Harmonic field systems that use large, fixed layouts (rectangular or circular loops or long grounded cables as sources) are among the oldest electromagnetic prospecting methods. These methods were developed to a large extent in Sweden by Karl Sundberg and his group during the two decades following the end of World War I in 1918 and were used mainly for ore prospecting (Sundberg, 1931) but also for structural investigations in oil fields (Sundberg and Hedström, 1934). They were still being used at the end of the 1940s but were gradually replaced by the small-loop (or dipole source) systems because the small loop systems were more flexible and, at the same time, adequate for the shallow exploration problems being encountered. The superiority of reflexion seismics over electromagnetics for oil prospecting was already manifest and the large-layout systems were no longer used in oil exploration.
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Electromagnetic Methods in Applied Geophysics, Volume I, Theory presented the mathematical and physical foundations common to all EM methods. The purpose of Volume I was to help facilitate the understanding of the theory involved and to provide a limited amount of interpretational aids. Volume II, Applications is devoted to a method-by-method treatment of the principal EM techniques in common use.