A Magnetometer Array Study in Southern Australia

Abstract

Summary The first magnetometer array study in Australia was in operation in the second half of I970 across the central southern part of the continent. Twenty-five three-component magnetometers simultaneously recorded disturbance fields of magnetic storms and substorms, in addition to the quiet daily variation. Curious anomalous effects were observed which showed partial consistency for horizontal fields of given polarization. Transfer functions were computed for the stations and these indicate, to first order, a long curved conductor running approximately north-south through the array and a second, more resistive conductor running eastwest to the north of the array. Other structures are also present. The north-south conductor, which runs close to the Flinders Ranges, may represent a feature important in the continental tectonics of Australia. 1. Observations Studies with two-dimensional arrays of three-component recording magnetometers enable transient induced electric currents and, by inference, electrically conductive regions to be mapped and to some extent interpreted in terms of conductive structure. Quantitative interpretation involves non-linear three-dimensional inversion and is very difficult, though appreciable progress has been made in some cases (Gough 1973). Structure in the electrical conductivity is of interest largely because this parameter is linked to temperature: anomalously hot regions of the upper mantle are highly conducting; conducting regions may be at high temperature. In parts of the western United States a correspondence between electrical conductivity, heat flow and seismic propagation parameters has been demonstrated (Reitzel et al. 1970; Porath & Gough 1971; Sass et al. 1971; Roy, Blackwell & Decker 1972; Hales & Doyle 1967). The location of the first magnetometer array study in Australia was chosen on two hypotheses. The first was that eastern and specially south-eastern Australia is tectonically more active than the shield of west Australia and might have higher temperatures in the underlying upper mantle. The second hypothesis was that the Flinders Ranges (Fig. 1) might constitute a surface expression of the boundary between these regions. The first hypothesis has considerable support from heat flows, which are near 2 HFU (pcal cmm2 s-l) at many sites in the south-east but typically nearer 1 HFU in the western shield (Jaeger 1970); from seismicity, which is generally higher in the south-east than in the west of the continent (Doyle, Everingham & Sutton 1968); from geological evidence of recent vulcanism in the south-east (Gill 1967); and from geochronology, which indicates ages less than 400 My in the southeast and Precambrian ages in the west (Brown, Campbell & Crook 1968). The second hypothesis was much less secure than the first. There was no evidence to indicate where the tectonic boundary, required by the first hypothesis, might be;