The Electrical Methods of Geophysical Exploration
electric resistance
The resistivity method is useful for the study of horizontal and vertical variations in the electrical properties of the subsurface. This method does utilise either the direct current or a low frequency alternate current. This method is being primarily utilised in ground water exploration, mineral exploration, oil exploration, and waste site exploration.
In the resistivity method, current is driven through a pair of electrodes and the potential established in the subsurface by this current can be measured by the second pair of electrodes connected to a voltmeter. Based on the responses, effective or apparent resistivity of can be evaluated. In-homogenous layers within the subsurface can be interpreted by the fact that they deflect the current and distort the normal values of potentials. The electrical potentials can also be developed within the subsurface by electrochemical reactions, from any external source, telluric currents, etc. The resistivity methods can be useful to find the depth as well as the shape of several types of ore bodies.
The resistance offered by any object to the current flow can be expressed in terms of resistivity. The electrode configurations can be of several types such as Wenner’s, Schlumberger’s, dipole-dipole, etc.
The igneous rocks are the most resistant. The sedimentary rocks are the most conductive. The young volcanic rocks do possess lesser resistivity when compared to the old volcanic rocks. The resistivity of the rocks depends upon several factors such as porosity, saturation, content of clay, etc.
The metallic sulfide deposits are often be termed as the zones of anomalously low resistance.
Potentials, currents, and electromagnetic fields can be measured by using several electrical methods of geophysical prospecting. These methods do yield the best results in the upper few hundred meters of the surface and in the zones containing fresh un-weathered rock bodies.
The induced polarisation method is the only geophysical method which is capable of detecting concealed and disseminated sulphide deposits. This method is based on the fact that the earth materials discharge after the applied current is switched off. When the applied primary current is switched off, the decay of the secondary voltage can be detected and thus provides the measurement of the size and the position of the chargeable rock body. This method does utilise the electrochemical effects produced by currents, which pass through the disseminated sulphide metal deposits. The flow of electric current in any rock body may make parts of it to be electrically polarised. This effect is mostly seen in metallic sulphides and graphite deposits.
This method does measure the magnitude of polarisation and thus can be an excellent indicator of the clay content in the formations.