Obtaining soil stratification information of soda saline-alkali soils based on ground penetrating radar

The large area of soda saline-alkali soils in the western part of Songnen Plain of Northeast China is difficult to utilize, which seriously restricts the development of local agricultural productivity. The status of soil layers affects the movement of water and salt in soil. It is of great significance to quickly obtain the stratification information in this area for evaluating, utilizing and improving soda saline-alkali soils. This study took the typical saline-alkali soils in this area as the research object. Firstly, soda saline-alkali soils with different salt contents were selected to conduct ground penetrating radar (GPR) detection experiment. Then the stratification time domain location was determined by radar image waveform and Hilbert spectrum instantaneous attributes, respectively. Soil layering method based on radar image waveform diagram, determined the time domain position of the overall soil layer division of the studied plot by combining multi-point single channel waveform diagrams with radar time profiles. Soil layering method based on Hilbert spectral instantaneous attributes, compared the "three instantaneous" attributes of radar signals (instantaneous amplitude, instantaneous frequency, instantaneous phase), and ultimately decided to use a combination of multi point instantaneous phase maps and instantaneous phase profiles to determine the time domain position of the overall soil layer division of the studied plot. Then, the dielectric constant of each soil layer is calculated by using the extended Dobson dielectric constant model, and the propagation velocity of electromagnetic wave in each soil layer is estimated according to the dielectric constant, and the thickness of each soil layer is calculated by combining the time domain position of the soil layer determined by the above two methods. Finally, the calculation results of the two methods are compared with the field excavation profile (the stratification results of the field excavation profile are judged by the soil classification expert according to the visual characteristics and tactile characteristics of the soil combined with his understanding of the soil occurrence process). The results indicated that: 1) The soil salinity content had a significant impact on the ground penetrating radar signal. The electromagnetic wave amplitude became very small when the two-way travel of GPR exceeded 7 ns. 2) Both methods (i.e. method based on radar image waveform and method based on Hilbert spectral instantaneous attributes) could accurately identify the time domain position of the plow layer (Ap). However, due to the attenuation of electromagnetic wave energy, the method based on waveform was difficult to recognize the layers below the plow layer. On the contrary, the method based on Hilbert spectral instantaneous phase could accurately identify the most soil layers within 60 cm, except for a few transition layers; 3) Except for a few transition layers, the absolute error and relative error of soil thickness obtained based on the Hilbert spectrum instantaneous phase method and measured based on real soil profile was basically within 5 cm, 15%, respectively, which could basically meet the needs of field exploration of saline-alkali land. This study can provide reference for rapid and nondestructive access to the information of salinized soil layers.