Characterisation of the structural heterogeneity of the soil tilled layer by using in situ 2D and 3D electrical resistivity measurements

Compaction is responsible for soil physical degradation by affecting soil structure, i.e. the arrangement of soil particles in space. Under wheel tracks, the soil density increases and the porosity decreases, which affects soil physical properties. Conventional methods of measuring soil compaction are time consuming, destructive and give misinterpretations by using 2D informations. We then need non-destructive tools that enable the description of the soil structure in the field at a centimetric or decimetric resolution. The aim of this study is to discuss the feasibility of using a geophysical method, the electrical resistivity one, to describe the complex 3D structure of the cultivated layer in an agricultural field. The studied zone – a 2.4 m × 1.4 m field plot – consists in a compacted band, first created by a heavy tractor, then fragmented by ploughing. 3D information about the structure was given by the analysis of ten successive morphological profiles spaced 10 cm apart. They have shown that the structure was composed by dense aggregates, loose material and large voids. The studied zone was characterised by electrical prospectings thanks to both 2D investigation (Wenner arrays of 24 electrodes) and 3D investigation (square arrays of 48 electrodes), with electrodes spaced 10 cm apart. The analysis of apparent resistivity and interpreted resistivity data has shown that we can localise zones with large voids, loose material or compacted aggregates. These results have been confirmed by direct observations on soil profiles. The 3D electrical resistivity prospecting method thus enables the description of the 3D heterogeneity of the tilled layer at a decimetric resolution and constitutes then an alternative tool to the X-ray tomography for studies in the field.