Self Potential Technique:

Self-potential methods measure naturally occurring electrical potentials in the earth. One source of these self-potentials is the "streaming potential" (or electrokinetic potential) which arises from the flow of fluid (e.g. groundwater) through a porous medium.

For this reason Self-Potential is used in groundwater investigations and in geotechnical engineering applications for seepage studies.

Self-potential field surveys are conducted by measuring electrical potential differences between pairs of electrodes that contact the surface of the earth (or water, in water-covered

areas) at a number of survey stations in the area of interest.These stations may be along profiles or spaced so as to obtain areal coverage. One station is selected as a base station and all potentials are referenced to that point. The base station should be located at a point removed from expected anomalous activity. Potential (voltage) measurements are made by contacting the earth with non-polarizing electrodes. These electrodes, often called "porous pots," are designed so as not to create any spurious chemical potentials upon contact with the ground. Measurements are made by connecting a high impedance voltmeter between two electrodes, usually the base station and a roving electrode.

Self-potential interpretation can range from a simple qualitative inspection of the plotted Self-potential profiles to complex computer modeling involving subtle interactions between temperature, electrochemical reactions and earth geometry. For most groundwater investigations a simple qualitative analysis will provide the desired information about groundwater flow paths. Data are plotted as profiles (observed potential versus distance along the profile) or, if the data provides sufficient areal coverage, as contour plots. All else being equal, the anomaly location corresponds to the point of maximum groundwater flow. For "point sources," some estimate of the depth of the source may be obtained from the width of the anomaly.

There are several other sources of Self-potential variations which may act as noise or interference when mapping streaming potentials for a groundwater investigations. These include: buried metal, temperature variations, soil property variations, electrochemical variations, topographic effects, and tellurics, (naturally occurring time-varying electric potentials caused by distant thunder storms and ionospheric disturbances. NGA's geophysical team has the expertise to deal with these interference and create sound geophysical interpretations.