200 MHz,400MHz and900 MHz antennas are used to detect loam and clay soils with two different textures.The velocity analysis method is used to calculate the propagation velocity of ground waves in soils with different moisture content and different textures.According to the ground The wave velocity calculates the dielectric constant corresponding to the soil with different moisture content,and modifies the empirical formula suitable for detecting loam and clay.From the perspective of amplitude analysis,the AEA method is used to calculate the reciprocal value of the a envelope mplitude corresponding to the time window range of the ground wave,and the regression analysis fits the relationship between the reciprocal amplitude envelope and the soil moisture content.The results show that there is a strong relationship between the two Linear relationship.(3)In terms of the estimation of the effective sample depth of ground waves,we also start from the perspective of velocity analysis and amplitude analysis.The research results show that the effective sample depth of ground waves is related to antenna frequency,soil moisture content and soil particle content,and it fits the moisture measured by antennas at different frequencies.The relationship between the detection rate and the sample depth,the maximum sample depth of the three antennas in loam is0.41 m,0.24 m and 0.13 m,respectively.The maximum sample depth in clay is 0.35 m,0.20 m and 0.10 m,respectively.Amplitude analysis calculates the residual percentage of the envelope amplitude and also estimates the maximum sample depth of the ground wave,and the difference in the effective detection depth of the ground wave calculated by the two methods is not large.Based on the two methods to calculate the ground wave to measure the soil moisture content and determine the effective sample depth,the application range of the ground wave method is expanded,and it provides a theoretical basis for the further application of the ground penetrating radar ground wave method in farmland and engineering.