On the physical principles underlying electromagnetic induction

Abstract

This paper provides a theoretical overview of some of the fundamental concepts underlying electromagnetic (EM) induction exploration methods using marine controlled-source EMs as an exemplar. In particular, it will be shown, from different vantage points, that EM induction operates in the magnetoquasistatic regime in which inductive effects dominate, capacitive effects are ignored, and the displacement current is negligible; hence, charge polarization and dielectric phenomena play no role. We determine some of the major physical consequences of this approximation, and we make a distinction between wave physics and diffusive behavior, which is of particular interest in the special case of time-periodic excitation. We distinguish the fundamentally different roles of mobile charge carriers and bound charges in EM induction. It is emphasized that EM induction cannot be fully understood by comparing and contrasting Maxwell’s equations with governing equations from other disciplines that possess a similar mathematical structure. It is suggested that visualizations of energy flow using the Poynting vector and the Joule heating parameters provide a powerful tool to understand how the geologic medium shapes EM induction responses.