AusMT / surveys / Black Hill 2005-06

Black Hill 2005-06 BBMT

Magnetotelluric survey · South Australia · Geological Survey of South Australia

A broadband magnetotelluric survey over the Black Hill area of South Australia, on a footprint running from 67 to 157 km north east of Adelaide with its centroid about 100 km out, acquired by Quantec Consulting Inc.

About this survey

A broadband magnetotelluric survey over the Black Hill area of South Australia, on a footprint

running from 67 to 157 km north east of Adelaide with its centroid about 100 km out, acquired by

Quantec Consulting Inc. for Inco Resources (Australia) Pty Ltd as project QCL-414 in two

campaigns, 19 October to 14 December 2005 and 17 January to 15 February 2006. The target was

magmatic nickel, copper and platinum group element mineralisation in the Black Hill Complex, a

sulphide saturated mafic body considered prospective for Voisey's Bay style deposits, lying along

a cross fault that cuts the structural corridor between the Adelaide Fold Belt and the Kanmantoo

Fold Belt. Magnetotellurics was chosen because Murray Basin cover is thick and conductive enough

that an earlier airborne electromagnetic survey was judged effective over only about a third of

the area.

Soundings were remote referenced tensor measurements on an L shaped array with 100 m electric

dipoles and three induction coils per site, drawn from a fleet of six EMI coils of models BF-4,

BF-7 and BF-21, recorded on Quantec RT-130 loggers at 1000 samples per second for 2 to 14 hours

per site. The report's text describes the dipoles as Ex east and Ey north, but the delivered

channel geometry says otherwise in all 229 delivered files: Ex runs east and Ey runs south, with

the magnetic pair at 90 and 180 degrees. That layout is what makes the acquisition frame a clean

quarter turn from the standard one, and it is the geometry the rotation below relies on. Two

loggers ran simultaneously for the site and its remote reference, synchronised by GPS. Station

spacing was a nominal 500 m with infill to 250 m on some lines. Processing was full waveform time

series to the frequency domain with QGL QuickLay 2.25.28, and the transfer functions were exported

by Geotools 2.3.

This package covers all 17 lines the survey ran: 409 transfer functions over 406 distinct sites

plus three repeat occupations on line 8, on 22 to 63 periods with a median of 35, spanning roughly

351 seconds to 239 Hz. It is assembled from two sources of very different standing, and every

station declares in its own file which of the two it is, under the key

ausmt.station.tf_provenance.

229 stations on lines 2, 4, 8, 10, 19, 20, 21, 22, 23, 24, 27 and 28 are contractor-processed.

They are served as a derived rendition of the custodian's delivery, which is cross-spectra stored

in an acquisition frame with the electric x axis east, declared by ROTSPEC 90 and by sensor

azimuths of 90 and 180. What is served is the impedance tensor computed from those spectra and

rotated into the standard magnetotelluric frame, x north and y east, by the transformation

Z_std = R Z R transpose with theta the negative of the declared frame. Each file states its source

file and that file's sha256, the rotation applied, and the convention by which the variances were

carried through. These files carry a complete tensor: diagonal and off diagonal impedance with

variances, and tipper with variances, at every frequency.

180 stations on lines 11, 12, 16, 25 and 26 are figure-recovered. The custodian never delivered

them as data and no transfer function for them survives in any SARIG system; their only surviving

form is the sounding curve figures in the contractor's reports, 52 of them in the report delivered

with the data and 128 in the EL 3122 annual technical report. Those figures were read from the

reports' own vector drawing operators, with no raster image and no character recognition, and

rebuilt as transfer functions. The method was scored first against the 219 stations that appear

both as a figure and as a delivered file, and agrees with them to a median of 0.0010 of a decade

in apparent resistivity and 0.05 of a degree in phase, with no systematic sign to the disagreement

in either. These 180 stations carry the off diagonal impedance and the tipper only: the report

figures plot no Zxx and no Zyy, so they have no diagonal impedance, no phase tensor and no

dimensionality call, and none has been inferred. Their uncertainty is the contractor's stated

survey wide data accuracy rather than a per station measurement, and they carry no tipper

uncertainty because the figures draw none. Every absent value in those files is written as the

literal nan, because the format's own EMPTY marker is read back as a measured zero by the common

reader of this format; nan is not a token the SEG EDI standard defines, and a strict third party

reader may reject it or mishandle it. No value in the 229 contractor-processed files is nan.

Data origin. The 229 contractor-processed transfer functions are SARIG's delivery for electrical

survey 22, BLACK HILL, 229 sites on tenements EL 3304 and EL 3305. Two identifiers are needed

together. The dataset record Magnetotelluric and geomagnetic depth sounding surveys,

https://pid.sarig.sa.gov.au/dataset/mesac1113, is what magnetotelluric transfer functions are

published under statewide and is the licence of record; all 229 of those stations sit in its

electrical survey layer, but it names no downloadable resource for this survey, because SARIG

generates this delivery on demand rather than attaching it. The survey itself is documented in

South Australian open file company report envelope 11269,

https://pid.sarig.sa.gov.au/document/mesac21595, and the contractor's logistics report is

delivered with the data. The 180 figure-recovered transfer functions come from Appendix B,

Sounding Curves, of that same logistics report for line 11, and from Appendix B of the logistics

report bound inside EL 3122 open file envelope 11133,

https://pid.sarig.sa.gov.au/document/mesac25554, for lines 12, 16, 25 and 26. Release is open file

by the Geological Survey of South Australia.

Data and downloads

Stations (409)

Contributors and organisations

Organisation

Geological Survey of South Australia

Licence

CC BY 4.0

Instruments

Quantec RT-130
EMI BF-4
EMI BF-7
EMI BF-21

Processing

QGL QuickLay 2.25.28

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0