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This paper on airborne Electromagnetic(AEM) techniques deals with a number oftopics relating to airborne EM surveysystems and methods. These AEM topicsinclude: Basic Principles, TransientAirborne Electromagnetics,FrequencyDomain Airborne Electromagnetics,Airborne VLF Electromagnetics,FactorsAffecting Detectability, CombinedAEM/Magnetometer Surveys, Survey DataPresentation and Interpretation.
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6/18/2015 ConsultingPotentialFieldGeophysics,AirborneElectroMagneticSurveysWorkshop
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GeoExploLtda. GeophysicalAirborneSurveyElectroMagneticMethods(EM)SantiagoChile BacktoHomePageTel(562)3265116Email:[email protected] Dr.W.E.S.(Ted)Urquhart
ElectroMagneticAirborneSurveys
Abstract
ThispaperonairborneElectromagnetic(AEM)techniquesdealswithanumberoftopicsrelatingtoairborneEMsurveysystemsandmethods.TheseAEMtopicsinclude:BasicPrinciples,TransientAirborneElectromagnetics,FrequencyDomainAirborneElectromagnetics,AirborneVLFElectromagnetics,FactorsAffectingDetectability,CombinedAEM/MagnetometerSurveys,SurveyDataPresentationandInterpretation.
OtherUsefulLinksTheBerkeleyCourseinAppliedGeophysics(EM)
TheBerkeleyCourseinAppliedGeophysics(EM)
TableofContents
3.AIRBORNEELECTROMAGNETICSURVEYS
3.1BasicPrinciples3.1aTransientAirborneElectromagnetics
3.1bFrequencyDomainAirborneElectromagnetics
3.1cAirborneVLFElectromagnetics3.2FactorsAffectingDetectability
3.3CombinedAEM/MagnetometerSurveys3.4SurveyDataPresentation
3.5Interpretation
3.5aOtherInterpretationMethodsAppendix1.TypicalElectricalProperties
SelectedBibliography
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3.AirborneElectromagneticSurveysThegeneralobjectiveofAEM(AirborneElectroMagnetic) SURVEYS istoconductarapidandrelativelylowcostsearchformetallicconductors,e.g.massivesulphides,locatedinbedrockandoftenunderacoverofoverburdenand/orfreshwater.Thismethodcanbeappliedinmostgeologicalenvironmentsexceptwherethecountryrockishighlyconductiveorwhereoverburdenisboththickandconductive.Itisequallywellsuitedandappliedtogeneralgeologicmapping,aswellastoavarietyofengineeringproblems(e.g.,freshwaterexploration.)
Semiaridareas,particularlywithinternaldrainage,areusuallypoorAEMenvironments.Tidalcoasts
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andestuariesshouldbeavoided.Weatheredmaificflowscanprovidestronglyconductivebackgrounds,particularlyflowsofTertiaryorQuaternaryage.
Conductivitiesofgeologicalmaterialsrangeoversevenordersofmagnitude,withthestrongestEMresponsescomingfrommassivesulphides,followedindecreasingorderofintensitybygraphite,unconsolidatedsediments(clay,tills,andgravel/sand),andigneousandmetamorphicrocks.Consolidatedsedimentaryrockscanrangeinconductivityfromthelevelofgraphite(e.g.shales)downtolessthanthemostresistiveigneousmaterials(e.g.dolomitesandlimestones).Freshwaterishighlyresistive.However,whencontaminatedbydecaymaterial,suchlakebottomsediments,swamps,etc.,itmaydisplayconductivityroughlyequivalenttoclayandsaltwatertographiteandsulphides.
Typically,graphite,pyriteandorpyrrhotiteareresponsiblefortheobservedbedrockAEMresponses.ThefollowingexamplessuggestpossibletargettypesandwehaveindicatethegradeoftheAEMresponsethatcanbeexpectedfromthesetargets.
