b'Minerals geophysics Minerals geophysicsmetal and iron ore deposits, geophysicalfields generated by energisation from logging of pattern drillholes can bea transmitter loop laid on the ground used for detailed classification of rocksurface. This can significantly extend the properties and correlation of units. Thereradius of investigation of the drillhole, will then be less need for an emphasis onand is an approach that has been used physical sample recovery for geologicalwith great success to guide future drilling logging, allowing a cheaper drillingwhere the targeted mineralisation is technique to be used. Geophysicalelectrically conductive.logging also has the advantage that petrophysical properties are measured inIn mineral exploration, drillhole-situ, reducing the risk of contaminationbased surveying may have particular from adjacent material, which is aapplication where surface conditions are concern in some drillhole samplingnot favourable for the application of a Terry Harveyprocedures. In the best case scenario,particular geophysical technique. With IP-Associate Editor for appropriate geophysical logging (withresistivity surveying, for example, where Minerals geophysicssuitable calibration controls and checksinjection of electrical current through terry.v.harvey@glencore.com.au of course!) may significantly reduce orhighly resistive surface material such as even replace the need to sample andsilcrete in Australia or desiccated thick assay ahead of mining. Considerablegravels in the Atacama Desert in Chile Drillhole geophysics investigative work on geophysicalcan be problematic, placing electrodes logging for mining was undertaken atin drillholes can by-pass these layers. Drillhole based geophysics can bethe Century Zinc Mine in NorthwestSimilarly, if conductive cover diverts most subdivided into two styles: logging andQueensland, for instance. electric current away from the underlying surveying. Logging seeks to measureresistive basement, strategically the petrophysical properties of the rocksDrillhole-based surveying, whereplaced drill-hole based electrodes will through which the drillhole passes.advantage is taken of sub-surfaceensure that a larger proportion of the Surveying uses the extra access providedaccess provided by drillholes, is quitetransmitted current penetrates the by the drillhole to extend and/orwidely used in mineral exploration andbasement and traverses the target zone. enhance the geophysical investigation ofto some extent in the mining industry.Sometimes these drillholes may not the surrounding terrain. Drillhole-based surveying may involvehave been drilled for this purpose, as either passive or, more commonly, activefor example in systematic RAB/Aircore Drillhole logging was pioneered in thegeophysical techniques. drilling programmes for basement oil and gas industry, where the need togeochemical sampling. However, with a maximise the return from very deep andIn drillhole-based surveying with passivebit of forward planning they may provide expensive drillholes is paramount. The oilgeophysical techniques, extra accessan opportunistic means of improving the and gas industry has developed and nowafforded by drillholes is utilised to extendeffectiveness of your survey.routinely uses sophisticated geophysicalthe range of sampling sites beneath the logging tools to measure or derive aground surface. In particular, magneticIn mining operations, where multiple range of petrophysical properties for rockor gravitational field measurementsdrillholes are available, hole-to-hole type characterisation, with particularmay be systematically recorded withinas well as hole-to-surface and hole-to-emphasis on reservoir-critical propertiesthe drillhole(s). This may provide criticalmining-void geophysical surveys can be such as porosity, permeability, fluidinput into inversion routines whichused to investigate intervening hitherto identification and quantification, etc. would otherwise have to rely solely onuntested regions. Specialised techniques above surface measurements. An ofteninclude radio frequency imaging (RIM) In mineral exploration, geophysicaloverlooked source of passive geophysicaland seismic tomography (which both drillhole logging is more likely tomeasurements is the drillhole orientationutilise computational concepts originally be directed at the petro-physicalsurvey where total magnetic fielddeveloped for medical imaging) to map properties that our ground and airborneintensity is recorded as part of the dipout target zones and structures in the geophysical techniques are targeting:and azimuth measurement process. region under investigation.density, magnetic susceptibility, resistivity, IP effect, gamma rayIn drillhole-based surveying with activeSo, if you have the luxury of a drillhole radioactivity, etc. Such logging may begeophysical techniques, drillhole accessor two in your exploration project area, aimed at verifying that a geophysicalis utilised to position either transmitterand it is appropriate, why not take the target has been intersected and thusor receiver sites (or both) beneath theopportunity to run drillhole geophysical explained, or perhaps providingground surface to improve depth orlogs to measure in situ petrophysical petrophysical property measurements forprecision of investigation. The mostproperties of the rock types making input into controlled inversion routines. widely used example of this approach isup the geological environment, or electromagnetic (EM) drillhole surveying,take advantage of improved access to In mining operations, particularly if thewhere an in-hole receiver probe is usedundertake a drillhole-based geophysical mineralisation is well ordered, such asto systematically sample along thesurvey to investigate wider, deeper and/in some bedded evaporite, coal, baselength of the drillhole the secondary EMor with more precision?43 PREVIEW DECEMBER 2020'