AusMT / surveys / Tasman Project land line 1983-84

Tasman Project land line 1983-84 GDS

Geomagnetic depth sounding survey · New South Wales and Australian Capital Territory · Australian National University

Part of the Australia legacy GDS collection

The Tasman Project of Seafloor Magnetotelluric Exploration was planned by Frederick (Ted) Lilley of the Australian National University and Jean Filloux of the Scripps Institution of Oceanography under the US/Australia Agreement for Science and Technology, with the Royal Australian Navy providing HMAS Cook for the two cruises and the US National Science Foundation funding the American side.

About this survey

The Tasman Project of Seafloor Magnetotelluric Exploration was planned by Frederick (Ted) Lilley of the Australian National University and Jean Filloux of the Scripps Institution of Oceanography under the US/Australia Agreement for Science and Technology, with the Royal Australian Navy providing HMAS Cook for the two cruises and the US National Science Foundation funding the American side. Its aim was to measure electromagnetic induction across a whole continent-to-ocean system, extending the earlier ANU continental array results into the deep ocean. The line was chosen to cross the Tasman Sea from the Australian shelf over the abyssal plain, the Tasmantid Seamount Chain and the fossil spreading ridge to the submerged continental crust of the Lord Howe Rise, and to meet the coast where earlier ANU arrays had suggested the margin was simply two-dimensional. Seven ANU Gough-Reitzel magnetometers were buried along a line from Durras on the south coast inland through Clyde Mountain, Braidwood, Canberra, Coolac and Barellan to Ivanhoe, at the edge of the Murray Basin. The Bureau of Mineral Resources' Canberra Magnetic Observatory near Bungendore was the eighth station. Spacing widened from about 30 km near the coast to 274 km inland, over a line 640 km long.

The land instruments went into the ground in July 1983 so that their suspension fibres could settle, and recorded photographically at sixty readings an hour through the principal period of December 1983 to April 1984 while nine seafloor instruments recorded across the Tasman Sea, seven of them with horizontal electric channels. Ted Lilley, Ian Ferguson and N. L. Bindoff made service visits before, during and after the recording period. The return was almost complete, and digitising and reducing the film took about three quarters of a man-year at the ANU. Every transfer function in the project, land and sea, was referenced to the horizontal field at the Canberra observatory, so the land line provided the simultaneous reference for the marine experiment, and with the Eyrewell observatory in New Zealand on the line's eastward extension it showed the source field coherent over 2,000 km. Ferguson's analysis of the arrows found a strong coast effect at the Australian coastline that grew towards the sea at periods between 3,600 and 34,560 s, reached its maximum just offshore, between Durras and seafloor site TP8, and faded inland much more slowly than seaward; the profile suggested a deep conductivity contrast and confirmed Bennett's 1972 model of the margin. A weaker coast effect sat over the Lord Howe Rise, and at longer periods the arrows near the coast carried components parallel to it, evidence that induction at this margin is a three-dimensional process. Whether local induction or channelled ocean current produced the effect was left open. R. L. Kellett reactivated magnetometers at four of its sites for the Continental Slope Experiment of 1986, two of which returned usable data, and built his two-dimensional model of the margin from the land line and that experiment together. The CICADA97 array of 1997 re-occupied six of the eight sites and found the real arrow azimuths agreeing, on AusMT's reading, to within a few degrees fourteen years later. The experiment was published by Lilley, Filloux, Ferguson, Bindoff and Mulhearn in 1989 and the Tasman Sea result by Ferguson, Lilley and Filloux in 1990.

A ninth land instrument at Cobar, already in the ground from an earlier array, recorded through the project, but Ferguson's thesis tabulates no induction arrows for it, so it is not served here. The shortest period should be read with caution. At 684 s six of the seven field magnetometers give arrows a tenth or less of the observatory's, and Durras about half, and the CICADA97 array reproduced the same pattern, so on AusMT's reading the likely cause is uncorrected instrument response rather than a property of the earth.

The served values are induction arrows, one set for each of the eight stations at fourteen periods from 684 to 117,000 s (0.190 to 32.5 hours in Table A5.1), every one of them referenced to the horizontal field at the Canberra Magnetic Observatory. They are the arrows printed in Appendix 5, Table A5.1 of Ferguson (1988), Australian National University PhD thesis, taken from that table and rotated from magnetic to geographic north with the declinations Ferguson prints in Table 6.7 of the same thesis; the 95 per cent confidence limits given in Table A5.1 travel with the values. The station coordinates are the positions printed in Table 2.1 of the thesis. Ferguson records that the reduced time series were archived on magnetic tape, and the original recordings are not held.

Data and downloads

Stations (8)

Contributors and organisations

Organisation

Australian National University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0