AusMT / surveys / Eyre Peninsula Northern Extension 1988

Eyre Peninsula Northern Extension 1988 GDS

Geomagnetic depth sounding survey · South Australia · Flinders University

Part of the Australia legacy GDS collection

The southern Eyre Peninsula arrays had mapped the Eyre Peninsula Anomaly for about 100 km inland from the south coast, trending just east of north along the boundary between Archaean basement to the west and the Proterozoic Hutchison Group to the east.

About this survey

The southern Eyre Peninsula arrays had mapped the Eyre Peninsula Anomaly for about 100 km inland from the south coast, trending just east of north along the boundary between Archaean basement to the west and the Proterozoic Hutchison Group to the east. In 1988 Peter Milligan, of the Bureau of Mineral Resources by the time the work was published, with Antony White and Francois Chamalaun of Flinders University, set out to map its northward continuation with twenty-two of the Flinders digital fluxgate magnetometers, on the hypothesis that the conductor's axis should follow the Precambrian structural trends and swing to the north-west in the north of the peninsula. The work also had a practical aim. A conductive zone of this kind adds errors of several nanotesla to aeromagnetic surveys, and closely spaced magnetometer arrays can be used to predict and remove those errors; the Gawler Craton is a potential source of major economic minerals, and where the conductor runs bears on the tectonic development of the region.

The instruments, buried upright in augered holes and recording three components, were deployed in three stages from February 1988 at 54 sites across the north-eastern peninsula. Recording began on four southern lines at one-minute sampling, and when those were finished half the fleet moved to two northern lines, both stages referenced to Cowell as a common base. The third stage was a closely spaced array at ten-second sampling, deployed by personnel from the South Australian Department of Mines and Energy. Profiles for the first two stages were set perpendicular to the conductor's known orientation, at about 10 km between sites and 20 km between lines. The published magnetograms are drawn from a storm on 23 February 1988. Hypothetical-event contours and transfer-function vectors showed the anomaly continuing northward, swinging east and then turning north-north-west in conformity with the structural trends, with its current axis centred near Cornubia Dam. Across the axis the vertical and total-field components changed in amplitude and phase, and along it the horizontal components were enhanced. The anomalous-to-normal ratios were smaller than in the south, which Milligan, White and Chamalaun read as the induced currents either driven deeper or becoming more diffuse, and they left the northern extent to be mapped. S. K. Kusi's arrays of 1993 to 1995 re-analysed these records together with the earlier southern ones and followed the conductor to its northern end at the Polda Trough. The modelled conductance near Cornubia Dam is about 10 000 S for the top 10 km, at the upper end of the 3000 to 10 000 S range that study reported. In that reanalysis the northern arrows showed no clear continuation of the conductor, only a boundary between a resistive region in the west and a more conductive one in the east, with the currents diffusing north of 33.6 degrees south. The 1998 work therefore weakened the conclusion the 1989 paper had drawn from these records.

Of the 54 sites occupied in 1988, records from only seventeen, Cowell included, are known to survive. The sixteen served here are the values printed by S. K. Kusi in 1996. Cowell was kept as the continuous reference through the 1993 to 1995 arrays. The responses from the remaining sites were published only as vectors drawn on Figure 2 of the 1989 paper.

The served numbers are vertical magnetic field transfer functions, real and imaginary parts, at six periods from 320 to 10,200 s, with a standard deviation printed for each value. They were printed in Appendix III, Transfer functions and Parkinson arrows, of Kusi (1996), PhD thesis, The Flinders University of South Australia; on AusMT's reading these are Kusi's own reprocessing of the February 1988 records against the horizontal fields at Cowell, by the Egbert and Booker robust method, and those printed standard deviations travel with the values served here. The station names and coordinates are the positions printed in Table 3.1 of the same thesis. The original recordings are not held: Appendix II of that thesis, p. 202, records the February 1988 field data as archived on an optical disk kept in 1996 by the Marine Geology and Geophysics section of the School of Earth Sciences at Flinders University, and the primary publication, Milligan, White and Chamalaun (1989), Exploration Geophysics 20, 187 to 190, prints no data tables.

Data and downloads

Stations (16)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0