In this paper, transient electromagnetic response excited by ramp-shaped current using a conical source is studied for the sensing of underground environments. The transient time responses from the conical source with either multi-turn coils or a single loop coil are investigated and compared. Although with the same magnetic moment, the turn-off time for conical source is longer than that of the single loop-this is due to the fact that the time of entering different testing stages using a multi-turn coil system is often earlier than that for a single loop system. To correct turn-off error, the conical source type transmitter is used and the one-dimensional modeling of a layered media, in which the calculation of the turn-off response is carried over the entire time range by solving directly an upper limit-variable integral. The turn-off correction method matched the resistivity and the layer thickness, which is critical for the detection of shallow depth problems. Finally, a field example is presented to illustrate the effect of turn-off time on the interpretation of transient sounding data for a conical source system.
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