Self demagnetisation corrections in magnetic modelling: some examples

Abstract

Self demagnetisation, also referred to as shape demagnetisation, affects both the induced and remanent magnetisation of any magnetised body. In the case of weak-intermediate susceptibility (k < 0.1 SI), its effects are insignificant and can be neglected in magnetic forwarding modelling. However, such effects are important when modelling bodies with high susceptibility. The effects of self demagnetisation in magnetic modelling are considered in a few publications, but very few detailed case studies have been presented. In this paper, we present three examples of magnetic modelling that illustrate the importance of the phenomenon when source bodies are highly magnetic. The first example concerns a magnetite-iron deposit. In this case accounting for self demagnetisation in terms of a single, simple body is sufficient to recognise that the discrepancies between observed and calculated magnetic anomalies are due to self demagnetisation of the orebody. The second example, a volcanic-hosted iron deposit, demonstrates that ellipsoidal models are applicable as a satisfactory approximation of modelling for equi-dimensional and/or lens-like bodies. The third case, an ultramafic-hosted nickel and copper deposit, demonstrates that under favourable conditions applying demagnetisation corrections can help to identify the effects of mineralisation within a larger anomaly.