Introduction

Electrical measurements of various types are made at the surface of the earth to investigate subsurface conditions in an area. In the most commonly used method, an electric current is driven through the ground and the resulting potential differences are measured at the surface. Anomalous conditions or inhomogeneities within the ground, such as electrically better or poorer conducting layers, are inferred from the fact that they deflect the current and distort the normal potentials. This is the underlying principle of measuring subsurface variation in electrical resistivity (reciprocal of electrical conductivity) within the earth. The technique of resistivity surveying was developed by Conrad Schlumberger, who conducted the first experiments (1912) in the fields of Normandy.

Electric potentials in the ground can, however, be caused by phenomena other than the simple ohmic conduction of the applied current. For example, electric charges sometimes accumulate on the interfaces between certain minerals as a result of the flow of electric current from an external source. The method of ‘induced polarization’ (IP) is based on this phenomenon in the search for disseminated ores, clay minerals, and groundwater. Moreover, electric potentials can be developed in the ground by electrochemical actions between minerals and the solutions with which they are in contact, or by electrokinetic processes involving the flow of fluids (discussed in Chapter 5). In addition, slowly varying potentials are caused by natural (‘telluric’) currents flowing inside the earth.