Applying Transmission Line Theory to Study the Transmitting Turn-OFF Current in a Long Grounded Wire

A long grounded wire in the transient electromagnetic (TEM) surveys is usually considered by using the lumped parameter circuit model, which fails to explicate the early turn-OFF current with crucial importance for TEM surveys. In this paper, the transmission line model is introduced for the first time to explicate the early turn-OFF current in the long grounded wire considering the high frequency at early turn-OFF stage. The transmission line model suitable for the long grounded wire is constructed, then the calculation formulas of the distributed parameters and load impedance are analyzed, and the transfer function utilized to simulate the current at any position of the transmission line is also derived. The simulation is performed in the frequency domain with subsequent transformation into the time domain, and the simulation results are in good agreement with the experimental data. The early turn-OFF currents depending on the earth resistivity vary with the position in the long grounded wire. Therefore, the early time TEM response differs from that predicted by the classical theory of the TEM surveys. This proposed model can guide the realization of the minimum current turn-OFF duration, which will help to reduce the detection blind spot and this paper provides new ideas and prospects for early response data and current waveform data for interpretation in the geophysics detection.