The response measured by transient electromagnetic (TEM) systems is normally of a predictable sign. This is particularly true for coincident-loop ground TEM systems, where the response should in theory always be positive when the ground conductivity is a constant function of frequency and the quasi-static assumption is satisfied. In most cases, these are good assumptions, but, occasionally, coincident-loop negative transients are ob-served. These negatives can be explained if material in the ground has a conductivity that changes with frequency, or, equivalently, has nonzero polarizability.

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