Structural heterogeneity of the soil tilled layer as characterized by 2D electrical resistivity surveying

Abstract

Our research was aimed at analysing the possibilities of using a geophysical method, the electrical resistivity method, to describe the structure of a cultivated loamy soil. Soil electrical resistivity was measured in laboratory conditions on two soil blocks (0.30 m × 0.30 m × 0.20 m) with 2D Wenner configuration using an inter-electrode spacing of 0.015 m. The two soil blocs exhibited different structure: one with a compacted structure (bulk density equal to 1.59 Mg m −3 with a standard deviation of 0.05 Mg m −3) and the second with a porous structure (bulk density equal to 1.39 Mg m −3 with a standard deviation of 0.04 Mg m −3). The electrical resistivity results showed a significant 10 Ω m difference between the compacted block (30 Ω m) and the porous block (40 Ω m) due to the difference in their bulk density. This structural distinction by electrical resistivity needs temperature correction using the Campbell equation. The soil electrical resistivity was also measured in the field with a 2D Wenner configuration using an inter-electrode spacing of 0.10 m along a 3.20 m transect. After the electrical measurements, a pit was dug and the contours of porous and compacted zones in the ploughed layer were identified, the boundaries between the ploughed layer, the plough pan and the pedological horizons were defined. Comparisons between inverted electrical resistivity maps and visual morphological descriptions showed the ability of electrical resistivity to detect wheels tracks. However, electrical surveying in a heterogeneous field after ploughing did not correspond to the visual morphological description, the latter being 2D whereas the electrical resistivity map is a 2D projection of a 3D sensing. As a non-destructive method, the electrical resistivity method could improve the quantitative description of the tilled layer and permit a temporal survey.