AusMT / surveys / CICADA97 induction array 1997-98

CICADA97 induction array 1997-98 GDS

Geomagnetic depth sounding survey · New South Wales · Australian National University

Part of the Australia legacy GDS collection

CICADA, Clarifying Induction Contributions to Aeromagnetic Data, was a programme of the Australian National University's Research School of Earth Sciences with the Australian Geological Survey Organisation and Flinders University to find out how electromagnetic induction in the Earth contaminates total-field magnetic mapping.

About this survey

CICADA, Clarifying Induction Contributions to Aeromagnetic Data, was a programme of the Australian National University's Research School of Earth Sciences with the Australian Geological Survey Organisation and Flinders University to find out how electromagnetic induction in the Earth contaminates total-field magnetic mapping. Earlier work had measured the coast effect down to periods of about 600 s. Magnetometers sampling every five seconds and resolving a tenth of a nanotesla carried the measurement into the micropulsation band. Variations that short cannot be removed by tie-line levelling or a single base station, and aeromagnetic surveying was by then crossing from land to sea. In September 1997 Adrian Hitchman, for his doctorate under Frederick (Ted) Lilley, laid a line of ten simultaneously recording three-component vector magnetometers from Barellen in the Riverina, about 360 km inland, across the Southern Tablelands through Coolac, the Canberra observatory, Bombay Bridge and Clyde Mountain to Currowan and Durras on the south coast, then offshore to Solo on the continental slope in 523 m of water and to two abyssal sites at about 4,800 m that also served the Study of Ocean Dynamo Action. Six of the land sites were those of the Tasman Project of 1983 - 84 and the Continental Slope Experiment that followed it, chosen so the campaigns could be compared where their periods overlap.

At the six land field sites the instruments were the Flinders University ring-core fluxgate magnetometers of Francois Chamalaun and Bob Walker, enhanced by Antony White and Graham Heinson, buried at each site and recording for up to fifteen days in each of two deployments. The seventh land station was the Canberra Magnetic Observatory. The seafloor instruments were of similar design in waterproof housings, deployed and recovered by CSIRO's RV Franklin. The Canberra observatory's Narod magnetometer supplied the horizontal reference for every transfer function, so the values relate each site's vertical field to the observatory's horizontal field. Magnitudes near and above unity at the coast are normal on this reference, not anomalous. Transfer functions were estimated over a common three-day window at the start of October 1997, at twenty periods from 6,840 s down to 10 s, and at eighteen periods at the two abyssal sites; Barellen had stopped recording early in that window, so its transfer functions come from the preceding day. The shortest period observed came down from about 600 s to about 10 s. The long-period coast effect reached even Barellen, and its influence extended to progressively shorter periods at stations nearer the coast. One negative excursion of the total field measured 9 nT inland at Coolac and about 87 nT at Solo. At periods below about a thousand seconds the arrows at Durras swung abruptly away from those at Currowan, only 15 km away, most likely from induction in Batemans Bay just to the south, and Durras behaved as a magnetic amphidrome, with the power in its total-field variations among the lowest on the line. A two-dimensional model of the margin fitted the long-period arrows well and failed at the coast below a thousand seconds, where the structure is plainly three-dimensional. For magnetic surveying that meant long-period variations of the total field are enhanced for hundreds of kilometres inland, while short-period patterns depend on features as small as a bay, so a single record of the temporal variation across a coastal survey area may be unobtainable. The arrows agreed with the Tasman Project's of fourteen years earlier where the periods overlap, and the amphidrome argument for Durras is set out in the thesis and repeated in the 2000 paper.

At Solo the magnetometer was set to the wrong sampling interval, which cost a minute of data every 390 seconds, and those gaps were filled from the Canberra observatory record that also serves as the reference. Hitchman found the response there below about 200 seconds hard to determine, and those bands should not be weighted as though they were sound.

The values served here are vertical-field transfer functions, real and quadrature, at twenty periods from 6826.7 down to 10.0 seconds, and at eighteen periods at the two abyssal sites. They were printed by Hitchman (1999), a doctoral thesis of the Australian National University (hdl:1885/147975), as Table B.2 on printed pages 169 to 172, already rotated by the author into the geographic frame using the AGRF95 declinations of his Table B.1 on printed page 168, and the standard errors printed beside them travel with every value. Station positions and elevations come from the thesis Table 6.2 on printed page 47, measured by GPS except at the Canberra observatory, whose position is taken from Hopgood and McEwin (1997), at Currowan and Durras, which were scaled from the Ulladulla 1:250,000 topographic map, and the three seafloor depths, which were read from the RV Franklin's depth sounder. The original recordings are not held.

Data and downloads

Stations (10)

Contributors and organisations

Organisation

Australian National University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0