3. Part B—Conduction of Electricity in Rocks

1. Basic Concepts of Electrical Conduction Electrical conductivity is the movement of electrical charge from one location to another. Because the charge may be carried by ions or electrons, whose mobilities vary from material to material, there is a full spectrum of conductivities ranging from highly conducting metals to nearly perfect insulators, as illustrated in Figure 1. Electrical conductivity can be derived from the relation $$\sigma =ne\mu$$ where n is the number of charge carriers in a material, e is the charge carried by each, and μ is the mobility of the carriers. The mobility is defined as the drift velocity per unit electric field. Since the charge carriers may be ions or electrons (or “holes” as we shall see later), we classify conduction in solids as ionic or electronic within the range 1 to 10⁸ mho/m. Below this range of conductivity, materials may be semiconductors or insulators. For porous media, such as rocks at the earth's surface, conductors extend into the range normally covered by solid semiconductors.