A tutorial overview is presented of controlled-source electromagnetic (CSEM) geophysical methods as they are applied in petroleum exploration. CSEM methods utilize man-made sources to investigate the variation of electrical conductivity in the Earth, typically in the depth range of hundreds of meters to several kilometers. Over the frequency range used in CSEM (typically 0.1 Hz to 10 KHz), displacement currents can usually be ignored so the EM fields are diffusive in nature. The basic theory for CSEM methods and some simple approximate solutions which provide insight into the behavior of quasistatic EM fields are given. An overview of data inversion methods is also given, as well as examples of typical applications. It is argued that CSEM methods offer a number of advantages compared to natural-source electromagnetic (NSEM) methods such as magnetotellurics (MT). These include relative insensitivity to regional effects and small near-surface inhomogeneities which adversely affect MT, and the ability to utilize different source types and orientations to improve resolution. NSEM methods can, however, probe deeper conductivity structures than CSEM methods.
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