Abstract

The negative transients observed in coincident-loop measurements are believed to be caused by polarizable conductive structures (i.e., structures whose in-phase conductivity increases with frequency). For most structures, it is not possible to predict negative transients as large as some of those observed in the field unless the polarizability is exceptionally large. Previous work shows that modest polarizabilities can be used to predict negative transients of the correct magnitudes when the conducting structures are such that the following qualitative conditions apply: (1) the transmitter is positioned proximal to the conducting structure in a manner which ensures a large early-time fundamental inductive current is induced in the structure; (2) the polarization current is proximal to the receiver and results in a measurable negative polarization response; and (3) the positive response associated with the fundamental inductive current decays away sufficiently rapidly that the total response can change sign. These conditions are termed the 'favorable coupling conditions.' A polarizable half-plane model is a structure which satisfies these conditions. The positions near the edge of the half-plane, where the favorable coupling conditions are best satisfied, are those which are most conducive to observing negative transients. Field examples of negative transients associated with either an overburden which comes to an edge or a dipping sheet-like structure can be modeled with a weakly polarizable half-plane. We believe that most of the negative transients observed in the field are likely to be associated with structures which satisfy the favorable coupling conditions.

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