Electricity and Geophysics

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electricity

The resistivity method is used in the study of horizontal and vertical discontinuities in the electrical properties of the subsurface and also in the detection of three-dimensional bodies of anomalous electrical conductivity. This method is mainly used in engineering and hydrogeological investigations.

In the resistivity method, artificially generated electric currents are introduced into the subsurface and the resulting potential differences are measured at the surface. The potential difference is defined as the difference in electrical potential between two points. Deviations from the pattern of potential differences are expected from homogenous ground provide information on the form and electrical properties of the subsurface inhomogeneities.

The resistivity of a material is defined as the resistance in ohms between the opposite faces of a unit cube of the material. The resistance offered by a material to current flow is expressed in terms of resistivity. The SI unit of resistivity is the ohm-meter and the reciprocal of resistivity is termed conductivity. Seimens/ meter and mho are some of the units for its measurement.

Native minerals and Graphite do conduct electricity through the passage of electrons. Most of the rock-forming minerals are however insulators and hence, the electricity is carried through them mainly by the passage of ions in pore waters. Thus most of the rocks do conduct electricity through electrolytic rather than electronic processes. Hence porosity is the major controlling factor of the resistivity of rocks and thus the resistivity generally increases as porosity decreases. The law of Archie does show the inverse relationship between resistivity and the porosity.

Even the crystalline rocks with negligible intergranular porosity are conductive along cracks and fissures. But among the common rock types, there is a considerable overlap between different rock types and thus identification of rock type is not possible solely on the basis of resistivity data alone!. The apparent or the effective resistivity ( the resistivity of the rock and its pore water) can also be expressed in terms of resistivity and the volume of pore water present.

Resistivity method does measure the magnitude of conduction, whereas the induced polarisation measures the magnitude of polarisation.

Groundwater exploration, mineral exploration, detection of cavities, waste site exploration, oil exploration, etc are some of the major applications of resistivity surveying.

The induced polarisation method does make use of the capacitative action of the subsurface to locate zones where the conductive materials are disseminated within their host rocks. The induced polarisation phenomenon was first discovered in 1912 by Schlumberger. The electrically conductive materials exhibit: 1. delayed voltage response, and 2. over-voltage effect. In simple terms, the induced polarisation response does reflect the degree to which the subsurface is able to store the electrical charge analogous to a capacitor. Polarisation does result from a redistribution of ions along interfaces ( metal-fluid or nonmetal- fluid) following application of an electric current. Residual current flow occurs as ions relax to equilibrium following the removal of the electric field. Induced polarisation method does indicate the presence of clay content.

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Source by Tirumala Prasad