AusMT / surveys / Robe-Mildura Coast Effect Profile 1976

Robe-Mildura Coast Effect Profile 1976 GDS

Geomagnetic depth sounding survey · South Australia, Victoria · Flinders University

Part of the Australia legacy GDS collection

In November and December 1976 Antony White and O.

About this survey

In November and December 1976 Antony White and O. W. Polatajko of Flinders University laid a line of magnetometers perpendicular to the south-east coast of South Australia: a seafloor instrument in about 140 m of water on the edge of the continental shelf, 40 km south-west of Robe, then the coastal station at Robe and five inland stations, to a site near Mildura about 350 km from the sea. It was the first of several profiles they planned across the South Australian coastline, chosen because the edge of the continent there is nearly straight, and its purpose was to test whether the way the coast effect decays inland is a signature of the geological history of the continental crust, as Bennett and Frederick (Ted) Lilley had argued from the New South Wales coast. Kellett's 1989 thesis records their seafloor magnetometer as the first ocean-bottom magnetometer used in Australia.

All the instruments used fluxgate elements resolving one nanotesla, five of them three-component and three vertical-only, recording on chart or cassette recorders and digitised at two-minute intervals. The seafloor instrument ran for fourteen days from 18 November, until its battery pack failed. No recordings were made at the station near Mildura during that time, its battery cable having short-circuited. The transfer functions were computed against the horizontal variations at Robe, from up to six stretches of simultaneous recording. The coast effect they found was unusually simple. The in-phase arrows held perpendicular to the continental edge across the band, and their amplitude fell steadily inland. At a period of 3,600 s the decay matched Bennett and Ted Lilley's south-east Australian coast closely enough that the authors took it as confirming the coast-effect signature depends on the geological history of the crust. The Recent volcanic province to the south-east had no effect on the data at all. Only the value at Robe sat lower than expected, and the authors could not account for it. The result set the course for the programme's later profiles westward across older crust, whose Port Lincoln and Elliston lines went on to find the Eyre Peninsula Anomaly.

The paper's analysis window ran from 14,400 to 900 s and its table prints four periods within that band, so the served periods are fewer than the profile yielded. White and Graham Heinson returned to the line in 1989 with three new seafloor stations and found the arrows at the coast and on the shelf agreeing with the 1976 values, and the 1995 SAMAS array was sited along this corridor for the same comparison. Both were separate experiments.

The seven served transfer functions were digitised from Table 1 of White and Polatajko (1978), Journal of Geomagnetism and Geoelectricity 30, 109 to 120, printed page 116, which gives in-phase and quadrature amplitude ratios and induction angles at periods of 14,400, 7,200, 3,600 and 1,800 s for SF, ROBE, MSC, XMR, CON, GSD and PAN. The uncertainties printed with that table, 0.05 in amplitude and 5 degrees in angle, travel with every served value; MIL made no recordings during the seafloor deployment and has no row in the table. The station coordinates are the degree and minute positions printed in Table 1 of White and Polatajko (1985), Geophysical Journal of the Royal Astronomical Society 80, 757 to 771, printed page 759. The original recordings are not held.

Data and downloads

Stations (7)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0