b'Minerals geophysicsMinerals geophysicsbroader coherent magnetic anomalyOn the positive side, enhanced patterns from basement targets\x08 weathering related to sulphides or On the positive side, in somestructural fracturing, and supergene circumstances these maghaemiteenrichment, may generate detectable responses can be beneficial, for exampleEM targets if other weathering-related where surficial iron concentrations reflectresponses do not dominate the survey underlying geological features of interestresults\x08 Where a conductive basement such as transgressive structures andtarget is in electrical contact with the shallow palaeo-channels\x08 Maghaemiteweathered zone, current flow in the effects in magnetic survey results cantarget, and hence detectability, may be be reduced by filtering the data, byincreased\x08 Maghaemite effects in ground physically raising the sensor furtherEM survey results may be reduced Terry Harveyby modifying survey parameters, by Associate Editor forabove the land surface in ground surveys, or by using an airborne platform\x08 changing the relative disposition of the Mineralsgeophysicstransmitter loop and receiver sensor, or terry.v.harvey@glencore.com.au GravityIntense and prolongedby using an airborne platform\x08weathering destroys rock fabric, increasing porosity and permeability,IP-ResistivityIn the presence of a low The Australian land surface which can dramatically lower rockresistivity weathered zone, much of the densities\x08 Variations in the thickness andtransmitted electric current in an IP-The geology of much of the Australianintensity of this weathering will generateresistivity survey will fail to penetrate an land surface can be quite different tounwanted effects in the measuredunderlying resistive basement, degrading that of the rest of the world\x08 This impactsgravity field, the more so because of theirthe investigation of basement target zones; strongly on the application of mineralrelative proximity to the gravity meter\x08associated electromagnetic coupling exploration geophysics, as explorationThese effects can distort or even obscuremay also corrupt IP values\x08 As a further companies discovered when geophysicalthe broader coherent gravity anomalycomplication, a highly resistive capping techniques developed for use in thepatterns from basement targets\x08 such as silcrete will make it difficult to inject Northern Hemisphere were first appliedelectric current into the ground, further to Australian mineral exploration\x08 On the positive side, if enhancedreducing the strength of any current weathering is associated with targetedpenetrating into the basement\x08Australia is often described as thesulphides or structural fracturing, the oldest continent in the world, and itresulting lower densities may actuallyOn the positive side, enhanced weathering does have some of the worlds oldestfacilitate their detection with gravity\x08related to sulphides or structural rocks\x08 But it is really the land surface,In addition, secondary sulphidesfracturing, and supergene enrichment, parts of which date back more thangenerated by weathering-relatedmay present detectable targets for 90 million years, that is truly ancient\x08supergene enrichment may presentresistivity if other weathering-related The extremely prolonged exposurea positive gravity target in their ownresponses do not dominate the survey of this land surface to weathering hasright\x08 Weathering-related effects inresults; sulphides generated by supergene resulted in a distinctive regolith - angravity survey results may be reducedenrichment are also a potential IP target\x08 indurated crust (variously silcrete,by filtering the data, by incorporatingWhere a low resistivity basement target is ferricrete, calcrete, etc) overlying ainformation on the weathering profilein electrical contact with the weathered thick zone, often more than 100 metressuch as depths to bedrock derivedzone, current flow in the target, and thick, of intensely weathered materialfrom other techniques (passive seismichence resistivity and IP detectability, (saprolite)\x08 It is this combinationsurveys, magnetics, etc\x08) or by resortingmay be increased\x08 Weathered-zone-of resistant crust and underlyingto an airborne platform\x08 related transmitter current injection and saprolites, now set in an aridElectromagneticsIncreased porositypenetration problems in IP-resistivity environment, which poses particularand permeability in the weathered zonesurvey results may be reduced by challenges to the application of mineralcan dramatically lower the resistivity ofexpending extra effort on transmitter geophysics in much of Australia\x08 Howthe affected rock\x08 In electromagneticelectrode preparation to boost current does this weathered land surface(EM) surveying, this low resistivity layerstrength, employing a more powerful, affect the application of geophysicalwill absorb much of the EM sourcehigher voltage transmitter, and by utilising exploration techniques? field, generating unwanted anomalousa much larger transmitter dipole size (eg MagneticsIron compounds,responses and limiting the strength ofpole-dipole array)\x08concentrated in the indurated crustthe EM field that penetrates into theIn summary, geophysical exploration (e\x08g\x08 ferricrete) by prolonged exposurebasement\x08 Localised variations in thein deeply weathered environments can to weathering, can be converted tothickness and degree of weatheringbe anything but straightforward\x08 Extra highly magnetic maghaemite by thewill further complicate the EMthought may be needed to tweak the effects of lightning strikes, bushfires,response patterns\x08 In addition, surficialsurvey parameters to handle potential etc\x08 This maghaemite generates strongmaghaemite concentrations in thedifficulties\x08 In some circumstances, if and sharply erratic magnetic responses,indurated crust can generate anomaly- youre lucky, the effects of extreme particularly in ground magnetic surveys,like responses in EM survey results due toweathering may even work to your which can distort or even obscure thesuperparamagnetic effects\x08 advantage\x08 Good hunting!FEBRUARY 2021 PREVIEW 28'