MassivevolcanosedimentarystrataboundsulphideoresofCu,Pb,Zn,(andpreciousmetals),usuallywithpyriteand/orpyrrhotite.FairtogoodAEMtargetsACCOUNTING forthemajorityofAEMsurveys.CarbonatehostedPbZn,oftenwithmarcasite,pyrite,orpyrrhotite,andsometimesassociatedwithgraphitichorizons.FairtopoorAEMtargets.MassivepyrrhotitepentlanditebodiescontainingNiandsometimesCuandpreciousmetalsassociatedwithnoriticorothermafic/ultramaficintrusiverocks.FairtogoodAEMtargets.VeinDEPOSITS ofAg,oftenwithSb,Cu,Co,Ni,andpyriteinvolcanicandsedimentaryrocks.GenerallypoorAEMtargets.QuartzveinscontainingAuwithpyrite,sometimesalsowithSb,Ag,Bi,etc.,involcanicorsedimentary(andpossiblyintrusive)rocks.PoorAEMtargets.SkarnDEPOSITS ofCu,Zn,Pb,andpreciousmetals,usuallywithpyriteandmagnetite,aroundigneousintrusions.FairtopoorAEMtargets.
Conductivetargetscanbeconcealedbyothergeologicalconductors,"geologicalnoise",suchas:
Lateralvariationsinconductiveoverburden.Graphiticbandsinmetamorphosedcountryrock.Altered(toclayfacies)maficultramaificrocks.Faultsandshearzonescarryingappreciablegroundwaterand/orclaygouge.Magnetitebandsinserpentinizedultramafics.
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3.1BasicPrinciplesElectromagneticinductionprospectingmethods,bothairborneand(most)groundtechniques,makeuseofmanmadeprimaryelectromagneticfieldsin,roughly,thefollowingway:Analternatingmagneticfieldisestablishedbypassingacurrentthroughacoil,(oralongalongwire).Thefieldismeasuredwithareceiverconsistingofasensitiveelectronicamplifierandmeterorpotentiometerbridge.Thefrequencyofthealternatingcurrentischosensuchthataninsignificanteddycurrentfieldisinducedinthegroundifithasanaverageelectricalconductivity,
Ifthesourceandreceiverarebroughtnearamoreconductivezone,strongereddycurrentsmaybecausedtocirculatewithinitandanappreciablesecondarymagneticfieldwilltherebybecreated.Closeto
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theconductor,thissecondaryoranomalousfieldmaybecomparedinmagnitudetothe PRIMARY ornormalfield(whichprevailsintheabsenceofconductors),inwhichcaseitcanbedetectedbythereceiver.Thesecondaryfieldstrength,Hs,isusuallymeasuredasaproportionoftheprimaryfieldstrength,Hp,atthereceiverinpercentorppm(partspermillion).
Anomaly=Hs/Hp.
Increasingtheprimaryfieldstrengthincreasesthesecondaryfieldstrengthproportionallybutthe"anomaly"measuredinppmorpercentremainsthesame.
Figure3.11,fromGRANT andWest,illustratesthegeneralprincipleofelectromagneticprospecting.
Figure3.11:Ageneralizedpictureofelectromagneticinductionprospecting.
Prospectingforanomalouszonesiscarriedoutbysystematicallytraversingthegroundeitherwiththereceiveraloneorwiththesourceandreceiverincombination,dependingonthesysteminuse.Inthecaseofairbornesystems,thereceivercoilsareusuallyinatowedbirdandthetransmittermaybealargecoilencirclingafixedwingaircraft,e.g.INPUTsystems,oroneormoresmallcoilsinthesamebirdthathousesthetransmittingcoils,e.g.mostHEM(HelicopterEM)systems.
Therearetwodifferentbasicsystemscommonlyusedtogenerateandreceivetheelectromagneticfield:
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transientor"timedomain"systemslikeINPUT,GEOTEMandMEGATEManda/c."frequencydomain"systemslikemostHEMsystems.
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TransientAirborneElectromagneticsHistorically,themostcommonlyencounteredsystemofthistypewastheINPUTsystem.ThenewersystemsGEOTEMandMEGATEM(FugroAirborneSurveys)functioninasimilarwaytoINPUT,ThusforsymplicitywewillexamineonlytheINPUTsystem.ForthosewhowouldliketoknowmoreaboutthenewersystemspleaselinktoGEOTEM,MEGATEM,orTEMPESTofFugroAirborneSurveys.
