AusMT / surveys / Central Australia 1976-77

Central Australia 1976-77 GDS

Geomagnetic depth sounding survey · Northern Territory, South Australia, Queensland · Australian National University

Part of the Australia legacy GDS collection

In the winter of 1976 the Australian National University's set of twenty-one Gough-Reitzel variometers was spread across central Australia, from northern South Australia through the southern Northern Territory into south-western Queensland, at spacings of 100 to 200 km over an area of about 400 by 900 km.

About this survey

In the winter of 1976 the Australian National University's set of twenty-one Gough-Reitzel variometers was spread across central Australia, from northern South Australia through the southern Northern Territory into south-western Queensland, at spacings of 100 to 200 km over an area of about 400 by 900 km. Frederick (Ted) Lilley planned and ran the experiment and Dennis Woods, whose doctorate it became, assisted. The array was a search for a northward continuation of the Flinders conductor that the 1970 array had found in South Australia, and in particular for whether a conductor followed the seismically active zone of the interior. The instruments were buried upright in two-metre auger holes at landing strips, some of them cleared on desert clay pans for the purpose, and were installed by road and light aircraft and serviced by air, recording from July to September.

Across the whole area the in-phase induction arrows at substorm periods pointed consistently towards a conductor in south-west Queensland, and at Mt Leonard, the array's extreme south-eastern station, the vertical field reversed, which meant current was concentrated between that station and the rest. The following year a second array of twenty-one instruments, this time serviced by vehicle along the roads of south-west Queensland, closed the spacing to less than 20 km around the anomaly and referenced every station to Bulloo in the east, well away from it. The detailed arrows showed not a single line current but the response over the edge of a conducting sheet, with sharp changes over 20 km that placed the causative structures at depths of order 20 km or less, and current-sheet modelling that put them within the top 5 to 10 km. Woods and Lilley interpreted it as magnetic activity inducing a coherent concentration of current through the low-resistivity sediments of the Great Artesian Basin along its western margin, channelled locally by basement structures such as the Boulia Shelf and the Betoota Dome, and they proposed that the current path might continue north to the Gulf of Carpentaria and so cross the continent. The 1976 array also gave, from the quiet daily variation, a tentative conductivity of order 0.1 to 1 S/m at a depth of order 300 to 400 km beneath the interior, a depth Lilley, Woods and Sloane revised to about 500 km in 1981. The 1995 Carpentaria arrays were designed to trace the Southwest Queensland Anomaly northward. They found the anomaly there, and Liejun Wang's inversion showed the two joining.

The arrays were read at periods from 600 to 6,000 s, and only the 3,000 s arrows survive as numbers, the set Woods and Lilley map in their 1980 paper, one value per station. Forty-one of the forty-two occupations are served. The forty-second, Bulloo, was the 1977 reference station, whose record was subtracted from the others, so it has no response of its own.

The served numbers are the vertical field response arrows of the 1976 and 1977 arrays at a period of 3,000 s (50 minutes in the paper), one value for each of the forty-one stations; no uncertainties were recorded with them. They are held at reference code 13 of Liejun Wang's Australian Electromagnetic Database, which Wang (1998) Table 2.7 attributes to Woods (1979), the Australian National University doctoral thesis, and the same arrows are printed as Figure 2 of Woods and Lilley (1980). The coordinates are the register's positions, which agree to about a kilometre with the station table printed in Woods (1979) Tables 2.1a and 2.1b and in Woods and Lilley (1980) Table 1, and Tieyon's longitude is the thesis value of 133.867 degrees east. The original recordings, photographic film from the Australian National University's set of twenty-one Gough-Reitzel variometers, are not held.

Data and downloads

Stations (41)

Contributors and organisations

Organisation

Australian National University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0