Electrical resistivity techniques are routinely used to solve hydrogeological (e.g., finding water bearing units (aquifers)), environmental (e.g., delineating groundwater plumes), and engineering (e.g., locating subsurface voids) problems.  Electrical methods measure the flow of electric current in the ground thereby investigating electrical properties of the subsurface.  The resulting subsurface geoelectrical image (represented as changes in electrical resistivity with depth) can be interpreted in terms of lithology (different rock types) and their properties such as porosity, saturation, and the chemistry of the fluids filling the pores (Figure 1).  Therefore, in order to interpret electrical resistivity data, it is important to understand the electrical properties of rocks and soils.