Electrical conductivity anomalies in the Australian lithosphere: effects on magnetic gradiometer surveys

Abstract

The use of recording magnetometers to observe natural magnetic fluctuations across different parts of Australia is delineating areas where the fluctuations are anomalous. These areas are termed 'conductivity anomalies', as the fluctuation patterns must be due to regions in the lithosphere of high electrical conductivity, where the naturally induced electric currents are concentrated. The main causes of such enhanced lithospheric conductivity are thought to be (i) water, with impurities (especially salt), and perhaps with well-connected paths of conduction due to fracturing of rock; (ii) highly conducting minerals such as graphite, and (iii) heat (which may cause partial melting at the lithosphere base). The fluctuating magnetic fields near a conductivity anomaly may cause difficulty for the data-reduction of a high-resolution aeromagnetic survey which is carried out during magnetic activity. Making gradiometer survey measurements avoids such effects, to the extent that the magnetic fluctuations are spatially uniform. This paper examines the maximum spatial magnetic gradients likely to be associated with a strong (and shallow) continental conductivity anomaly, and determines values in the range 1 to 10 nTkm-1. Such gradients in the static magnetic field are typical of a sedimentary basin, but are much less than those which occur where the magnetic relief is strong. While the magnetic fluctuation fields of conductivity anomalies should be kept in mind during magnetic surveying, gradiometer measurements should generally be effective in minimizing their influence.