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Natural source methods take advantage of naturally occurring electrical potentials and electromagnetic fields as energy sources. Advantages of natural source methods are that there is no dependence on an artificial energy source and that the natural electromagnetic field is well understood. The principal disadvantages are the unpredictability and lack of control over energy levels and the attendant effects of cultural noise on the signal to noise ratio.

Natural source methods take advantage of naturally occurring electrical potentials and electromagnetic fields as energy sources. Advantages of natural source methods are that there is no dependence on an artificial energy source and that the natural electromagnetic field is well understood. The principal disadvantages are the unpredictability and lack of control over energy levels and the attendant effects of cultural noise on the signal to noise ratio.

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.