b'Minerals geophysics Minerals geophysicstransported cover (for electromagneticsMore rarely, you may have the luxury and IP-resistivity), etc.? of a potential underspend. Have you compromised too much? Does reducing And what about the geologicalthe risk of missing something warrant environment itself? Is it benign oran upgrade in survey parameters and malign? Graphitic, carbonaceous andan increase in survey expenditure? Are pyritic rock types can be a graveyardthere other concomitant benefits to be for electrical and electromagnetichad in spending more, such as tenure exploration. Are there rock types thatexpenditure requirements? Could you will complicate magnetic or gravitysensibly increase the extent of the survey responses, rendering the expected target(access permitting); would interpretation anomaly just one of a multitude of similarbenefit from more extensive coverage? style targets? Are there other adjacent areas worth Terry HarveyMaybe youll have to do subsidiaryinvestigating? Can you take advantage Associate Editor forsurveys to fine tune the interpretation.of this survey to commission additional MineralsgeophysicsPerhaps passive seismic to map depth towork, perhaps undertaking other small terry.v.harvey@glencore.com.au basement to help discriminate betweensurveys which by themselves would be basement topography effects anduneconomic?Factors in survey design genuine in-basement gravity responses,We once commissioned a helicopter or IP-resistivity surveys to identify whichelectromagnetic survey with lines of the potential field anomalies havespaced closer than that normally How do you go about undertaking asulphidic source material. considered necessary for 100% geophysical survey? You identify whichThen there is the access for the surveycoverage. We considered this would technique and what survey parametersitself. Are there environmental issuesgive us more confidence to site drill-you need to employ, estimate howthat limit what you can do in the way ofholes directly from the airborne survey much it is going to cost, decide onground preparation and track clearing? Itresults without the luxury of ground the contractor and the timing, andmay not be realistic to plan a high powersurvey confirmation, which would commission the work. unfortunately, it isground electromagnetic survey if accesshave been difficult in the rugged rarely that simple! restrictions limit you to hand-carryingtopography of the survey area. A everything for kilometres just to get onsecondary benefit was that the extra Heres the scenario. The explorationto site. You may have to use airborneline kilometres required for the closer team has identified an area of interestaccess to the site, or look to using anline spacing took us over the threshold and come up with a target style. Thisairborne method. for a lower per line kilometre rate. We should give you some parameters togot significantly more data for not work with. First of all, what is the relevantWhat about man-made features?much more expenditure. Yes, we did petrophysical property that you canPower lines, earthed fences and thespend more on the survey, but we got utilise to identify your target? Then therelike can play havoc with electrical andan arguably more appropriate product. is the expected size of the target, orelectromagnetic surveys. Mine sitesAnd we did successfully intersect all perhaps the minimum size that the teamcan be a difficult place to undertakethe targets.is interested in. What about the requiredgeophysics, although gravity may offer depth of investigationhow deep ispossibilities, but only if it is appropriateThere can be non-technical factors, too deep? So far so good. With enoughto your target. Also, are there flyingtoo. Prevailing attitudes for example. information, you may be able to modelheight restrictions and no-go areas forIs there pressure to use a particular some expected geophysical responsesairborne surveys because of topography,technique because it is currently in to back your judgement that your surveyresidential areas, power lines, etc.? vogue, or because someone has had can do the job. past success or even just has some Having selected your method, there willfamiliarity with it? Conversely, is there Now, what about the near-surfacebe other considerations. Cost and timingan aversion to using a particular environment. Any nasties there? Do youare the two most obvious. technique because of past bad have to contend with deep and severeexperiences? At one stage there was weathering? Large and erratic densityDo you have the budget for this?resistance to the re-introduction of variations within the weathered zone canIf necessary, can you reasonablyone technique into our geophysical cruel a gravity survey (unless they relatecompromise survey design parametersexploration arsenal because of previous to a near-surface expression of what youto reduce the survey expenditure; doesissues where the benefits of the are after). Are there unfavourable rockit really have to be the Rolls Royce ofmethod had been oversold.types at shallow depths between you andgeophysical surveys? Perhaps there is the potential targetsa magnetic basaltenough flexibility with survey timing toIf your recommendations stack up (for aeromagnetics), surficial maghaemiteopportunistically take advantage of thetechnically and financially, and it (for ground magnetics), highly resistivepresence of a survey crew already inis prudent to do so, back your own silcrete (for IP-resistivity), conductivethe area and share mob-demob costs. judgement! And good luck!29 PREVIEW JuNE 2020'