Changes

Controlled source methods use generated currents or electromagnetic fields as energy sources. An advantage is the control over energy levels and the attendant positive effects on signal to noise ratio in areas of high cultural noise. A disadvantage of controlled source methods is that the complex nature of the source field geometry (the geometry of the electromagnetic field or currents induced with the earth by the transmitter) may present quantitative interpretation problems in areas of complex geology.

Controlled source methods use generated currents or electromagnetic fields as energy sources. An advantage is the control over energy levels and the attendant positive effects on signal to noise ratio in areas of high cultural noise. A disadvantage of controlled source methods is that the complex nature of the source field geometry (the geometry of the electromagnetic field or currents induced with the earth by the transmitter) may present quantitative interpretation problems in areas of complex geology.

In the ''DC method'', a current (usually a very low frequency square wave and not actually direct current) is injected into the earth through a pair of current electrodes, and the resulting potential field is mapped. Various geometries of current and potential electrodes have been employed, with the choice primarily based upon the depth and geometry of the survey target. The measured surface potential field is interpreted in terms of the subsurface resistivity distribution through modeling and inversion techniques<ref name=pt07r66>Zody, A. A. R., 1989, A new method for the automatic interpretation of Schlumberger and Wenner sounding curves: Geophysics, v. 54, n. 2, p. 245–253., 10., 1190/1., 1442648</ref>. ''Induced polarization'' (''IP'') and ''complex resistivity'' (''CR'') techniques are special cases of the DC method in which the induced potential field is measured and interpreted in terms of mineralogy and/or soil characteristics. IP and CR have been applied with some success to hydrocarbon exploration through the measurement of geochemical alteration halos that have been found to be related to reservoirs under some conditions.

In the ''DC method'', a current (usually a very low frequency square wave and not actually direct current) is injected into the earth through a pair of current electrodes, and the resulting potential field is mapped. Various geometries of current and potential electrodes have been employed, with the choice primarily based upon the depth and geometry of the survey target. The measured surface potential field is interpreted in terms of the subsurface resistivity distribution through modeling and inversion techniques.<ref name=pt07r66>Zody, A. A. R., 1989, A new method for the automatic interpretation of Schlumberger and Wenner sounding curves: Geophysics, v. 54, n. 2, p. 245–253., 10., 1190/1., 1442648</ref> ''Induced polarization'' (''IP'') and ''complex resistivity'' (''CR'') techniques are special cases of the DC method in which the induced potential field is measured and interpreted in terms of mineralogy and/or soil characteristics. IP and CR have been applied with some success to hydrocarbon exploration through the measurement of geochemical alteration halos that have been found to be related to reservoirs under some conditions.

In the ''electromagnetic'' (''EM'') ''method'', an electromagnetic field is produced on or above the surface of the ground<ref name=pt07r39>Nekut, A. G., Spies, B. R., 1989, Petroleum exploration using controlled source electromagnetic methods: Proceedings of the IEEE, v. 77, n. 2, p. 338–362., 10., 1109/5., 18630</ref>. This primary EM field induces currents in subsurface conductors. The induced currents in turn reradiate secondary EM fields. These secondary fields can be detected on or above the surface as either a distortion in the primary field (frequency domain methods) or as they decay following the turning off of the primary field (time domain methods). Both loops and grounded wires are used to generate the source field. Resistivities are calculated from the observed electromagnetic field data using modeling and inversion techniques.

In the ''electromagnetic'' (''EM'') ''method'', an electromagnetic field is produced on or above the surface of the ground.<ref name=pt07r39>Nekut, A. G., Spies, B. R., 1989, Petroleum exploration using controlled source electromagnetic methods: Proceedings of the IEEE, v. 77, n. 2, p. 338–362., 10., 1109/5., 18630</ref> This primary EM field induces currents in subsurface conductors. The induced currents in turn reradiate secondary EM fields. These secondary fields can be detected on or above the surface as either a distortion in the primary field (frequency domain methods) or as they decay following the turning off of the primary field (time domain methods). Both loops and grounded wires are used to generate the source field. Resistivities are calculated from the observed electromagnetic field data using modeling and inversion techniques.

EM techniques have been adapted to a variety of surface and airborne configuration, with the airborne instruments generally limited in penetration to 100 to [[length::200 m]]. Airborne electromagnetic surveys have proven very effective for mapping the shallow resistivity distribution, leading to cost-effective surveys over large areas. Surface loop or grounded wire systems are applicable to depths well in excess of [[length::1 km]], although high power transmitters are required as depth increases. The resolution attainable is normally considered as a percentage of penetration depth, such that absolute resolution decreases with depth.

EM techniques have been adapted to a variety of surface and airborne configuration, with the airborne instruments generally limited in penetration to 100 to [[length::200 m]]. Airborne electromagnetic surveys have proven very effective for mapping the shallow resistivity distribution, leading to cost-effective surveys over large areas. Surface loop or grounded wire systems are applicable to depths well in excess of [[length::1 km]], although high power transmitters are required as depth increases. The resolution attainable is normally considered as a percentage of penetration depth, such that absolute resolution decreases with depth.