Due to the effect of transmitting current turn-off time, transient process of receiver coil and long sampling interval, the transient electromagnetic method fails in resolving the shallow target in the vertical direction. A numerical method calculating the all-time apparent resistivity for transient electromagnetic method was proposed, which could be applied to calculate apparent resistivity with central loop configuration with the turn-off time as one of system parameters. As a result, the turn-off effect was eliminated and the discriminability was enhanced. The simulation and further experiments proved the practicability and accuracy of this method. Compared to conventional methods, this direct numerical method was more accurate and costed less computation time. With the development of linear ramp turn-off source, this method was of some important practical values.
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