The effect of magnetic anomalies on transient electromagnetic data

Abstract

The effect of magnetically polarisable material on transient electromagnetic (TEM) responses is discussed using data calculated for a layered-Earth model and a sphere in free-space. The layered-Earth models are characterised by a resistive surface layer of elevated magnetic permeability to represent lateritic duricrust and ironstone. The corresponding results indicate that data measured by TEM systems such as SIROTEM, UTEM, QUESTEM and TEMPEST will only be marginally affected by the magnetic permeability of the surface layer. The sphere model is characterised by high conductivity and high magnetic permeability to represent a pyrrhotite-rich mineralisation. The corresponding TEMPEST responses as well as results from singular value decomposition analysis indicate TEMPEST data to be sensitive to magnetic permeability variations exceeding 0.2 µ0, but only for conductive and sizeable structures. TEMPEST data acquired crossing a banded iron formation-related magnetic anomaly of 8000 nT were found to be devoid of any indication of magnetic induction. The anomaly was modelled with a dike-shaped polygon suggesting a magnetic permeability of 2.0 µ0 with the corresponding EM data indicating a zone of low conductivity. The EM responses caused by dragging the receiver coils through the associated magnetic gradient is removed by the stacking process, but is retrievable from the streamed data.