Using ground-penetrating radar to investigate the thickness of mollic epipedons developed from loessial parent material

•Permittivities of Mollisols and the underlying loess were predicted by two equations.•GPR can be used to detect the thickness of the Mollisol layer.•GPR was used to detect the continuous soil thickness distribution on the slope. The information of mollic epipedon (ME) thickness was essential for evaluation and protection of Mollisol resource. However, the traditional methods including the soil profile method, soil probes, and the drilling method, are low efficient and accurate for large area survey of soil thickness. In this study, the ground penetrating radar (GPR) was employed to detect the thickness of ME with underlying loessial parent material (LPM) along three slopes in straight, convex, and concave shapes in northeast China. The accuracy of GPR for measuring the ME thickness was verified by using excavation profile and pre-buried iron pipes. The effects of soil moisture and bulk density on soil permittivity were also investigated based on indoor experiments. The results indicated that the soil permittivity increased and decreased with the increase of bulk density and soil moisture, respectively. The relationships among soil moisture, bulk density and permittivity of both Mollisol and LPM could be described by two fitted equations, and the errors of the prediction equations was in the range of 0.32 %–5.10 %. The errors of GPR to measure the ME thickness was in the range of 3.72 %–10.64 %. The spatial distribution of the ME thickness along the three slopes varied, and the concave slope had a thicker ME than the convex and straight slope. Meanwhile, similar trends were shown that the sedimentation at the slope foot led to the thickest ME, while soil erosion at the slope shoulder and back resulted in the thinnest ME. This study could provide an efficient and accurate method to investigate soil thickness for further evaluation and protection of soil resources.