AusMT / surveys / Southern Adelaide Geosyncline Pilot 1981-82

Southern Adelaide Geosyncline Pilot 1981-82 GDS

Geomagnetic depth sounding survey · South Australia · Flinders University

Part of the Australia legacy GDS collection

From October 1981 Antony White and Peter Milligan of Flinders University worked twelve magnetometer sites, occupied in turn over the following eight months, along two intersecting profiles across the southern Adelaide Geosyncline and the younger sedimentary cover to its east.

About this survey

From October 1981 Antony White and Peter Milligan of Flinders University worked twelve magnetometer sites, occupied in turn over the following eight months, along two intersecting profiles across the southern Adelaide Geosyncline and the younger sedimentary cover to its east. One line ran inland from the university's geophysical vault south of Adelaide, perpendicular to the edge of the continental shelf, a cross-line ran over the Murray Basin straddling the River Murray, and four sites lay north-east of Burra in the northern Mount Lofty Ranges. The nine fluxgate magnetometers were being tested under field conditions before work in more remote country. The sites were also chosen to test two ideas. The southern Geosyncline, with its complex history, its seismicity and its high heat flow, was expected to concentrate telluric current. And the Flinders Anomaly found by the 1970 array was thought to swing south-west rather than continue south, as White and O. W. Polatajko had proposed.

Four of the nine instruments recorded three components and five the vertical component only, buried against temperature changes and recording on paper chart recorders. Recorder troubles and quiet magnetic conditions stretched the campaign to June 1982, and one site in the Mount Lofty Ranges had its recorder damaged by heavy rain. Because the horizontal variations proved identical across the area, the transfer functions could be referenced to whichever three-component site was recording, with Bower the common site linking the southern and northern phases, and active stretches of six to twenty-seven hours were digitised at two-minute intervals. The southern Geosyncline showed surprisingly little lateral variation in conductivity, despite geological and geophysical evidence pointing the other way. At the longer periods the vectors show a strong coast effect, and at the shorter periods they rotate westward towards a current concentration lying west of every site. There was no sign of a current path through the sites themselves, so the Flinders Anomaly does not continue south to the east of the Mount Lofty Ranges. Its south-western extension was located more precisely, along the axis White and Polatajko had drawn. The hypothetical event contours suggest the currents run on into Spencer Gulf, which the authors said a closer study at the head of the gulf would be needed to confirm, and the results confirmed White and Polatajko's proposal that the crystalline Houghton Anticlinal Zone acts as a barrier deflecting the northern currents to the south-west. Francois Chamalaun's 1985 array in the central Flinders Ranges confirmed the south-westerly trend. Milligan's later Eyre Peninsula array, run with the same instruments once most of their faults had been fixed, found a zone of highly conductive material under the ancient craton of southern Eyre Peninsula. The younger geosyncline has none.

Riverton's record is unreliable by the thesis's own account, its power supply defective and its traces erratic at some periods, so the small azimuth difference the paper reports there should not be read as a finding.

The twelve tipper transfer functions served here were taken from Table D.2 in Appendix D of Milligan (1989), Geomagnetic variations in South Australia: the Eyre Peninsula Anomaly, the Flinders University doctoral thesis in which this pilot is Area A; the table prints in-phase and quadrature vector lengths and azimuths for the twelve stations at five periods from 7,200 to 900 s (the thesis's 0.5 to 4.0 cycles per hour); the azimuths are in the magnetic frame on AusMT's reading. It prints no uncertainties, so none travel with the served values. The site positions and deployment dates are those printed in Table C.1 of the same thesis, with two degree-digit misprints, Riverton's longitude and Canegrass's latitude, corrected against the neighbouring sites and Figure 2 of White and Milligan (1985). The original recordings are not held: the paper chart records and the two-minute digitisations made from them survive only as figures in Appendix B of the thesis, and the analysis drawn from them is published in White and Milligan (1985), Journal of Geomagnetism and Geoelectricity 37, 715 to 728.

Data and downloads

Stations (12)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0