b'Minerals geophysics The signal-to-noise ratio for the electricaldistort the secondary electromagneticwith a top of mineralisation at around field measurements is considered muchfield data for methods using natural fields. 400 m depth from the surface.higher than the signal-to-noise ratio for the magnetic field, which is one of theAn excellent example of the advantagesHistorically, several active source advantages of the MobileMT system. Theof MobileMT technology was acquired atairborne EM surveys have been flown in electric component of MobileMT is oneCoda Minerals Elizabeth Creek project,South Australia, including VLF (very low of the features that distinguish it from itslocated in the Olympic Dam district,frequency) and frequency domain and predecessors. which is a belt of Cu-enriched basementtime domain surveys. Due to the presence of the Gawler Craton, South Australia.of highly conductive cover, the success The technical solutions realised in theThe districts Mesoproterozoic and olderof these methods to aid in the location airborne MobileMT technology providedcrystalline basement is overlain by a thickof ore deposits has been limited. The exploration capabilities that overcome thesuccession of Neoproterozoic, Cambrian,Gawler Province is an area approximately limitations of other AEM systems based onand younger sedimentary basin rocksthe size of France, with very little fresh different principles, including those usingknown as the Stuart Shelf. The copper- rock outcrop. As such, understanding the controlled primary field sources. cobalt deposits (fine-grained sulphides)subsurface relies on information from are hosted by flat-lying, undeformeddrill holes and non-invasive geophysical The specific technical advancements thatLate Proterozoic sedimentary rocksmethods. Despite the conductive cover/enable exploration advantages include: deposited on the Stuart Shelf. Theseregolith, which limited the success of Measurement of magnetic fieldplatform sediments are known as theother airborne EM technologies, the variations with three orthogonal coilsCover Sequence, they unconformablyMobileMT data successfully mapped (total field). This provides sensitivityoverlie the complexly deformed andthe strata-bound zones with related to any direction of geoelectricalmetamorphosed igneous rocks of themineralisation at depth.boundary, from horizontal to vertical. Archaean basement. The Emmie Bluff prospect in the north of the ElizabethFigure 2 shows the resistivity section Measurements are obtained over threederived from 1D inversions with the decades of frequency, from 19 Hz toCreek project is an underground target 26 kHz. This allows imaging of near-surface structures and those at 1 km depth, depending on the conductance of the geologic environment.The frequency range is divided into 30 windows that provide high in-depth resolution and a good opportunity for data selection, depending on cultural noise sources, natural EM field signal, and exploration goals.The high sampling rate of the airborne data and the base station data result in bias-free and denoised data.The main advantages of the natural field method, in general, include:The depth of investigation consistently exceeds the capabilities of systems with controlled sources.The method is sensitive to conductors and resistivity differences in the range of thousands and tens of thousands of ohm-m, which is challenging for existing time-domain systems. At the other end, for time-domain systems, the response from superconductors (hundreds and thousands of Siemens) is not visible in the off-time channels of the dB/dt stream. For the natural field EM principle, it is not a limitation.There is no critical dependence on the terrain clearance of the system. This allows for less aggressive flying in rugged terrain conditions, improving the overall safety of data acquisition.IP and SPM parasitic effects are inherent to impulse time-domain systems thatFigure 2.MobileMT apparent conductivity profiles (a); resistivity section from the 1D model with contours badly influence the inductive response.of the data inversion logarithm of sensitivity (b); resistivity section from the 2D model (c) along a survey line These effects are not formed and do notfrom the Emmie Bluff prospect over a known mineralisation.35 PREVIEW AUGUST 2023'