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The electrical geophysical methods are used to determine the electrical resistivity of the earth's subsurface. Thus, electrical methods are employed for those applications in which a knowledge of resistivity or the resistivity distribution will solve or shed light on the problem at hand. The resolution, depth, and areal extent of investigation are functions of the particular electrical method employed. Once resistivity data have been acquired, the resistivity distribution of the subsurface can be interpreted in terms of soil characteristics and/or rock type and geological structure. Resistivity data are usually integrated with other geophysical results and with surface and subsurface geological data to arrive at an interpretation.

The electrical [[geophysical methods]] are used to determine the electrical resistivity of the earth's subsurface. Thus, electrical methods are employed for those applications in which a knowledge of resistivity or the resistivity distribution will solve or shed light on the problem at hand. The resolution, depth, and areal extent of investigation are functions of the particular electrical method employed. Once resistivity data have been acquired, the resistivity distribution of the subsurface can be interpreted in terms of soil characteristics and/or rock type and geological structure. Resistivity data are usually integrated with other geophysical results and with surface and subsurface geological data to arrive at an interpretation.

Electrical methods can be broadly classified into two groups: those using a controlled (human-generated) energy source and those using naturally occurring electrical or electromagnetic energy as a source. The controlled source methods are most commonly used for shallow investigations, from characterizing surficial materials to investigating resistivities down to depths as great as 1 to [[length::2 km]], although greater depths of investigation are possible with some techniques and under some conditions. The natural source methods are applicable from depths of tens of meters to great depths well beyond those of interest to hydrocarbon development.

Electrical methods can be broadly classified into two groups: those using a controlled (human-generated) energy source and those using naturally occurring electrical or electromagnetic energy as a source. The controlled source methods are most commonly used for shallow investigations, from characterizing surficial materials to investigating resistivities down to depths as great as 1 to [[length::2 km]], although greater depths of investigation are possible with some techniques and under some conditions. The natural source methods are applicable from depths of tens of meters to great depths well beyond those of interest to hydrocarbon development.

The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock<ref name=pt07r46>Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.</ref> (see [[Determination of water resistivity]]). This is because the dry soil or rock matrix is a virtual insulator at DC and near DC frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.

The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock<ref name=pt07r46>Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.</ref> (see [[Determination of water resistivity]]). This is because the dry soil or rock matrix is a virtual insulator at DC and near DC frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.

Faulting or [[Fracture|fracturing]] of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture [[porosity]] changes the already high porosity by only a small percentage. However, in very tight rocks, such as igneous, metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see [[Porosity]]).

Faulting or [[Fracture|fracturing]] of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture [[porosity]] changes the already high porosity by only a small percentage. However, in very tight rocks, such as [[igneous]], metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see [[Porosity]]).

In summary, the factors affecting ''in situ'' average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.

In summary, the factors affecting ''in situ'' average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.

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., and B. R. Spies, 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.

The ''self-potential'' (''SP'') ''method'' examines the slowly varying surface potential field caused by electrochemical and electrokinetic actions in near-surface materials.<ref name=pt07r52>Sill, W. R., 1983, Self-potential modeling from primary flows: Geophysics, v. 48, n. 1, p. 76–86., 10., 1190/1., 1441409</ref> Potentials can form, for example, at interfaces between materials containing fluids with different ion contents, or they can be caused by moving groundwater or by differential oxidation of ore bodies. The method has been applied successfully in geothermal and mineral exploration and in the delineation of certain groundwater contaminants. Field procedures are straightforward, with the potential measured between carefully designed electrodes using what is essentially a highly sensitive DC voltmeter. The potential field is mapped along profiles or on a grid of measurement stations. Interpretation is generally qualitative, with SP anomalies interpreted in terms of the shape and depth of the causative body or fluid flow.

The ''self-potential'' (''SP'') ''method'' examines the slowly varying surface potential field caused by electrochemical and electrokinetic actions in near-surface materials.<ref name=pt07r52>Sill, W. R., 1983, Self-potential modeling from primary flows: Geophysics, v. 48, n. 1, p. 76–86., 10., 1190/1., 1441409</ref> Potentials can form, for example, at interfaces between materials containing fluids with different ion contents, or they can be caused by moving groundwater or by differential oxidation of ore bodies. The method has been applied successfully in geothermal and mineral exploration and in the delineation of certain groundwater contaminants. Field procedures are straightforward, with the potential measured between carefully designed electrodes using what is essentially a highly sensitive DC voltmeter. The potential field is mapped along profiles or on a grid of measurement stations. Interpretation is generally qualitative, with SP anomalies interpreted in terms of the shape and depth of the causative body or fluid flow.

