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

A physical model study was conducted of the responses provided by moving-so urce and fixed-loop frequency-domain electromagnetic prospecting systems wh en operated over the same target conductor located in a conductive host env ironment. The results indicate that the fixed-loop responses display enhanc ement due to the current gathering effect that exceeds that seen in the mov ing-source responses by at least an order of magnitude for all the source-t o-receiver separations tested for the moving-source system. The results als o 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 reduc tion with increase of target depth for the fixed-loop system but an increas e 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.