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What do electromagnetic induction responses measure?

Texas A&M University, College Station, USA

Corresponding author: everett@geo.tamu.edu

The first 20% of the full text of this article appears below.

It is common knowledge to applied geophysicists that, for determining spatially variable electromagnetic properties of the ground, the magnetic method is sensitive to magnetic permeability µ, the ground-penetrating radar method responds to dielectric permittivity , and the dc resistivity method provides a good indication of , the electrical resistivity. It is also commonly acknowledged that the electromagnetic induction (EMI) technique responds to electrical conductivity . The choice of which variable to report, or its reciprocal , is largely a matter of convention decided upon by vocal adherents of the respective methods.

The concept that EMI is simply a means to register the bulk electrical conductivity of the ground warrants further attention. Electrical conductivity itself is a somewhat nebulous physical property. By most standard definitions, it is the capability of a material to sustain long-term electric current flow. However, field and laboratory experiments continue to indicate that electrical conductivity of heterogeneous geologic materials is frequency dependent. The precise cause of this frequency dependence and its impact on measured EMI responses has been under intense investigation of late. However, this is not the topic I wish to consider here. I want to focus on the central but poorly understood role of the ground inductance L in shaping EMI responses.

Inductance is a purely geometric quantity, as any student of physics or engineering knows. The inductance L of a circuit is a measure of the electromotive force (emf) raised in the circuit by a time-varying magnetic flux.

The emf is not a force per se, but rather a voltage. If the circuit has finite resistance R, an electrical current of the form exp(–t/) decaying exponentially with time constant =L/R is induced to flow in the circuit so as to discharge the emf.

The preceeding discussion does not . . . [Full Text of this Article]