Inductive interaction between polarizable conductors: An explanation of a negative coincident‐loop transient electromagnetic response

Abstract

Theoretically, the coincident‐loop transient electromagnetic (TEM) voltage responses of an earth with frequency‐invariant conductivities and permeabilities cannot change sign. In Australia, where prospecting is often done with the SIROTEM system operating in coincident‐loop mode, the response occasionally changes sign at late times from the normal positive transient to negative values. Because it has been shown that the observed negatives cannot be explained by poor instrument design, normal displacement currents, dispersive permeabilities, or geometric effects, the mechanism causing the negatives is now inferred to be a frequency dependence (dispersion) of the conductivity. To obtain negative values, the conductivity must increase with frequency over the spectral range covered by the TEM survey (∼10–1000 Hz). Slightly dispersive conductivities are observed by induced‐polarization (IP) surveys and are geologically common; however, field experience suggests that whereas some negatives are associated with IP anomalies, others are not. We present an interpretation of SIROTEM field data in which a negative response is observed between the positive responses of two steeply dipping conductors. Although no IP survey has been performed in this area, field experience in similar areas suggests that a significant IP anomaly is not likely to be directly associated with the negative anomaly. The negatives in the field data can be explained (i) if at least one of the two conductors is moderately dispersive in the frequency range 10–1000 Hz and (ii) if the inductive coupling between the conductors is taken into account. The magnitude of the dispersion is such that no significant IP anomaly would be measured in the region where the TEM negative transient is observed. The least complicated model that can fit the field data comprises two circular, polarizable wire‐filament circuits, which are inductively coupled to each other. The geometry of the field situation has been accounted for quantitatively by mutual inductance calculations, but the conductivity and polarizability of the bodies are modeled only in terms of simple circuit analogies.