Test on nondestructive detection of farmland solum structure in fluvo-aquic soil area using ground penetrating radar

Abstract: Solum structure is an important index to evaluate the cultivated land quality. How to obtain the solum structure information quickly and accurately has been the hot spot in the fields of soil, land, water conservancy, agriculture, and so on. In order to explore the feasibility of rapid and nondestructive detection of solum structure in natural soil by ground penetrating radar (GPR), this paper first used GprMax2D software based on finite difference time domain (FDTD) to simulate 4 soil models including 3 soil layers with different dielectric characteristics, and recognized the objective change rules of amplitude and phase of the radar electromagnetic wave in those soil layers with different dielectric characteristics. Then, 2 kinds of soil profile patterns i.e. clay interlayer type and sand bottom type with obvious soil layers in Quzhou County of Hebei Province, located in the alluvial soil area of North China Plain, were selected for GPR detection experiment in 2016, and soil profiles were excavated. The clay interlayer type solum structure indicated that a thin clay layer was in the soil mass, and the sand bottom type solum structure indicated that there was a sand layer at the bottom of soil mass with 1 m depth. Radar images were processed through radar image processing software and MATLAB programming to extract waveform data. According to the objective rules obtained by simulation, soil layer identification was carried out, and the detected thickness and measured thickness of every layer were compared. The results showed that the reflection of radar electromagnetic wave occurred at the interface of different media, and the greater the difference of dielectric properties between 2 sides, the greater the amplitude of reflected wave. If the dielectric constant of the upper layer was smaller than that of the lower layer, positive reflection would occur; otherwise negative reflection would occur. The measured waveform was more chaotic than the simulated waveform. The relative error (<9%) for identifying the thickness of the thicker layers of the clay interlayer type solum structure with clear layer interface was less than the relative error (>9%) for identifying the thickness of the sand bottom type solum structure by the amplitude and phase change of the electromagnetic wave. The reflection coefficients of the identified hierarchical interfaces were all greater than 0.02, indicating that the 2 sides of those interfaces had larger dielectric properties difference. The hierarchy of solum structure for farmland fluvo-aquic soil can be identified by the objective interpretation basis, but the tidy and clear degree of the hierarchical interface, difference degree of soil between the 2 sides of the interface and the thickness of adjacent soil layers affect the GPR detection effect. This article can provide reference for the related research and provide basis for rapid monitoring of cultivated land quality with solum structure as an important index.