Acquiring soil stratification of soda saline-alkali soils using ground penetrating radar

A large area of soda saline-alkali soil has seriously restricted the local agricultural productivity in the western Songnen Plain of Northeast China. The soil layers can also dominate the movement of water and salt in soil. It is of great significance to rapidly obtain the stratification in this area, in order to evaluate and improve the soda saline-alkali soils. Taking the typical saline-alkali soils in this area as the research object, this study aims to acquire the soil stratification using ground penetrating radar. Firstly, the soda saline-alkali soils with different salt contents were selected to conduct the ground penetrating radar (GPR) detection experiment. Then, the time domain location of stratification was then determined by radar image waveform and Hilbert spectrum instantaneous attributes. In waveform diagram of radar image, multi-point single channel waveform diagrams were combined with the radar time profiles. The soil layering was then determined as the time domain position in the overall soil layer division of the studied plot. The "three instantaneous" attributes of radar signals (instantaneous amplitude, frequency, and phase) were compared to determine the soil layering, according to Hilbert spectral instantaneous attributes. As such, the multi-point instantaneous phase maps and profiles were combined to determine the time domain position of the overall soil layer division in the studied plot. Then, the dielectric constant of each soil layer was calculated using the extended Dobson dielectric constant model. The propagation velocity of electromagnetic wave was estimated in each soil layer, according to the dielectric constant. The thickness of each soil layer was calculated to combine the time domain position of the soil layer. Finally, the field excavation profile was compared after data acquirement. The stratification of the field excavation profile was also evaluated by the soil classification experts, according to the visual and tactile characteristics of the soil. The results indicated that: 1) The content of soil salinity shared a significant impact on the ground penetrating radar signal. There was the very small amplitude of electromagnetic wave, when the two-way travel of GPR exceeded 7 ns. 2) Both radar image waveform and Hilbert spectral instantaneous attributes were accurately identified the time domain position of the plow layer (Ap). However, the waveform failed to recognize the layers below the plow layer, due to the attenuation of electromagnetic wave energy. On the contrary, the Hilbert spectral instantaneous phase was accurately identified the most soil layers within 60 cm, except for a few transition layers; 3) The absolute and relative errors of soil thickness were basically within 5 cm, and 15%, respectively, using Hilbert spectrum instantaneous phase and real soil profile. The performance was fully met the needs in the field exploration of saline-alkali land. This finding can provide a strong reference for the rapid and nondestructive access to the salinized soil layers.