AusMT / surveys / SWAGGIE land array 1998

SWAGGIE land array 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 mapped on land by ten geomagnetic depth-sounding arrays between 1984 and 1989, and S.

About this survey

By 1998 the Eyre Peninsula Anomaly had been mapped on land by ten geomagnetic depth-sounding arrays between 1984 and 1989, and S. K. Kusi's inversions had found a conductance of 3,000 to 10,000 S rising southward with a hint of an offshore extension. Land studies alone could not say which cause was right. The candidates were saline fluids in fractures connecting to the ocean, graphite in a shear zone, or current channelled from the sea into the continent. SWAGGIE, the Southern Waters of Australia Geoelectric and Geomagnetic Induction Experiment, was mounted to answer that question. Flinders University ran it jointly with the Scripps Institution of Oceanography, the Australian Geological Survey Organisation and the Australian National University, with Australian Research Council funding. The declared aims were to determine the strike and depth of the conductor, to follow it across the continental shelf and slope with seafloor magnetotelluric and magnetometer instruments deployed from the ORV Franklin, and to define the structure of the margin across the Duntroon Basin. Twenty-three magnetometers made up the onshore half, spread across the southernmost peninsula and recording while the marine instruments worked offshore, to resolve the anomaly beneath the land.

The land instruments resolved one nanotesla and sampled every ten seconds at about 5 km spacing in a two-dimensional cluster some 35 km across, between about 135.39 and 135.78 degrees east and 34.66 and 34.93 degrees south, sixteen switched on together on 12 April 1998 and five more on 3 May, most recording for three to four weeks. The served transfer functions run to twenty periods, from 40 to 27,307 s at all but one site; MAB alone spans 22.8 to 15,565 s. All of the paper's results come from the marine instruments. The anomaly appeared continuous to the edge of the continental shelf, and its vertical-field response exceeded that of induction in 5 km of ocean, which put its conductance at 15,000 S or more in a narrow, near-vertical zone. Offshore its axis lies between the seafloor sites FMT2 and M1 across the inner shelf and between SMT4 and FMT5 on the outer shelf. It showed far more clearly in the depth-sounding arrows than in the magnetotelluric impedances. The land array recorded through the campaign to resolve the anomaly beneath the southern peninsula. Igor Popkov and his co-authors left the cause open, saline fluids or graphite, and said plainly that their results were preliminary. L. Loader re-occupied the structure in 2018 with a 39-site broadband survey for an honours thesis at the University of Adelaide.

Two of the twenty-three instruments ran for a day or less and produced no response; their positions are not known. Five of the sites stand within about a kilometre of localities used by Antony White and Peter Milligan on the Eyre Peninsula in the 1980s, four of them within two kilometres of stations of the Southern Eyre Dense Array of 1980 to 1984. On AusMT's reading of the files, the transfer function at Fishery Bay (FBY) reaches a magnitude above three, more than twice the next largest on the array; it is well determined and smooth in period, but it should be checked before the array is used for arrow amplitudes. The seafloor half of the same campaign appears separately as SWAGGIE seafloor GDS 1998.

The served numbers are real vertical-field transfer functions at twenty periods, each with its own per-frequency variance, so the uncertainties travel with the values. Popkov and his co-authors (2000), Exploration Geophysics 31, 187 to 191, plot induction arrows for the marine instruments only and tabulate no land site, so these values were never printed. On AusMT's reading they are the campaign's own remote-reference processing of the ten second magnetometer series, by the method the paper sets out on page 189; the files were archived at the University of Adelaide, and because no land station table was published the positions come from the files themselves. The original recordings, ten second time series for twenty-one of the twenty-three sites, are held in the same University of Adelaide archive.

Data and downloads

Stations (21)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0