IntheINPUTsystemthetransmittingcoil,usuallyencirclingafixedwingaircraft,isenergizedbywhatis,essentially,astepcurrent.Intheabsenceofconductors,asharptransientpulseproportionaltothetimederivativeofthemagneticfieldisinducedinthereceiver.Whenaconductorispresent,however,asuddenchangeinmagneticfieldintensitywillinduceinitaflowofcurrentintheconductorwhichwilltendtoslowthedecayofthefield.Figure3.12illustratesthissituation.Theswitchingisrepeatedseveraltimesasecondastheaircraftfollowsitsflightline,sothatthesignalisvirtuallyCONTINUOUS .
Thereceiver"listens"onlywhilethetransmitteris"quiet"sothatproblemsarisingoutofrelativemotionbetweentransmitterandreceiver,becausethereceiveristowedinabirdbehindtheaircraft,arevirtuallyeliminated.Moreover,iftheentiredecayofthesecondaryfieldcouldbeobserved,theresponsewouldbeequivalenttoACmeasurementsmadeoverthewholeofthefrequencyspectrum.Itisimportanttonoteinthisconnection,however,thatnotthedecayfunctionitselfbutonlyitstimederivativecanberecordedifacoilisusedasthedetector.Thismeansthattheanomalousfieldswhichdecayveryslowlyaresuppressedinamplitudemorethantheothers,andsincethesearetheveryonesgenerallyassociatedwithgoodconductors,therewouldseemtobeaninherentweaknessinthissystem.Becauseitisdifficulttopreciselysynchronizetheinstantwhenthetransmitterbecomes"quiet"withtheinstantthatthereceiverBEGINS to"listen",itisnearlyimpossibletorecordtheentirefunction.Thisisequivalenttobeingunabletorecordmanyofthelowerfrequenciesintheacspectrum.Thshouldbenoted,however,thatinthepastseveralyears,significantprogresshasbeenmadeinmeasuringtheearlytimeresponse.
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Figure3.12:AsketchoftheINPUTtransientairborneEM SYSTEM operation.Theprimaryfieldisastepfunctionandthereceiverrecordsthedecayofthefieldafterthetransmitterstopstransmitting.
(GrantandWest1965)
Typically,thetimederivativeofthedecayfunctionismeasuredusingfromsixtotwelvedifferenttimedelaysfromtheinstantthattransmitterstopstransmittingbeforerecordingthesignalreceived.
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3.1bFrequencyDomainAirborneElectromagneticsHistorically,allhelicopterborneelectromagnetic(HEM)systems,whereofthistype.ThereareanumberofnewersystemsthatemploythetransienttechniquesimilartotheINPUTsystembutthesewillnotbe
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discussedhereastheyareasyetnotwidelyused.
InthetypicalfrequencydomainhelicopterEM SYSTEM (HEM)boththetransmittingcoilsetandthereceivercoilsetarehousedinarigidboomor"bird"thatistowedbeneaththehelicopter.Commonly,thisboomisfromthreetofivemeterslongandcontainsfromtwotosixcoilpairs.Usually,halfofthecoilsineachofthetransmittersetandthereceiversetare"coaxial",i.e.anaxisnormaltotheplaneofthecoilspassesthroughthecentreofbothcoils.Thesecondhalfofthecoilsetsarenormally"coplanar",beingequivalenttoboththetransmittingandreceivingcoillyingflatontheground.Othercoplanarorientationshavebeenusedoccasionally.Adiagramofthissystemisshowninfigure3.13.Forclarity,theboomisshownoversizedinthisdiagram.Notethestabilizingairfoilattachedtooneendofthebird.
Figure3.13:SketchofatypicalHEM SYSTEM configuration.
