Abstract:
The time-domain electromagnetic response from multiple conductors in a resistive environment is studied by using wire loops in free space to model conductive sheets. This approach provides a relatively simple way of qualitatively examining the transient effects that two closely separated conductors have on the measured response. Results from model studies indicate that the response from a multiple conductor system is not the sum of the individual conductor responses. When two conductors occur near a transmitter loop, the influence that exists between them can have a detrimental effect on the resolvability of the conductors. A conductor between a transmitter loop and a second conductor causes the response of the second conductor to be partially suppressed. This masking phenomena is particularly severe when the conductor that is closer to the transmitter loop has a lower conductance than the other conductor. When dealing with a multiple conductor system where the conductors have conductances of similar magnitudes, the distance separating the conductors has to exceed their depths if both conductors are to be resolved from the electromagnetic profile plots. Only when the separation/depth ratio approaches 2, can the presence of two conductors be established with any certainty. When large differences (in the order of a few tenths of a Siemens) exist in model conductances, the cross-overs and amplitude peaks shift in the direction of the conductor of higher conductance. These shifts are observed even when the conductor separations are very small (less than 1. 5 m) . Calculating pseudo decay (time) constants, for each station along the survey profiles, can provide additional information that is helpful to determine whether a multiple conductor situation exists. The pseudo decay constants can indicate the presence of multiple conductors even when the separation/depth ratio is less than unity.