''Magnetotellurics'' (''MT'') is an electrical method of geophysical exploration that makes use of naturally occurring electromagnetic energy propagating into the earth to determine the electrical resistivity of the subsurface.<ref name=pt07r42>Orange, A. S., 1989, Magnetotelluric exploration of hydrocarbons: Proceedings of the IEEE, v. 77, n. 2, p. 287–317., 10., 1109/5., 18627</ref><ref name=pt07r61>Vozoff, K., ed., 1986, Magnetotelluric methods: Tulsa, OK, Society of Exploration Geophysicists, Geophysics Reprint Series, 763 p.</ref> The low frequency electromagnetic field is measured, and the wave impedance is calculated and expressed in terms of the resistivity of the subsurface. The depth of investigation is a function of the frequency of the electromagnetic wave, taking advantage of the fundamental principle that the lower the frequency of a wave, the deeper the penetration into the crust. MT surveys generally involve applications that range in depth from a few hundred meters to [[length::10 km]] or more.

''[[Magnetotellurics]]'' (''MT'') is an electrical method of geophysical exploration that makes use of naturally occurring electromagnetic energy propagating into the earth to determine the electrical resistivity of the subsurface.<ref name=pt07r42>Orange, A. S., 1989, Magnetotelluric exploration of hydrocarbons: Proceedings of the IEEE, v. 77, n. 2, p. 287–317., 10., 1109/5., 18627</ref><ref name=pt07r61>Vozoff, K., ed., 1986, Magnetotelluric methods: Tulsa, OK, Society of Exploration Geophysicists, Geophysics Reprint Series, 763 p.</ref> The low frequency electromagnetic field is measured, and the wave impedance is calculated and expressed in terms of the resistivity of the subsurface. The depth of investigation is a function of the frequency of the electromagnetic wave, taking advantage of the fundamental principle that the lower the frequency of a wave, the deeper the penetration into the [[crust]]. MT surveys generally involve applications that range in depth from a few hundred meters to [[length::10 km]] or more.

The resistivity versus depth [[cross section]] developed from MT data can be interpreted in terms of rock type. Spatial variations in the resistivity-depth relationship observed at closely spaced locations on the surface can be interpreted in terms of subsurface geological structure. While MT cannot be used to detect oil directly, the identification of favorable rock types and the presence of geological structure capable of trapping hydrocarbons is critical to successful exploration. MT data are interpreted using forward and inverse modeling techniques. Resolution is considered low when compared with exploration or exploitation seismology, but may be adequate in certain instances to provide valuable information concerning reservoir geometry, rock characteristics, and a regional geological framework. For the larger deep reservoirs, MT may be considered as a possible candidate for EOR monitoring if model studies indicate that the resistivity changes over time associated with the operation are within the resolving power of the method.

The resistivity versus depth [[cross section]] developed from MT data can be interpreted in terms of rock type. Spatial variations in the resistivity-depth relationship observed at closely spaced locations on the surface can be interpreted in terms of subsurface geological structure. While MT cannot be used to detect oil directly, the identification of favorable rock types and the presence of geological structure capable of trapping hydrocarbons is critical to successful exploration. MT data are interpreted using forward and inverse modeling techniques. Resolution is considered low when compared with exploration or exploitation seismology, but may be adequate in certain instances to provide valuable information concerning reservoir geometry, rock characteristics, and a regional geological framework. For the larger deep reservoirs, MT may be considered as a possible candidate for EOR monitoring if model studies indicate that the resistivity changes over time associated with the operation are within the resolving power of the method.

The following is a brief summary of some of the many possible applications of electrical methods of interest to the development geologist:

The following is a brief summary of some of the many possible applications of electrical methods of interest to the development geologist:

* Evaluation of various characteristics of the shallow environment. Examples of such characteristics are the classification of unconsolidated materials based on their electrical properties, the identification of a lateral and/or vertical freshwater-saltwater boundary, the depth to bedrock, and the identification and mapping of conductive groundwater contaminants.

* Evaluation of various characteristics of the shallow environment. Examples of such characteristics are the classification of unconsolidated materials based on their electrical properties, the identification of a [[lateral]] and/or vertical freshwater-saltwater boundary, the depth to bedrock, and the identification and mapping of conductive groundwater contaminants.

* Monitoring of reservoir [[stimulation]] and enhanced recovery projects, where the stimulants and propants or flood materials can be expected to modify the resistivity of the formations.

* Monitoring of reservoir [[stimulation]] and enhanced recovery projects, where the stimulants and propants or flood materials can be expected to modify the resistivity of the formations.

* Investigation of permafrost and ice characteristics in the Arctic.

* Investigation of permafrost and ice characteristics in the Arctic.

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]