AusMT / surveys / Southern Eyre Dense Array 1980-84

Southern Eyre Dense Array 1980-84 GDS

Geomagnetic depth sounding survey · South Australia · Flinders University

Part of the Australia legacy GDS collection

Antony White and O.

About this survey

Antony White and O. W. Polatajko's coast-effect profiles inland from Port Lincoln and Elliston in 1977 and 1978 had found the expected coast effect severely disrupted by what looked like a strong concentration of telluric current running inland between the two lines. In the summer of 1980 to 1981 White and his doctoral student Peter Milligan of Flinders University recorded at five coastal sites between Elliston and Port Lincoln to locate where that current met the coast. Near Coffin Bay the vertical field was severely attenuated, the phase changed across the zone and the horizontal east component was much enhanced, which put the axis of the current there. It appeared narrow, shallow and well within the crust of the Gawler Craton, one of the few such zones reported anywhere across Precambrian crystalline basement. The main experiment followed from July 1982 to January 1983, with a last line late in 1983.

Nine fluxgate magnetometers built at Flinders, four three-component and five single-component, were buried with their sensors about a metre down and recorded unattended for a month at a time onto chart recorders at a resolution of about one nanotesla. They were deployed as sub-linear arrays parallel to the south-west coast and moved inland to the north-east at the end of each phase, thirty-seven recording localities in all, most of them referenced to a fixed station at Port Lincoln whose horizontal field could be taken as normal. The charts were digitised and transfer functions computed for periods between 900 and 7,200 s. They delineated a linear zone of very high conductivity trending inland just east of north from the south coast between Coffin Bay and Sleaford. Above it the 1,800 s east component was more than twice its value 40 km away, and the vertical component was severely attenuated with a large phase change across the zone. The response peaked near a period of 1,800 s, and the daily variation and its first two harmonics were not anomalous. Line-current and current-channel models bounded the body at less than about 20 km deep and less than 25 km wide. Milligan read it as a steeply dipping fault or fracture zone holding saline pore water, or as a graphitic bed of the Hutchison Group, coincident with a gravity gradient and a line of shallow earthquake epicentres at what appears to be the boundary between Archaean and Proterozoic crust, and he judged its anomalous fields almost unique on a global scale. White and Milligan published the discovery in Nature in 1984. The later literature calls the feature the Eyre Peninsula Anomaly, and it became one of Australia's best-known crustal conductors. The 1988 northern extension traced it up the peninsula. S. K. Kusi's arrays of 1993 to 1995 followed it 160 km to its termination at the Polda Trough and put its conductance between 3,000 and 10,000 siemens. SWAGGIE carried it offshore to the edge of the continental shelf in 1998.

Milligan's thesis prints the transfer functions at six periods from 720 to 7,200 s, with one standard deviation on each value. Nine of those printed rows carry no data, so some stations have fewer than six periods. The thirty-seventh locality, Edillilie, recorded declination only and so has no vertical-field response.

The served values are the vertical-field transfer functions printed in Milligan (1989), Appendix D, Table D.1, at six periods from 720 to 7,200 s (12 to 120 minutes in Table D.1). They reached this collection through Liejun Wang's Australian Electromagnetic Database, whose entries for this array were computed from that table's printed components. The thesis prints a one standard deviation on each component and the database did not carry them, so no uncertainties travel with the values served here, and the coordinates are the positions printed in the thesis's Table C.2, Appendix C. The original recordings are not held, and nothing on record says whether the paper chart records survive.

Data and downloads

Stations (36)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0