AusMT / surveys / Ballarat 1992

Ballarat 1992 GDS

Geomagnetic depth sounding survey · Victoria · University of Adelaide

Part of the Australia legacy GDS collection

By 1992 airborne magnetometers could resolve less than a tenth of a nanotesla, and the Australian Geological Survey Organisation had re-equipped its survey aircraft, from August 1990, with a helium-vapour instrument an order of magnitude better than anything it had flown before.

About this survey

By 1992 airborne magnetometers could resolve less than a tenth of a nanotesla, and the Australian Geological Survey Organisation had re-equipped its survey aircraft, from August 1990, with a helium-vapour instrument an order of magnitude better than anything it had flown before. At that resolution the micropulsations of the geomagnetic field, with periods of less than a few hundred seconds and amplitudes of a few tens of nanotesla at most, are a likely source of noise in high-resolution aeromagnetic surveys, and AGSO set out to learn how those pulsations vary across the ground an aircraft covers. In March 1992, in a collaboration between AGSO and Flinders University reported by Peter Milligan, Antony White, Graham Heinson and Ross Brodie, a magnetometer array was laid across the Ordovician shales south of Ballarat in southern Victoria, chosen because the shales carry only a weak aeromagnetic signature and the ground is close to the airport from which AGSO was flying the Ballarat map sheet at the same time, in lines 400 m apart. It was the first experiment of its kind in AGSO's programme, and the induction effects at these short periods were expected to tell something of the subsurface conductivity as well.

Ten sites about 12 km apart covered an area some 40 km across. Flinders ring-core and cylindrical fluxgate magnetometers were buried against thermal drift and left recording unattended for a week, sampling every five and every ten seconds. A Narod magnetometer ran on mains power at the Ballarat University College, and the aircraft's spare helium-vapour sensor sat beside the proton base-station at the airfield. Long trains of Pc4 micropulsations of 60-second period and up to 3 nT peak to peak were recorded, and they proved uniform in phase and nearly uniform in amplitude across the whole array. Pulsations of that kind are the ones expected to put noise into airborne magnetic surveys. Variations of about 300 to 7,200 s were not uniform across the array. Over so small an area those differences must be induction, and they may come from the contrast between the Tertiary volcanics and the Ordovician shales. The in-phase induction vectors between 100 and 5000 seconds all pointed to the south-east, swinging from east towards south with period and lengthening from north-east to south-west across the array, and they agreed with the arrow Frederick (Ted) Lilley had measured at Ballarat in 1973 - 74, pointing to the same conductor to the south-east, the Otway conductor. The authors called for larger arrays of fast-sampling instruments across whole map sheets, and no follow-up is known.

The served numbers are ten vertical magnetic field transfer functions, one for each site, at ten or twelve periods between 80 and 8,192 s, and every value carries its own variance, so the uncertainties travel with the data. The paper prints no table, and its only induction vectors are the Figure 4 arrows at a single period of 500 seconds, drawn against a length key and carrying no numbers. The transfer functions come instead from The University of Adelaide Legacy Magnetotellurics Data, published by NCI Australia (doi:10.4225/41/5a618c3ae30ab), and the original recordings are not held.

Data and downloads

Stations (10)

Contributors and organisations

Organisation

University of Adelaide

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0