Field observations with a coincident loop transient EM frequently show that the measured decaying voltage changes its sign during the decay, reaches a maximum value with this changed sign, and then decays to zero. This change of sign has been ascribed to special distributions of magnetic permeability or conductivity (Spies, 1980), as well as to the presence of electrochemical polarizability (Lee, 1975, 1981). Gubatyenko and Tikshayev (1979) showed, however, that for any frequency-independent linear medium the induced voltage caused by a step current excitation is always of one sign. Weidelt (1982) extended the results of Gubatyenko and Tikshayev (1979), and established further constraints on the slope and curvature of the decay curve. It is thus quite clear that with a coincident loop system a change of the polarity of the decaying voltage cannot be caused by any distribution of conductivity or permeability in the ground.

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