Examination of the relative influence of current gathering on fixed loop and moving source electromagnetic surveys

Abstract

A physical model study was conducted of the responses provided by moving‐source and fixed‐loop frequency‐domain electromagnetic prospecting systems when operated over the same target conductor located in a conductive host environment. The results indicate that the fixed‐loop responses display enhancement due to the current gathering effect that exceeds that seen in the moving‐source responses by at least an order of magnitude for all the source‐to‐receiver separations tested for the moving‐source system. The results also indicate that as transmitter frequency is increased the current gathering effect displays an abrupt onset in the responses provided by both systems, but that this onset begins at a frequency which is a decade lower for the fixed‐loop system than the corresponding frequency of onset for the moving‐source system. The current gathering enhancement effects show a clear reduction with increase of target depth for the fixed‐loop system but an increase with depth for the moving‐source system. The model parameters employed in these studies are shown to be well related to typical conditions found in full‐scale surveys.