AusMT / surveys / Southeast Australia array 1971

Southeast Australia array 1971 GDS

Geomagnetic depth sounding survey · Victoria, New South Wales and South Australia · Australian National University

Part of the Australia legacy GDS collection

In February 1971 the Australian National University set out a regional magnetometer array across the south-east corner of the mainland.

About this survey

In February 1971 the Australian National University set out a regional magnetometer array across the south-east corner of the mainland. Twenty-five Gough-Reitzel variometers, lent by Ian Gough of the University of Alberta, were buried at country airstrips from Mount Gambier in South Australia across Victoria and southern New South Wales to the Tasman Sea coast, with the Bureau of Mineral Resources' Toolangi observatory as a twenty-sixth site. The array was the main project of David Bennett's doctorate under Frederick (Ted) Lilley. Every earlier array of this kind had been run far from any coast, so this one was sited to record the coast effect over a two-dimensional area. It was also to look for currents channelled through Bass Strait, and to test whether the high heat flow of the eastern highlands and the recent Newer Volcanics of western Victoria showed as elevated conductivity in the crust and upper mantle. It was the first Australian array experiment to be reported, and the second to be carried out: a 1970 array across south-central Australia was published later.

The instruments were installed by a road party in February, switched on by light aircraft within four days of one another and serviced every three weeks the same way, recording photographically on 35 mm film until retrieval in May, and better than nine tenths of the recording time was returned. Two substorms, three quiet days of daily variation and a micropulsation train were Fourier analysed, and vertical-field transfer functions were fitted in the geographic frame with the estimator of J. E. Everett and R. D. Hyndman. The major result was that the strong vertical coast effect along the New South Wales coast was not matched by anomalous horizontal variations, while the shallow Bass Strait produced no comparable enhancement of the vertical field, its own coast effect showing instead in the horizontal amplitudes. The study also introduced the fitting of a response surface to a whole array, and the use of the polarisation of the horizontal field to separate the responses of different structures. In the south-west corner of the array, at Hamilton, Derrinallum and Ararat, the induction vectors departed from the coastal pattern. Bennett and Lilley's 1974 synthesis named this the Victorian Anomaly and read it as a conductive zone of elevated temperature and partial melt associated with the Newer Volcanics, having ruled out the sediments of the Otway Basin as its cause. The same paper named the coastal feature the East Coast Anomaly and concluded that a zone of anomalously high conductivity lies beneath the western Tasman Sea off the New South Wales coast. These were the two major conductive regions the array outlined. A second Australian National University array across southern Victoria and Bass Strait in 1973 and 1974 confirmed the Victorian anomaly and placed the conductor perhaps 20 to 30 km further south-east, under or near the Otway Ranges. Lilley (1976) renamed it the Otway Anomaly, and it is one of the seven major conductivity anomalies of the continent.

The film records of the two substorms were digitised in 1971 and 1972 by two outside bureaux. The investigators worked across periods from about 360 to 86,400 s. Every station survives, but only at one period, 4,800 s.

The served numbers are one complex tipper per station at a period of 4,800 s (the paper's 80 minutes), printed in Table 1 on page 59 of Lilley and Bennett (1972), Geophysical Journal of the Royal Astronomical Society 29, 49 to 64; for all but one station they reach this record through the rows for this array in Liejun Wang's Australian Electromagnetic Database, whose values reproduce that table exactly, while Ararat's value comes straight from that printed page, which alone gives its components. No per-station uncertainty was ever published: the paper gives a single standard deviation of 0.3 for the whole array, and 0.5 at Nowra and Bombala, and that array-wide figure is the only uncertainty travelling with any served value. The coordinates come from that same database, except Ararat's, which comes from the thesis table; all 26 agree with the positions printed in Table 2.1, page 48, of Bennett (1972), the Australian National University doctoral thesis, to within a kilometre. The original recordings, 35 mm photographic film, are not held.

Data and downloads

Stations (26)

Contributors and organisations

Organisation

Australian National University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0