Electrical and electromagnetic methods are both used to map variations in the electrical properties of the subsurface. The main physical property involved is electrical conductivity, which is a measure of how easily electrical current can pass through a material. In the case of electrical methods, current is injected directly into the ground, whilst in electromagnetic methods, currents are induced in the ground by the passage of electromagnetic waves. The indirect nature of electromagnetic methods makes it possible to take measurements from the ground and also from specially adapted aircraft, whilst electrical methods are restricted to ground based measurements.
Subsurface materials exhibit a very large range of electrical conductivity values. Fresh rock is generally a poor conductor of electricity, but a select group of metallic minerals containing iron, copper or nickel are very good conductors. Layers of graphite are also very good conductors.
The examples of good conductors mentioned above are quite rare. The electrical properties of most rocks are governed to a large degree by the amount of water filling the gaps between the mineral grains and the amount of salt dissolved in this water. Pure water has a very low electrical conductivity. On the other hand, seawater, which contains high levels of dissolved salts such as NaCl, is a relatively good conductor of electrical current. Groundwater can vary in salt content from fresh through brackish (slightly salty) to saline (similar in salt content to seawater) through to hyper-saline (more salty than seawater).
Some electromagnetic methods take advantage of currents induced in the ground by natural electromagnetic waves. Lightning on a global scale is one of the strongest natural sources of electromagnetic energy used in this fashion. However, in the majority of cases, a man-made transmitter of electromagnetic waves is used. A simple example of an electromagnetic instrument is a metal detector, which is able to detect highly conductive metal objects buried at shallow depths. The same principles are used on a larger scale involving more powerful, well-calibrated instruments to map more subtle contrasts in electrical conductivity from the surface to depths of tens, hundreds or even thousands of metres.
Electrical methods use electrical current from batteries or generators injected into the ground through metal stakes to investigate the subsurface. The conductivity of the ground is deduced from measurements of the electrical potential difference (voltage) recorded between pairs of electrodes placed at different points on the surface.
Electrical and electromagnetic methods are used extensively in the search for deposits of copper, lead and zinc. The other main area of application is in groundwater studies, where the influence of water content and water salinity on the conductivity of the ground can be utilised. Increasingly, electrical and electromagnetic methods are being used in a more general manner to map different geological units based on differences in electrical conductivity.
The image above, covering an area 10km by 8km and depths from the surface to more than 400m, shows the 3D distribution of elevated conductivity determined from an airborne electromagnetic survey. The coloured mesh shows the topographic variation of the ground surface. The flat sheets are near-surface geological units with slightly elevated concentrations of saline groundwater. The other objects with elevated electrical conductivity are associated with base metal sulphide mineralisation.
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