Electromagnetic and DC-current geophysics for soil compaction assessment

Abstract. Monitoring soil structure is of paramount importance due to its key role in the critical zone as the foundation of terrestrial life. Variations in the arrangement of soil components significantly influence its hydro-mechanical properties, and therefore its impact on the surrounding ecosystem. In this context, soil compaction resulting from inappropriate agricultural practices not only affects soil ecological functions, but also decreases the water-use efficiency of plants by reducing porosity and increasing water loss through superficial runoff and enhanced evaporation. In this study, we compared the ability of electric and electromagnetic geophysical methods, i.e. Electrical Resistivity Tomography and Frequency-domain Electromagnetic Method, to assess the effects of compaction on agricultural soil. The objective was to highlight the electro-magnetic response caused by both heavy plastic soil deformations generated by a super-heavy vehicle and the more common tractor tramlines. DC-current prospecting has finer spatial resolution and allows a tomographic approach, requiring higher logistic demands and the need for ground galvanic contact. On the other hand, contactless electromagnetic induction methods can be quickly used to define the distribution of electrical conductivity in the shallow subsoil in an easier way. Results, validated with traditional soil characterization techniques (i.e. penetration resistance, bulk density and volumetric water content on collected samples), show the pros & cons of both techniques and how differences in their spatial resolution heavily influence the ability to characterize compacted areas with good confidence. This work aims at contributing to the methodological optimization of agro-geophysical acquisitions and data processing, in order to obtain accurate soil models through a non-invasive approach.