AusMT / surveys / Tasman Project seafloor line 1983-84

Tasman Project seafloor line 1983-84 GDS

Geomagnetic depth sounding survey · Tasman Sea (offshore New South Wales) · Australian National University

Part of the Australia legacy GDS collection

In the summer of 1983 to 1984 the Tasman Project of Seafloor Magnetotelluric Exploration set nine autonomous instruments on the floor of the Tasman Sea along a single line from the foot of the continental slope off south-east Australia to the Bellona Gap at the southern end of the Lord Howe Rise, about 1,440 km from the coast on our measurement, in water 2,550 to 4,980 m deep.

About this survey

In the summer of 1983 to 1984 the Tasman Project of Seafloor Magnetotelluric Exploration set nine autonomous instruments on the floor of the Tasman Sea along a single line from the foot of the continental slope off south-east Australia to the Bellona Gap at the southern end of the Lord Howe Rise, about 1,440 km from the coast on our measurement, in water 2,550 to 4,980 m deep. 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 United States participation funded by NSF grants OCE 83-01216 and OCE 83-12339, it was the first use of Filloux's free-fall seafloor instruments in the Tasman Sea, after twelve array studies in the Pacific and one in the Atlantic. The project set out to find the conductivity structure beneath a young ocean basin of simple lithospheric history, to look for a conducting asthenosphere, to examine the continental margin, the Tasmantid Seamount Chain and the submerged continental crust of the Lord Howe Rise, and to test whether a deep conductivity contrast between ocean and continent contributes to the coast effect. The line was set perpendicular to a margin the ANU's arrays of the 1970s had suggested was two-dimensional, through the Canberra Magnetic Observatory, with the project's land magnetometers continuing it inland and the Eyrewell observatory in New Zealand on its eastward extension.

HMAS Cook of the Royal Australian Navy deployed the instruments in a fourteen-day cruise and recovered them in rough seas over ten days in late March and early April. Each was a suspended-magnet magnetometer in a pressure cylinder that free-fell to a flat patch of seafloor chosen by echo sounder, recorded onto cassette for the four months, and released itself to the surface on a timer; seven sites also carried horizontal electric field recorders, and pressure, vertical electric field and current-meter instruments were deployed for the oceanographic side of the project. A warm-core ring of the East Australian Current passed over the array during the recording period. Nineteen of the twenty-one Scripps instruments came back and thirty-eight of the forty-five recordings were essentially complete; the reduction took an estimated two man-years, and about five million data points were edited. The induction arrows were referenced to the Canberra observatory's horizontal field rather than to the local seafloor field, using the remote-reference method to suppress oceanic noise. Ian Ferguson found the induction across the sea to be a three-dimensional process governed by the large-scale shape of the basin. The seafloor impedances were strongly anisotropic, with the same axes at every site, and the component perpendicular to the sea's trend was attenuated towards Australia. Ted Lilley and colleagues read that as induced current inhibited by the resistive coasts of Australia and New Zealand. The arrows showed the strong coast effect of the Australian margin, peaking between the coastal site and the first seafloor site, a weaker coast effect over the submerged Lord Howe Rise, and a smoothly varying pattern between. The impedances at the central Tasman sites TP3 and TP4 gave a reliable conductivity profile for the Tasman Sea, with a highly conducting layer at 120 to 150 km, comparable to Pacific lithosphere of the same age. The impedance components from the seafloor sites set a lower bound of ten million ohm-square-metres on the integrated resistance of the oceanic lithosphere. At TP4, the site nearest the Gascoyne Seamount, Ferguson found about three times the conductance of its neighbour, spread over at least the upper 20 to 50 km, and tentatively attributed it to heat beneath an active seamount chain. The results were published by Filloux and colleagues in 1985, Lilley and colleagues in 1989 and Ferguson, Lilley and Filloux in 1990.

Ferguson's thesis tabulates the induction arrows at fourteen periods from 684 to 117,000 s at the six western sites, and at eleven periods at the three easternmost. Those arrows are the magnetic half of a magnetotelluric experiment; the horizontal electric field measurements made at seven of the sites are not represented here.

The served values are the nine seafloor sites' tipper transfer functions, referenced to the horizontal magnetic field of the Canberra Magnetic Observatory between 1,009 and 1,530 km away rather than to the local seafloor field, so they are inter-site transfer functions and not local tippers. They were first printed as induction arrows in Table A5.1, Appendix 5 of Ferguson (1988), the Australian National University thesis on the Tasman Project, and are served rotated to geographic north with the declinations of its Table 6.7 and scaled by the calibration factor K = 1.215 set out in its Addendum of 20 August 1988; the coordinates are the site positions of its Table 2.1, fixed by HMAS Cook's satellite navigation to better than 0.5 km. The thesis prints 95 per cent confidence limits on the transfer functions, but no uncertainties travel with the served values. The original recordings are not held; the thesis records the reduced time series as stored on magnetic tape at the Australian National University in 1988.

Data and downloads

Stations (9)

Contributors and organisations

Organisation

Australian National University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0