Comparison of Theoretical and Physical Model Studies of the Responses of Moving Source and Fixed Loop Electromagnetic Exploration Systems

Abstract

Theoretical and physical scale model responses for moving source and fixed loop electromagnetic exploration systems are presented for the case of a vertical, thin, plate conductor located first in free-space and then in a conductive host. The results are presented in the form of anomaly index diagrams plotted in complex space. The two modeling approaches show general agreement on the behavior of the current gathering effect that operates when the conductive host is present. Both approaches show an onset of a strong positive effect at higher frequencies in the responses provided by both horizontal coplanar coil systems and by fixed loop systems. Agreement is also shown on an unanticipated negative effect that a conductive host causes in the responses provided by the vertical coplanar coincident coil system. Both modeling approaches demonstrate that the responses provided by fixed loop systems are predominantly caused by the current gathering effect. The extent of this effect for fixed loop systems depends on the size of the transmitter loop with respect to the target and on the depth of the target. The theoretical modeling demonstrates that both moving source and fixed loop systems operated over a target located in a conductive host, provide responses that become almost identical and independent of the conductance of the target at high frequency.