{
 "schema": "ausmt-survey-metadata",
 "version": "0.1",
 "survey_id": "halls-creek-2000",
 "title": "Halls Creek Orogen array 2000",
 "dates": {
  "coverage": {
   "year_start": 2000,
   "year_end": 2000
  }
 },
 "abstract": "The Halls Creek Orogen is a north-north-east trending Palaeoproterozoic belt in the east Kimberley of Western Australia, dated 1915 to 1785 million years, read as the record of oceanic subduction followed by continental collision that welded the Kimberley and North Australian cratons together about 1820 million years ago, though the suture itself had never been located with strong evidence. Sheppard and colleagues proposed in 1999 that the orogen marked a palaeosubduction zone, dipping either west or east. Little geophysics existed there beyond the national gravity and magnetic grids: Francois Chamalaun and Cunneen had mapped the Canning Basin Anomaly some 200 km to the south-west in 1990, and the continent-wide AWAGS array of 1989 - 90 had found no anomaly near Halls Creek but had given some indication that the area was worth a closer look. In August 2000 seventeen three-component fluxgate magnetometers of the Chamalaun and Bob Walker design, instruments of the Flinders University fleet, were buried across the orogen and the adjoining Ord Basin between about 127 and 129 degrees east and 16 and 19 degrees south, Walker assisting with the collection, to test the electrical structure of the belt against the palaeosubduction hypothesis.\nThe buried units resolved about 1 nT over a range of plus or minus 80,000 nT and sampled every 60 seconds; they could record for three months but were left for about one, and all fifteen sites in the thesis table were in the ground together for some 26 days, so the vertical and horizontal fields could be compared directly across the array without modelling a changing source. Two stations, ELV51 and ALD40, came back with identical coordinates that could not be resolved, so the analysis used fifteen. That analysis came eight years later, when Andrew Roberts worked the recordings for an honours thesis at the University of Adelaide under Graham Heinson, from transfer functions computed with the robust remote-reference code of Chave and colleagues. At 200 and 1,000 seconds the induction arrows point towards one another across a linear conductor in the Ord Basin, east of the orogen and trending north-north-east. The thesis abstract puts it at about 300 km long, parallel to the orogen's grain but offset some 50 km east. Nothing in the basin's gravity or magnetics matches it, and the short imaginary arrows indicate a shallow source. Three-dimensional forward modelling put it within 5 km of the surface, which together with the absence of any correlating structure made fluid conduction through the porous Cambrian sediments of the Ord Basin the primary candidate; Roberts left open that a deeper conductor, graphite along fault planes with no surface expression, could still be the palaeosubduction signature the array had gone looking for. At 10,000 seconds the arrows swing to the south-west and the northern arrows turn west-north-west; Roberts read the first as the Canning Basin Anomaly and the second as the conductivity contrast at the coast, with the Indian Ocean 500 km away. A separate analysis of the daily solar quiet variation found the deep structure uniform across the array but unlike that beneath Kakadu observatory, a difference Roberts attributed to olivine anisotropy rather than temperature.\nAll seventeen transfer functions survive, two more than the thesis itself used, each with real per-period variances; sixteen carry thirty periods from 80 to 61,440 s and WOL46 carries twenty-four from 320 seconds. The array touches no earlier station and sits as a dense infill inside the regional footprint of the AWAGS array whose result had prompted it.\nThe served numbers are vertical-field transfer functions at 17 stations, each period carrying its own uncertainty, released in the Western Australian open-file package for survey registration R72000, Kimberley Halls Creek MT 2000, on 11 March 2021. They were computed with the robust remote-reference code of Chave et al. (1987) and Chave and Thompson (1989) from the 60 second three-component fluxgate recordings made between 7 August and 11 September 2000, as stated on page 16 of Roberts (2008), An electromagnetic investigation of the Halls Creek Orogen, honours thesis, University of Adelaide. The coordinates, elevations and occupation windows for 15 of the 17 stations are those printed in Appendix A, page 30, of that thesis, and the positions of the remaining two come from the files themselves. The original recordings are not held.",
 "subjects": [
  {
   "code": "370602",
   "scheme": "ANZSRC-FoR-2020",
   "label": "Electrical and electromagnetic methods in geophysics",
   "uri": "https://linked.data.gov.au/def/anzsrc-for/2020/370602"
  }
 ],
 "creators": [
  {
   "name": "Andrew Roberts",
   "name_type": "person"
  },
  {
   "name": "Graham Heinson",
   "name_type": "person"
  },
  {
   "name": "F.H. Chamalaun",
   "name_type": "person"
  },
  {
   "name": "R. Walker",
   "name_type": "person"
  }
 ],
 "organisations": [
  {
   "name": "Flinders University",
   "ror": "https://ror.org/01kpzv902",
   "roles": [
    "custodian"
   ],
   "primary_custodian": true
  }
 ],
 "rights": {
  "license": "CC-BY-4.0",
  "access": "open"
 },
 "relationships": [
  {
   "identifier": "https://ausmt.auscope.org.au/collections/australia-legacy-gds",
   "identifier_type": "URL",
   "relation": "IsPartOf"
  }
 ],
 "provenance": {
  "generated": "2026-09-09T06:47:27Z",
  "generator": "ausmt/extract.build_portal 0.2.1"
 }
}