Axisymetrical infiltration in soil imaged by non-invasive electrical resistivimetry

To progress in the monitoring and understanding of soil water processes, geophysical methods can be helpful because they are non invasive and as such do not disturb either the structure or the water dynamics of the soil. In this study we attempt to image water infiltration dynamics under a tension infiltrometer in a vertical 2-D plane using Electrical Resistivity Tomography (ERT). Two-dimensional images of soil electrical resistivity at different times were obtained using a Wenner array with 32 electrodes and a 10-cm spacing during the infiltration of a Cl- or Br- solution at 0.36M and 0.34M, respectively. This procedure allowed monitoring the development of the infiltration bulb through time. Both unipotential and multipotential infiltrations were performed and used to derive soil hydraulic conductivity and sorptivity values. Anion concentrations in soil samples taken after infiltration were compared with resistivity values obtained with ERT measurements. The axisymetrical pattern of the multipotential infiltration exhibited large distortions, well detected and imaged by the ERT and consistent with the anion concentration data. In general, images derived from 2D inversions of ERT measurements underestimated the depth of the infiltration bulb. Axisymetrical inversion software would be helpful for the interpretation of ERT data taken below tension disc infiltrometers. ERT on a small scale seems a helpful and promising tool for deriving in situ soil hydraulic patterns and for monitoring infiltration processes without the need for placing sensors into the soil.