A Magnetotelluric Traverse in Southern Australia

Abstract

Telluric potentials were recorded at nine sites stretching 1000 km from east of Wilcannia (N.S.W.) to west of Coober Pedy (S.A.) in south central Australia. These were combined with data obtained from a simultaneous magnetic variometer study, and analysed to yield magnetotelluric impedance tensors. The estimation of probable errors for the impedance tensors was an important part of the calculations. Numerical calculations for two-dimensional models showed that the observed data could be explained by a conductive-resistive-conductive layering. The top conductive (10 ohm-m) layer is approximately 5 km deep and the ensuing resistive section (1000ohm-m) continues to a depth of about 230km. This is underlain by an indeterminably thick conductive layer. Embedded in this general structure is an extremely good conductor (0.1 ohm-m) which strikes north-south. The detection of this good conductor is the main result of this work. The good conductor lies just east of the Adelaide Geosyncline at a depth of a few kilometres, and is of minimum thickness 10 km. It is thought to be related to the accretion of the eastern portion of the Australian continent on to the stable western Precambrian platform by tectonic processes. The strong anisotropy present in the magnetotelluric data at all sites could not be explained by induction in conductive sediments, and is thought to be caused by localized conductivity inhomogeneities distorting current flow.

Magnetotelluric soundings over a Precambrian contact in Australia

“…2 ) . Data from the latter extend the results of Tammemagi & Lilley (1973). Both sets of data are consistent with the present observations at site 4-8.…”

“…An inherent crystalline anisotropy can be suggested by the lack of any systematic trends in estimates of apparent resistivity. Furthermore the data presented by Tammemagi & Lilley (1973) indicates that the orthogonal components continue to diverge west of Broken Hill. Similar anisotropies have been noted in Central Alberta (Reddy & Rankin 1971) where total divergence is comparable to the Australian data.…”

“…However, the observed anisotropy can not be generated by the geometry of this conductor alone. rvlost observations are made at distances exceeding 20 km and any perturbation is greatly suppressed (Tammemagi & Lilley 1973). Two models generating sufficient anisotropy are indicated in Fig.…”

Electrical conductivity anomalies and their relationship with the tectonics of South Australia

“…8); we suggest that this probably continues northward and may link up with the Queensland anomaly (Woods & Lilley 1980). Both magnetotelluric soundings (Tammemagi & Lilley 1973) and resistivity soundings (Constable et el. 1982;Constable, McElhinny & McFadden 1983;Constable 1983) have been taken in the Frome Embayment.…”

“…1). A zone of high electrical conductivity was delineated by the array study and also by telluric observations made simultaneously (Lilley & Tammemagi 1972;Tammemagi & Lilley 1973). Broadly speaking, the conductor follows the trend of the northern Adelaide Geosyncline and its associated seismicity, but it appears to lie slightly east of these features.…”