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Abstract The occasional occurrence of persistent sign reversals in coincident loop transient electromagnetic (TEM) measurements stimulates an investigation of possible causes for this effect. By examining the response in the complex frequency plane near the spectrum of freely decaying current modes, it is shown that for any physically reasonable frequency-independent distribution of electrical conductivity and magnetic permeability the voltage response to a step function driving current is of one sign only. Moreover, under the conditions mentioned above, the logarithm of the induced voltage is a decreasing convex function of time. These characteristics are retained for more general time functions of the driving current. The conservation of sign for frequency-independent material parameters supports the assumption of IP effects as a possible mechanism for sign reversals. The latter point is illustrated by a simplified example.
Peter Weidelt (Wed,) studied this question.