Soil cracks detection by 3D electrical resistivity

Soil cracks, whose formation are associated to natural climate phenomena such as swelling and shrinking, play an important role in water and gas transfers. Up to now, their 3D structure was characterised either by serial sections (Cousin, 1996) which is a destructive technique or X-ray tomography (Macedo et al., 1998) which is applicable on limited size sample. Three-dimensional electrical resistivity prospecting enables now to monitor crack development and to characterise their geometry without any destruction of the medium under study. Three-dimensional electrical resistivity surveys are commonly gathered by a network of in-line survey arrays, such as Wenner, Schlummberg, or dipole-dipole (Xu and Noel, 1993; Zhou et al., 2002). As emphasized by Meheni et al. (1996) the resulting apparent resistivity maps are often different depending on the array orientation related to an electrical discontinuity. Chambers et al. (2002) underline that in heterogeneous medium 3D electrical resistivity model resolution was sensitive to electrode configuration orientation. Indeed asymmetric bodies or anisotropic material exhibit different behaviours depending on whether the current passes through them in one direction or in another (Scollar et al., 1990). It would be all the more true for medium having very contrasted resistivities like cracking soil. In that case the electrical current does not encounter the same resistance when it passes perpendicular or parallel to the resistant bodies. Measurements of apparent resistivity depend then on the location and orientation of the current source relative to the body under study (Bibby, 1986). Studies conducted by Habberjam and Watkins (1967) emphasized that the square array provide a measurement of resistivity less orientationally dependent than that given by a in-line array investigation. Intending to lead a more 3D accurate inversion, we have chosen to focus our attention on a 3D electrical resistivity data acquisition. We present here a three-dimensional electrical survey carried out by a square array quadripole for characterising the soil cracks network developing during a desiccation period.