Abstract

—The theoretical studies and methodological guidelines for transient electromagnetic (TEM) surveys assume an ideal step turn-off of transmitter current. Yet, the pulse duration in the survey practice is nonzero: It depends on penetration depth and has economic or logistic limitations. The turn-off time, which cannot be too short for technical reasons, affects especially the late-time transient responses, while the transmitter waveform is more essential at early times. Therefore, forward calculations of voltage decay should take into account the real parameters of the turn-off ramp. Earlier it was suggested to use transmitter current of a special waveform to improve the resolution of TEM soundings, e.g., in the designed and successfully tested compensation method. In this study the problem is treated from a broader perspective, in terms of TE and TM polarization of the TEM field. The two components are excited by different sources and behave in different ways in responses of a two-layer earth with a resistive lower layer to transmitter current of different waveforms.

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