oa Galvanic excitation of the Cadjebut Pb-Zn ore body

Numerical modelling was used to investigate the results from a downhole transient electromagnetic (DHTEM) test survey, which was performed over the Cadjebut lead-zinc ore body in 1987. Unexpectedly, the response due to the inductively well-coupled near-loop contained no signature of the known ore-body, whilst the response due to the offset loop contained a prominent negative intersection anomaly.

A dual-slab block model representing the known mineralisation was constructed using a 3D integral equation program MARCO. This algorithm was used because it permitted modelling of both inductive and galvanic effects. The response generated from this model for both the near and far loops contained a weak and strong single-signed galvanic anomaly, respectively. Current channelling was recognised as the primary excitation mechanism after consideration of the anomaly sign and verification with decay analysis. Specifically, decay analysis of the anomaly within the offset loop data yielded decay indices a= -3.27 and a= -3.47 for the field and model data, respectively. These results are in agreement with the decay index identifiable with current channelling, namely a = -7/2.

The modelling has demonstrated the importance of offset loops in the production of detectable current channelling effects, which in turn enables the detection of weakly conducting targets which may otherwise be invisible to traditional inductive exploration methods.