Three-dimensional crack monitoring by electrical resistivity measurement.

Soil cracks formed by natural processes play a key role in water and gas transfer. Patterns of soil cracks are, however, difficult to characterize. Our aim here is to assess the effectiveness of three-dimensional electrical resistivity surveys in detecting soil crack networks. A three-dimensional electrical survey was carried out by a square array quadripole with Cu-CuSO4 electrodes (electrode spacing of 3 cm). The measurements were made with two orientations (0° and 90°) on a block (26 cm × 30 cm × 40 cm) of soil while it dried for 18 days under controlled conditions. Two indexes, calculated from the apparent resistivity values, were evaluated to detect the degree of soil heterogeneity: (i) an anisotropy index based on the ratio between the apparent resistivity at 0° and that at 90°; and (ii) the angle-array orientation corresponding to the preferential anisotropic orientation (maximum resistivity). The anisotropy index provided information on the presence of cracks and the orientation for crack width > 1 mm in the first pseudo-depth (i.e. depth of investigation), while the angle-array orientation provided information on crack extension for the whole pseudo-depth. Information about the presence, position, orientation and extension of cracks can be obtained from an analysis of apparent resistivity obtained by a three-dimensional electrical survey. Such direct analysis will help the resistivity inversion to detect the crack network.