COPROD2 revisited : Can the B-polarization phase distinguish between single- and multi-body anomalies ?

Abstract

The small local minimum in the long-period B-polarization phase responses above the NACP anomaly in the COPROD2 data is re-examined with a view to understanding its role in determining whether the anomaly is a multi-body or continuous conductive structure. It is found by trial-and-error numerical modelling and by further optimization of a model with continuous structure that the B-polarization phase response is not profoundly affected by the NACP anomaly itself, but is extremely sensitive to the resistivity distribution in the sedimentary layer to the extent that quite minor resistivity variations can turn a local minimum into a local maximum and vice versa, without necessarily affecting the short period responses in an unreasonable manner. A further modelling exercise with a multi-body anomaly shows that its response does not necessarily give rise to a local minimum in the phase response, as sometimes claimed. The investigation presented here is not intended to establish whether the NACP anomaly is a continuous or broken conductive structure but it does suggest that the evidence that has been cited in favour of a multi-body anomaly may not be sufficient by itself to settle the question with certainty.