Thissystemoperatesinpreciselythemannerdescribedinsection3.1.Thereceivermeasurestheinphaseandoutofphase,orquadrature,ofthesecondaryfield,expressedinppmoftheprimaryfield.Aswewilldiscussintheinterpretationsection,thetwodifferentcoilorientationsprovidedatathatisusefulindiscriminatingbetweendikelikeconductorsthathaveconsiderableverticalextentandmaybeorebodies,andhorizontalsheetlikeconductorsthataresimplyconductiveoverburden.Thetwocoilorientationsalsoprovideadditionalinformationaboutthegeometryofthetargetbody.Asisillustratedinthediagram,thesystemincludesasecondbirdcarryingamagnetometer.Themagneticdataisoftenusefulindiscriminatingbetweenmetallicandnonmetallicconductorsandtoassistininterpretingthe
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geologicalsettingoftheconductor.SometimesaVLFreceiverisalsoincluded.
Figure3.14showsaphotographofoneofatypicalHEMsystemsbeinglaunchedfor SURVEYoperations.Thissystemincludescoaxialandcoplanarcoilpairstomeasuretheelectromagneticfieldatfourfrequenciessimultaneously."Clicking"themouseoverthepicturewillenlargethepicture(27kb).
Figure3.14:ThetypicalHEMbirdconfigurationbeinglaunchedfor SURVEY operations.NotethatthissystemalsoincludesamagnetometerbirdbetweenthehelicopterandtheEMbird.
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3.1cAirborneVLFElectromagneticsWithVLFsystemstheprimaryfieldissuppliedbypowerfulradiotransmittersusedformilitarycommunicationsandnavigation.Thereceiverusuallyconsistsofacoilandsupportingelectronicstowedinabird.Figure3.15showsthepositionsofcurrentVLFEMtransmittersandapproximaterangesofreception.
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Figure3.15:ThelocationsandrangesofVLFEMtransmittingstations.
Becausetheavailablefrequenciesarehigh(1522Khz)thesystemsareparticularlysusceptibletogeologicnoise.Also,becausethetransmittersarecontrolledbythemilitary,theymaynotalwaysbeoperatingfortheentireperiodthata SURVEY isinprogress.Theyarealsolimitedintermsofavailableprimaryfielddirectionswhichwillnotalwaysbewellcoupledwiththefavorablegeologicstrike.
Note:Anumberofthestationsshownintheabovepicturearenolongeroperating.
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3.2FactorsAffectingDetectabilityThereareatleastsixfactorsthatdeterminewhetherornotaparticularconductorwillbedetectablewithanyEMsystem.
1.Signaltonoiseratio:
Inpractice,becauseof"systemnoise"(Ns)and"geologicalnoise"(Ng),theabilityofasystemtorecognizeandmeasureananomalyislimitedbythe"signaltonoise"ratio:
Signaltonoise=Hs/(Ns+Ng)
BecauseHsandNgareproportionaltotheprimaryfieldstrengthHp,andNs,infrequencydomainsystems,usuallycontainselementsproportionaltoHp,thereislittletobegainedbyincreasingthe
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primaryfieldpower.IntimedomainsystemsNsisnotgreatlyaffectedbyHp,soextrapowerdoesresultinincreasedsignaltonoise.Attemptstoincreasethesignaltonoisearesometimesmadebyincreasingthedistancebetweenthetransmitterandreceiver.ThisresultsinroughlythesameHsandNgbutoftenalowersystemnoiseNs.Howeverthelongerbirdrequiredtoachievethisismorepronetoflex,andthusmayactuallydisplayincreasedsystemnoiseNs.Inaddition,thelargerbirdisheavierandmoredifficulttohandleandthusmayreducesurveyproductivity,increasingcost.InconductiveareasNgmaybehigher,therebyoffsettinganyadvantageoflowerNs.
2.Penetration
ThepenetrationofanAEMsystemisitseffectivedepthofexploration.Commonly,thisistakentoincludetheelevationofthesystemaboveground,asthisisalsoaffectedbylocalenvironmentandflyingconditions.
Ingeneral,systemswithlargetransmitterreceivercoilseparation,usuallyreferredtoasTxRx,havegreaterpenetrationthanthosewithsmallseparations.Penetrationiscloselyrelatedtosignaltonoise,asthesystemthatproducesalargeranomalyfromagivenconductorcan,ofCOURSE ,lookfurtherintotheground.Penetrationisusuallydefinedasthemaximumdepthatwhichalargeverticalsheetwillproducearecognizableanomalyofatleasttwicetheamplitudeofthesystemnoise.
