Self-potential method in hydrogeological exploration of volcanic areas

In volcanic areas, when two conditions are satisfied, there is a linear correlation between the range of negative self-potential (SP) anomalies and the thickness of the unsaturated zone measured in borings. The first condition is a high ratio between the resistivity of the unsaturated zone, and the resistivities of the substratum and the water-saturated zone. The second condition is the homogeneity of the unsaturated zone. From this relation, we develop the concept of a geophysical surface (SPS surface) calculated from SP and topographic data. Under these conditions, the SPS surface is both an equipotential SP surface and the interface between the unsaturated zone and the saturated medium below. Borehole logs for three field sites show that drainage courses and valleys the ridge lines of the SPS surface.