AusMT / surveys / SWAGGIE seafloor GDS 1998

SWAGGIE seafloor GDS 1998 GDS

Geomagnetic depth sounding survey · South Australia · Flinders University

Part of the Australia legacy GDS collection

By 1998 the Eyre Peninsula Anomaly had been traced on land since the early 1980s.

About this survey

By 1998 the Eyre Peninsula Anomaly had been traced on land since the early 1980s. Geomagnetic arrays found it first, ten further arrays followed to 1989, and sixty magnetometers recorded between December 1993 and March 1995. Two-dimensional inversions of that later work gave a conductance of 3,000 to 10,000 S, increasing southward, with an indication that the anomaly continued offshore. Land observations alone could not decide between the candidate causes, saline fluids in fractures connected to the ocean, graphite in a shear zone, or ocean currents channelled into the continent. SWAGGIE, the Southern Waters of Australia Geoelectric and Geomagnetic Induction Experiment, took the question to sea. It was a joint project of Flinders University, the Scripps Institution of Oceanography, the Australian Geological Survey Organisation and the Australian National University, funded by the Australian Research Council, with three aims: to map the anomaly across the continental shelf by geomagnetic depth sounding, to find its depth and width by magnetotellurics, and to define the structure of the margin across the Duntroon Basin. The four stations here are one part of that experiment, the seafloor sites that recorded the magnetic field alone.

Twenty seafloor instruments and twenty-three land magnetometers were used. The seafloor instruments were laid from the ORV Franklin in early April 1998 along three lines: an inner shelf line in 60 to 100 m of water, an outer shelf line above the Duntroon Basin in 117 to 131 m, and a line down the continental slope from 512 to 4,580 m to see the transition from continent to ocean. Fifteen low-frequency magnetotelluric instruments from the Flinders and Scripps pool and three simple Australian National University magnetometers stayed on the bottom for the two months, spaced about 25 km apart to span the anomaly's possible width, while two high-frequency units roved between twelve short sites for two days at a time; the paper counts thirty seafloor sites in all. Induction arrows from the recovered magnetic records were computed by robust remote-reference processing, usually against the nearest magnetometer, and they tend to point towards the anomaly at every site that returned magnetic data, with the outer shelf arrows smaller over the conductive basin sediments and a rotation towards the anomaly at shorter periods that the land arrays had also seen. From them Popkov and his co-authors concluded that the Eyre Peninsula Anomaly runs unbroken to the edge of the continental shelf, that its vertical-field response exceeds that of induction in five kilometres of ocean so that its conductance must be at least 15,000 S, and that its symmetrical response marks a thin, near-vertical zone. On the inner shelf they placed its axis between sites FMT2 and M1, two of the four stations served here, and on the outer shelf between SMT4 and FMT5. The anomaly was far clearer in the depth-sounding arrows than in the magnetotelluric impedances, and its cause was left open between hot saline fluids and graphite.

Of the eighteen long-deployment seafloor positions tabulated in the paper, fifteen survive as transfer functions: one instrument was lost, one recovered no data, and one magnetic-only site beside FMT7 has no surviving transfer function. Eleven of the fifteen recorded electric fields and belong to the experiment's magnetotelluric half; the four served here are the sites that recovered the magnetic field alone. M1 and M3, on the inner and outer shelf, were the ANU's simple seafloor magnetometers and never carried electrodes; FMT2 on the inner shelf and FMT7 at 4,580 m on the slope were Flinders magnetotelluric instruments whose electric channels returned nothing. Each carries a vertical magnetic-field transfer function at eighteen periods, from 80 s at FMT2 to 163,840 s at M3. FMT7 starts at 160 seconds; the ocean above it damps the short periods, and on our reading that screening shows in the choice of sampling. At 1,260 s, by our own measurement, its arrow is less than a fifth of the one at the top of the slope. The onshore half of the experiment, twenty-three land magnetometers of which twenty-one survive as transfer functions, appears in this collection as the SWAGGIE land array 1998.

The four served transfer functions come from the University of Adelaide geophysics collection, computed from the SWAGGIE seafloor magnetic recordings of April and May 1998. Each carries the vertical magnetic field transfer function at eighteen periods with a variance for every period, so the uncertainties travel with the values. The coordinates and water depths match Table 1 on page 189 of Popkov, White, Heinson, Constable, Milligan and Lilley (2000), Exploration Geophysics 31, 187 to 191, at all four sites. The original recordings are held, as the raw seafloor time series for the campaign in the University of Adelaide geophysics collection.

Data and downloads

Stations (4)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0