3.Discrimination
ThediscriminationofanAEMsystemistheabilityofthesystemtodifferentiatebetweenconductorsofdifferentphysicalpropertiesorgeometricshapes.Discrimination,particularlybetweenflatlyingsurficialconductorsandsteeplydippingconductors,isvitallyimportant.GooddiscriminationcanbeachievedinHEMsystemsbyusingseveralfrequenciesandbothcoaxialandcoplanarcoilpairs.
4.Resolution
ResolutionreferstotheabilityofanAEMsystemtorecognizeandseparatetheinterferingeffectsofnearbyconductors.Asystemthatdoesthiswellalsoproducessharpanomaliesoverisolatedordiscreteconductors.Resolutiongenerallyincreaseswithdecreasingflightelevationandcoilseparation.TypicallytheHEMsystemshavebetterresolutionthanthefixedwingtimedomainsystems.
5.ConductivityWidthAperture
AllAEMsystemsare,tosomeextent,aperturelimited.Belowacertain"responsefactor",whichincludestheconductivityanddimensionsoftheconductor,theanomalyproducedbythesystemwillbebelowtherecognitionlevel.Attheupperendoftheresponsefactor,somesystemsarelimitedandothersarenot.Theonesthatarenotlimitedsometimesceasetobemultichannelsystemsandlosetheirdiscrimination.TimedomainsystemslikeINPUTareaperturelimited.
6.LateralCoverage
Inadditiontopenetration,thelateralcoverageofanAEMsystemisimportantbecauseitdictates,tosomeextent,themaximumdistancebetweensurveylines,whichinturnaffectsthecostofexploration.Alternatively,atagivensurveylinespacing,asystemwithgoodlateralcoveragewillhaveabetterchanceofdetectingaconductorthatliesbetweentwosurveylines.Likepenetration,lateralcoveragegenerallyincreaseswithincreasingcoilseparation.
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3.3CombinedAEM/MagnetometerSurveysIngeneraltherearethreestepsinvolvedinplanningasurveyofthistype.WewilloutlinethesestepsandthengiveafewexamplesofhowtoplanTHESURVEY .
Step1.DefinetheTargetandGeologicalEnvironment
Target Definingparameters
Largemassivesulphidelens Type,attitude,strikeandcomposition.Smallmassivesulphidelens Type,shape,attitudeandcompositionVeinsorotherdiscontinuous
mineralizationType,extent,strikeandmineral
assemblage.Shearzoneorfracturehostednon
conductors Type,strike,alteration,watercontent
Geologicalenvironmentcriteria.
Depthandconductivityoftheoverburden.Considertheunderlyingbedrockgeology,residualortransportedsoil,andtheQuaternaryhistoryofthearea.Theconductivityofthebedrockandthepresenceofbedrockconductors.Thestrikeanddipoftheformations.Thepossiblepresenceofmagneticbodies.Thedepthrangetotheconductorsof INTEREST .
Step2.DetermineFactorsAffecting SURVEY Performance
Thetopographyandphysiographyofthearea:
Istheareaflatorhilly?Onlyahelicoptercanmaintainrequiredgroundclearancesafelyinhillyterrain.Theextentandheightoftreecoverwilleffectflightelevation.Presenceofculturalfeatureslikepipelinesorotherconductorsandorinterferencefrompowerlinesmaybeimportant.Determineaccesstotheareaandtherequiredlogistics.Thesefactorswillaffect SURVEYproductionandthereforecost.Mobilizationtoandfromthearea:howfarandhowlong?FerrydistancefrombasecamptoTHESURVEY area(s).Theshapeandsizeof SURVEY block(s).Linelength,spacingandthetotalkilometeragewillaffectsurveyproductionandthereforecost.Presenceofobstructionssuchaspowerlinesortowersthatmycauseasafetyriskforlowflyingaircraftand/orbirdandcableassembly
Step3. SELECT theAEMSYSTEM
ThefollowingareexamplesofdifferenttargetsinthreeareasinCanada(SeminarpresentedbyDr.N.
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Paterson).Thetarget,andtopographic/physiographicconditionsalsodifferbetweentheseareas.
