AusMT / surveys / SAMAS 1995

SAMAS 1995 GDS

Geomagnetic depth sounding survey · South Australia · Flinders University

Part of the Australia legacy GDS collection

By the mid 1990s airborne magnetometers could resolve a thousandth of a nanotesla, and the spatial inhomogeneity of the fluctuating field had become the principal factor limiting the integrity of high-resolution aeromagnetic data, because the standard correction subtracts the record of a single base station and assumes it stands for the whole survey area.

About this survey

By the mid 1990s airborne magnetometers could resolve a thousandth of a nanotesla, and the spatial inhomogeneity of the fluctuating field had become the principal factor limiting the integrity of high-resolution aeromagnetic data, because the standard correction subtracts the record of a single base station and assumes it stands for the whole survey area. Jonathon Whellams' doctoral work at Flinders University, under Francois Chamalaun, set out to measure that spatial inhomogeneity, first with the continent-wide AWAGS array and a chain across the Canning Basin anomaly, then over ground with no known conductivity anomaly at all. SAMAS, the Synchronous Aeromagnetic-Magnetometer Array Study, was the third of these. In April and May 1995 twenty-five Flinders digital fluxgate magnetometers of the AWAGS type were laid out on a regular grid across the south-east of South Australia beneath more than 39,000 line-kilometres of the South Australian Exploration Initiative's area B30 survey, then being flown at 400 m line spacing from Naracoorte. A twenty-sixth instrument at the university's Tennyson Woods Observatory near Adelaide was the remote reference. The array was also designed as a conventional depth-sounding experiment. Its sites were placed with regard to the strike of the shelf edge, and several were sited to re-occupy the stations of Antony White and O. W. Polatajko's earlier coast-effect profile so that the two studies could be compared directly, though the sites as installed stand near the earlier ones rather than on them.

The grid, held by GPS to within a kilometre or two and covering about 15,900 square kilometres, ran at about 40 km spacing in the north across the Murray Basin and 25 km in the south over the Padthaway Ridge and the northern Otway Basin, where inhomogeneity was expected to be strongest and where petroleum exploration for the Penola and Robe troughs was under way. The instruments, buried a metre down and sampling every 20 seconds, recorded for five to six weeks through a magnetically quiet trough of the solar cycle, and every one came back intact with a complete record. Two further base stations run by the survey contractor, one at Naracoorte airport and another about 5 km north of it, brought the ground total to 28. Transfer functions were estimated with the robust method of Egbert and Booker from twenty days of data. The induction arrows showed an uncomplicated coast effect, shortening inland and pointing towards deep ocean. They agreed well with the earlier profile and confirmed that the array area was free of major induction anomalies. Even so, the aeromagnetic data still suffered from spatial inhomogeneity. Errors of 5 nT appeared even in airborne data recorded within 50 km of a base station, and during disturbances the coast effect produced total field gradients of 5 nT per 10 km across the survey area. Across 5,299 line intersections the standard deviation of the mis-ties fell from 5.6 nT with a single base station to 4.7 nT with the array, and Whellams concluded that multiple base stations were worth deploying. The array's one new conductivity finding was a shallow arcuate anomaly that appeared only during rapid fluctuations, along the northern margin of the Otway Basin where its conductive sediments meet the resistive Padthaway Ridge. Earlier work in the area gave no sign of it, which Whellams put down to the band of periods that work considered.

His induction arrows survive at seventeen periods, from about 390 to 27,300 s; he printed four of them in the thesis.

The served numbers are Jonathon Whellams' induction arrows at seventeen periods from 389 to 27,307 s (6.48 to 455.11 minutes), held under reference code 12, station ids 467 to 492, in Liejun Wang's Australian Electromagnetic Database. Whellams (1996) prints them at four of those periods in Appendix B, Tables B.1 to B.4, at 853, 1,807, 3,964 (printed there as 60.06 minutes, on AusMT's reading a printing defect) and 6,827 s, and the printed and the held values agree row for row; the other thirteen periods appear in no publication. No uncertainties were printed with the arrows and none travel with the served values. The station names and coordinates are those of Whellams (1996), Table 5.1, and the original 20-second recordings are not held.

Data and downloads

Stations (26)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0