Introduction

The self-potential (SP) method is based upon measuring the spontaneous or natural potentials developed in the earth by electrochemical actions between minerals and subsurface fluids or by electrokinetic processes involving the flow of ionic fluids. It differs from other geoelectric methods (Resistivity and Induced Polarization (IP) methods discussed in Chapter 6) which require electric currents to be artificially introduced in the ground. The SP method is one of the simplest and oldest among all geophysical techniques used to locate sulfide ore-bodies.

In recent years the SP method has found increasing use in geothermal, environmental, and engineering applications to help locate and delineate sources associated with the movement of thermal fluids and groundwater. Specific applications include mapping of seepage flow associated with dams, dikes, and reservoir floors, and delineation of flow patterns in the vicinity of water wells, faults, landslides, and sinkholes. As the method offers relatively rapid field data acquisition, it is often cost effective for initial investigation of an area prior to more intensive studies using other geophysical methods.

The following discussion is an overview of the principles and practice of the method and includes some examples of environmental and engineering applications.

Origin of self-potentials

Spontaneous potentials in the subsurface are caused by a number of processes which are not well understood at this time. However, the principal types of source mechanisms appear to be as follows.

Electrofiltration potential

The flow of fluid through a capillary or porous medium may generate an electric potential (called the electrofiltration or streaming potential) along the flow path.