Abstract:

Many ground electromagnetic (EM) systems have been deployed and under ideal conditions these systems are capable o f detecting large conductors to depths of approximately 700 m; however, more common detection limits are in the order o f a couple o f hundred meters (<400 m). Although these systems have been utilized with great success in Sudbury, they may experience two weaknesses for deeper conductors, poor coupling and small signal-to-noise ratios, both of which decrease the quality and interpretability o f the data. This paper describes a novel time-domain EM procedure that addresses these weaknesses. The coupling weakness is addressed through multiple transmitter locations and multiple receiver locations, and the signal-to-noise ratio is increased by spatial stacking of receiver measurements (from the various transmitter-receiver combinations). Reciprocity data from a field test show that the noise levels of the data are about ±0.004 pV/Am2, which is less than the level of 0.08 pV/Am2, which is a conservative estimate o f the noise attributed to geometric errors. Spatial stacking of the data could reduce the noise levels by a factor o f 7. This means that a small conductor previously only visible to 150 m could be seen to 275 m and a conductor visible to 300 m could be seen to 575 m. The large volumes of data collected using the procedure, which creates a challenge in visualizing the results that has been addressed by creating response depth sections. Synthetic and real response depth sections appear very similar and demonstrate that these images of the subsurface could be interpreted. The only weakness encountered thus far is that the features appear 50 m deeper than they should be.