Resistivity model. Ten electrodes (arrows) are placed at equispaced intervals of 1 m along the borehole, from the surface to a depth of 9 m. Background resistivity is 100 Ωm and anomalies A and B are 20 and 500 Ωm, respectively. The discretization is an irregular mesh, as suggested by the grid overlay, with cell size increasing as the distance from the survey. The mesh extends beyond what is shown here to accommodate boundary blocks needed for modelling accuracy (see Fig. 3). A total of 416 cells (model parameters) are used.