electromagnetic method

1. n. [Geophysics]
A group of techniques in which natural or artificially generated electric or magnetic fields are measured at the Earth's surface or in boreholes in order to map variations in the Earth's electrical properties (resistivity, permeability or permittivity). Most applications of surface electromagnetic methods today are for mineral and groundwater exploration or for shallow environmental mapping. Electromagnetic or electrical logging is, however, the main technique used in oil exploration to measure the amount of hydrocarbons in the pores of underground reservoirs. Inductive electromagnetic (EM) methods include a variety of low frequency (a few Hz to several kHz) techniques deploying large or small wire coils at or near the surface. In older usage, "electromagnetic method" tended to refer only to inductive methods. This term is now commonly used for any method employing electromagnetic fields, including methods that use direct current (electrical or resistivity methods) and induced polarization (IP), methods that use microwave frequencies (ground-penetrating radar), and methods that use natural electromagnetic fields (magnetotelluric methods).