AusMT / surveys / North East Tasmania induction study 1978-79

North East Tasmania induction study 1978-79 GDS

Geomagnetic depth sounding survey · Tasmania · University of Tasmania

Part of the Australia legacy GDS collection

Between February 1978 and October 1979 Nazhar Buyung, a Master's candidate under Wilfred Parkinson at the University of Tasmania, occupied eleven temporary magnetometer stations in north-east Tasmania, on both sides of the lower Tamar: Deloraine, Rosevale, Western Junction, West Frankford, Lilydale, Nabowla, Scottsdale, Pipers River, Beechford, Tayene and Forester.

About this survey

Between February 1978 and October 1979 Nazhar Buyung, a Master's candidate under Wilfred Parkinson at the University of Tasmania, occupied eleven temporary magnetometer stations in north-east Tasmania, on both sides of the lower Tamar: Deloraine, Rosevale, Western Junction, West Frankford, Lilydale, Nabowla, Scottsdale, Pipers River, Beechford, Tayene and Forester. Buyung counted only four induction vectors in Tasmania before he began: Parkinson's at Hobart, and three on the north coast from Frederick (Ted) Lilley's south-east Australia array of 1973 and 1974, of which the Bridport and Devonport arrows pointed towards the mouth of the Tamar and suggested a conductive zone inland. The study set out to record how geomagnetic variations respond to lateral changes of conductivity across that ground and to interpret the structure responsible.

This was not a simultaneous array. One instrument at a time moved from site to site. Each occupation ran from three weeks to three months, and but for a four-week gap after Scottsdale the next opened on the day the last closed. Two magnetometers were used over the campaign, a TOPMAG fluxgate with no temperature control whose detector was buried half a metre down, and from July 1978 a commercial EDA FM-100B fluxgate with built-in temperature compensation. Both recorded on chart recorders, and the charts were digitised by hand and by scaler at intervals of two, three or six minutes. Between nine and fifteen substorms and bays were analysed at each station and transfer functions solved by the least-squares method of Everett and Hyndman (1967). At periods above 3,840 s the arrows carried the coast effect of the deep ocean to the south-east, with an average transfer function of 0.22, against the 0.2 to 0.3 that Parkinson and Jones (1979) gave for a site 185 km from the continental slope. At 2,880 s and shorter the local structure dominated, and at periods shorter than 1,920 s the arrows turned inland, north-east on the western side of the Tamar and south-west or north-west on the eastern side, so that the ground between the two groups showed an anomaly. Buyung's two-dimensional modelling put a conductive surface zone between east of Deloraine and west of Scottsdale above a deep vertical boundary beneath the Tamar zone separating the eastern and western formations, and he noted that the arrows at Nabowla, Scottsdale and Tayene pointed at a zone of mineralisation. He recommended more stations to the south to prove the boundary's continuation. Rahmat Hermanto's Tamar array of 1983 and 1984 was set up to do that. Later work traced the anomaly to the south-east and named it the Tamar Conductivity Anomaly, and modern magnetotelluric work in Tasmania treats it as a major shallow to mid-crustal electrical structure separating electrically different parts of the island.

The transfer functions served here are those the thesis prints, at every period it tabulates, eight to nineteen periods per station between 720 and 7,680 s, each with the uncertainty printed beside it. The Tamar array of 1983 and 1984 later re-occupied the West Frankford site.

The values served here are the vertical field transfer functions A and B that Nazhar Buyung computed and printed in Appendix I, Calculated Transfer Functions, printed pages 82 to 101 of Buyung (1980), A Geomagnetic Induction Study in the North East of Tasmania, MSc thesis, University of Tasmania, doi:10.25959/23237105, together with the azimuths and lengths of the induction vectors printed in Appendix II, pages 103 to 105. The thesis prints one uncertainty for each transfer function at each period and states in the Plate II remarks, printed page 62, that it applies to the imaginary part as well as the real; those uncertainties travel with the values served here, although the thesis does not say whether they are standard deviations or standard errors. The coordinates are the station positions printed in the thesis's Table 3, with the map references of Appendix III, printed page 106. The original recordings are not held: the chart magnetograms, the punched paper tapes and the computer output of the 1978 and 1979 fieldwork are gone.

Data and downloads

Stations (11)

Contributors and organisations

Organisation

University of Tasmania

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0