b'Minerals geophysics Minerals geophysicsreceiver dipoles required for the MIMDASfor highly conductive targets. Our targets, dipole-pole pole-dipole IP-resistivity arraywhile substantial in size, were expected to meant that, with the added input from abe much less conductive, and might only set of magnetometer coils, a simplifiedexhibit relatively subtle contrasts with the EMAP style of MT could also be surveyed. geo-electrical environment.Initial in-house processing of the MT dataWe reasoned that MT had potential, employed OCCAM 1D inversion. As withtrading off its deep penetration capability any 1D inversion routine used to generateagainst the debilitating effects of the 2D sections, the resulting resistivityconductive cover; we also felt there sections included artefacts relating to 2Dmay be some inherent capacity for (and 3D) geometry of the environmentdiscrimination of relatively subtle (see Figure 1). Sections constructedresistivity variations within the basement. Terry Harveyfrom 1D inversions represented anThe spatial resolution of the technique Associate Editor foradvance on those constructed fromwas a concern, but 2D MT inversion Mineralsgeophysicsapparent resistivities, but difficultiesroutines were now well established terry.v.harvey@glencore.com.au were created when these 1D inversionand therefore artefacts from 2D effects sections were interpreted too literally,should be less of a problem. We were also leading to mismatches with the geologyproposing full five component MT (three Evolution and disappointing drill-test results. Themagnetic plus two electrical) rather than Geophysical techniques evolve over timepotential depth of investigation of MTthe abbreviated version used in the past. and their mode of usage can change.might not have been oversold, but theGiven our previous unfortunate history The evolution of magnetotellurics (MT)validity of the finer detail in the resistivitywith the method, some selling of this as a geophysical method for mineralsections certainly was. MT lost credibilityproposal was required!exploration here in NW Queensland is awith our geologists.Initial trials showed that the new MT good illustration. Ten years later, mineral exploration inmethod was apparently capable of MT was traditionally viewed as theNW Queensland had expanded awaymapping quite subtle variations in electromagnetic method for deepfrom the areas of outcrop and shallowbasement electrical properties beneath investigation on a regional scale. Some(75 m) transported cover. We were nowsubstantial conductive cover. A station trial MT surveys were done over knowntasked with exploring for bulk tonnagespacing of 250 m produced sufficient detail ore deposits with a view to evaluatingtargets beneath 200 m or more offor our purposes; spatial discrimination at their regional setting, but MT wasnttransported cover. Magnetics and gravitythis scale seemed valid (see Figure 2).really seen as a method suited to detailedwere the obvious choices, and while systematic surveys. they did provide detectable anomalies,Overall, results were encouraging enough distinguishing these from the plethora offor MT to become an accepted technique The development of MIMDAS (MIMother similar features was proving difficult.in the exploration for bulk tonnage distributed acquisition system) in the lateWe needed another petrophysical propertytargets in this challenging environment. 1990s drove a resurgence in our use of IP- contrast and resistivity/conductivity offeredPast experience, a change in attitude, resistivity in NW Queensland, in part due tothat possibility. The difficulty was that theand need, drove the evolution of MT as a the relative cheapness and simplicity of thecover was not only very thick, but also verytool in our geophysical arsenal, facilitated sensors (stainless steel plates) comparedconductive (maxima over 1 mho/m). Highby improvements in instrumentation with those needed for electromagneticspower TEM with Squid sensors had hadand advances in data processing and (receiver coils). The string of collinearsome success in this environment, but onlypresentation.Figure 1.A 2000 MT OCCAM 1D inversion sectionsection extent 2 km x 1.1 km Figure 2.A 2012 MT 2D inversion sectionsection extent 4 km x 2.1 km49 PREVIEW DECEMBER 2021'