The study of electricity and geophysics
The resistivity method is employed within the study of horizontal and vertical discontinuities within the electrical properties of the subsurface and also within the detection of three-dimensional bodies of anomalous electrical conductivity. This method is especially utilized in engineering and hydrogeological investigations.
In the resistivity method, artificially generated electric currents are introduced into the subsurface and therefore the resulting potential differences are measured at the surface. The electric potential is defined because of the difference in electrical potential between two points. Deviations from the pattern of potential differences are expected from the homogenous ground to supply information on the form and electrical properties of the subsurface inhomogeneities.
The resistivity of a cloth is defined because of the resistance in ohms between the other faces of a unit cube of the fabric. The resistance offered by a cloth to current flow is expressed in terms of resistivity. The SI unit of resistivity is that the ohm-meter and therefore the reciprocal of resistivity is termed conductivity. Siemens/ meter and mho are a number 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 instead of electronic processes. Hence porosity is that 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 therefore the porosity.
Even the crystalline rocks with negligible intergranular porosity are conductive along cracks and fissures. But among the common rock types, there's a substantial overlap between different rock types and thus identification of rock type isn't possible solely on the idea of resistivity data alone!. The apparent or the effective resistivity ( the resistivity of the rock and its pore water) also can be expressed in terms of resistivity and therefore the volume of pore water present.
Resistivity method does measure the magnitude of conduction, whereas the induced polarization measures the magnitude of polarization.
Groundwater exploration, mineral exploration, detection of cavities, waste site exploration, oil exploration, etc are a number of the main applications of resistivity surveying.
The induced polarization 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 polarization 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 polarization response does reflect the degree to which the subsurface is in a position to store the electrical charge analogous to a capacitor. Polarization does result from a redistribution of ions along interfaces ( metal-fluid or nonmetal- fluid) following the application of an electrical current. Residual current flow occurs as ions relax to equilibrium following the removal of the electrical field. The induced polarization method does indicate the presence of clay content.