Target1:
AlargestrataboundvolcanogenicCuZnsulphidebodysomewhereina1000km2areainnorthwestQuebec.Thick(30to60meter),partlyconductiveoverburdencoversacountryrockthatisamixtureoffelsicandintermediatemetavolcanics,greywacke,quartzite,bandedironformation,intrusivegraniteandminorgabbro.TheareaisflatandswampyandtheonlyaccessisfromMattagami,150km.away.Theareaistobeflowninsummer.
AEMsystemrequirements:
Goodpenetration.Tolerancetoconductiveoverburden.Gooddiscriminationbecausegeologicconductorssuchasgraphite,sulphideandironformationarelikely.Goodlateralcoverageandaperturearedesirable.Lowflyingcostifpossible.
Appropriatesystems:
HelicopterEM:willrequireaflycampandgasolinedump.Mayberelativelyexpensive,especiallyifthelinespacingmustbereducedbecauseoflimitedlateralcoverageofthesystem.Thissystemwillproducethebestdiscriminationbetweengraphiticandsulphideconductorsandhasgoodsurficialtobedrockdiscrimination.INPUT:hasthenecessarycharacteristicsbutcouldhaveaproblemwithatmosphericnoiseinthesummermonths.A400meterlinespacingwouldbeappropriatesocostwouldberelativelylowbutdoesnothaveasgooddiscriminationastheHEMsystem.
Target2:
AlargestrataboundmassivePbZnbodyin150km2areaintheYukon.Verysteeptopography.Littleoverburdenexceptinvalleys.Hightreecover.Countryrockisphyllite,argillite,shist,intermediatevolcanicsandgranite.Theareais130km.fromRossRiver,Yukonand25kmfromaprivateairstripatAnvil.Theareaistobesurveyedinsummer.
AEMsystemrequirements:
Gooddiscriminationandresolutionbecausetheexpectedgraphiticconductorsareimportantmarkers.Goodsensitivitytopoorconductors.Thisrequiresthathighfrequenciesbeavailable.Goodperformanceinsteepterrain.Adequatepenetrationofatleast75meters.Flightlinesareshortsotheaircraftmusthavegoodturnaroundcapability.Becausethe PROGRAM issmall,mobilizationcostsmustbelow.
Appropriatesystems:
MulticoilHelicopterEMwithatleastonefrequencyover3000Hzhasallofthenecessarycharacteristics.Thistypeofsystemcanbe INSTALLED inalocalhelicopterarepreferableinordertoreducemobilizationcosts.150mlinespacingisappropriate.
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Target3:
SmallCuZnsulphidelensessomewhereina500km2kilometreareaofnorthwestNewfoundland.Thebedrockisintermediatemaficmetavolcanicswithsomeultramaficintrusivesandminormetasediments.Theterrainismoderatelyhillycoveredby1020meterhightrees.Thereislittleoverburdenandwhatthereis,isvirtuallynonconductive.Thereisgoodaccesstovillagesintheareabyroadandthenearestairstrip,Cornerbrook,is120km.away.Theareaistobeflowninwinter.
AEMsystemrequirements:
Goodresolutionbecausethemineralizationis,typically,insmallpods,ofteningraphitichostrockandsometimesassteeplydippingpipes.Goodresponsetopoorconductors.Typicalmassivesulphideconductanceintheareais13mhos.Thehostrockandtheoverburdenarerelativelynonconductive.Goodlateralcoverageisrequiredbecausetheconductorsareofirregularstrikeanddip.Adepthofpenetrationofabout75metersisadequate.
Appropriatesystems:
AmulticoilhelicopterEM SYSTEM withatleastonefrequencyover3000hzand,perhaps,withVLF.InthiscasetheVLFmayaddaperture,lateralcoverageandpenetration,andhelptodiscriminatebetweenlong(formational)andshort(lenstype)conductorsatverylowadditionalcost.Thesystemcanbebasedinavillageinthearea.
