very-low-frequency method

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An electromagnetic prospecting method in which permanent, high-power, military transmitters are used as sources of unmodulated carrier waves which induce secondary fields within subsurface conductors that may be many hundreds of kilometres from the source. Various orthogonal components of the secondary field are measured, as a function of the primary field, to obtain information about the nature and position of the subsurface conductors.

Subjects: Earth Sciences and Geography.

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