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3.4SurveyDataPresentationInadditiontoadigitaldatafile,theresultsofanAEMsurvey,thedata,ispresentedinavarietyofformats.SomecontractorsonlypresenttheEManomalylocationsplottedontheflightpathmaps,togetherwithacodingindicatinganomalystrengthsandcertainparametersderivedbycomputermodelingtheanomalysourcesasverticalsheets.Beforetheadventofpersonalcomputerswiththeirinteractivedisplaycapabilities,stackedprofilesoftheEM,altimeter,magnetic,andsometimes,sphericnoisedatausedtobeacommonformofdatapresentation.However,becausehandlingthelargeamountofpaperinvolvedwasalwaysanoneroustaskandmostexplorationistscannowdisplayprofiles,usingtheircomputer,directlyfromthedigitaldatabase,itisnolongercommontoproducehardcopyprofiledisplays.
TypicallycontractorspresentEMdataintwoprincipleformats:
Asprofilemapsshowingtheinphaseandquadraturecomponentsofcomplimentarycoaxialandcoplanarfrequencypairsplottedascolouredprofilesontheflightpath.ThismapalsoshowsthelocationsofsignificantEManomaliesdisplayedusinganiconcodeto INDICATE theCALCULATED conductivitythicknessproductofthesourceassumingthatitisaverticalsheet.Theprocessof"picking"andmodelingtheseanomalieswillbedescribedinmoredetailintheinterpretationsection.AsacolouredmapoftheapparentresistivitywithembeddedcontourscalculatedfromthecoplanarorcoaxialEMdata.Thismapshowstheapparentgroundresistivityassumingthegroundtobeofuniformconductivitybothlaterallyandvertically.Thesemapsarehelpfulinoutliningconductiveoverburdenandshowingdiscretebedrockconductors.Actualvaluesofresistivitybearlittle
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relationtothetrueresistivitiesoftheoverburdenorbedrockfeatures.
Figure3.41illustratesatypicalsuiteoffinalmapsofboththemagneticdataandtheEMdata,includingtheinterpretationmap,thatsurveycontractorwoulddeliveraftertheCOMPLETION ofa
combinationMagneticHEMsurveyoperationsandtherequiredcompilationandinterpretationphasesofdataanalysis.Movingthemouseoverthepictureswillallowyoutoseedifferentpresentations.
"Clicking"onthevisibleversionwillproduceanenlargement.
Figure3.41:AtypicalInterpretationmapthatresultcombinedHEMandmagnetic SURVEY .
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3.5InterpretationMostsurveycontractorslimittheirinterpretationtoasystematicanalysisofthemorepromisinganomaliesusingaverticalsheetastheconductormodel.Thisisnormallydone,usingaCOMPUTERPROGRAM ,afterthelocalbaselevelforestimatinganomalyamplitudeshasbeencarefullydetermined.Anomalyselectionisdoneby,judiciously,usingtheshapeofcalculatedmodelsofvariousconductors,verticalsheets,flatlyingsurficialsheets,etc.similartotheonesshowninfigure3.51.
Figure3.51:AsketchillustratingthetheoreticalHEManomaliescausedbysimpleconductorshapes.Whenmultipleconductorsare PRESENT ,theshapesillustratedwillbemodifiedbyneighbouring
anomalies.
Nomogramsexist,suchastheoneillustratedinfigure3.52bywhichsimilaranalysiscanbemadefromprofiledata.Bothproceduresproduceestimatesofconductance,calledtheconductivitythicknessproduct(whichistheproductoftheconductivityofthetabularsourceanditsthickness),andthedepthtothesourcefromthesensor.Thesensorheight,asrecordedbytheradaraltimeter,isthensubtractedfromthedepthtogiveanapparentdepthbelowground.
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Somecontractorshavedevelopedinteractivecomputerprogramsthatallowstheinterpreterto"pick"theanomaliesdirectlyfromadisplayonthecomputerscreenandimmediatelyseetheresultsoftheconductance/depthcalculation.Thispermitstheinterpretertoalterboththemapscaleandtheprofiledatascalequicklyto INSURE thatallfeatures,regardlessofamplitude,arefullyassessed.
Figure3.52:Anomogramusedtoestimatetheconductivitythickness PRODUCT anddepthtothesource.
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Figure3.53:Thisprofilemap SHOWS atypicalbedrockconductoranomaly.
Infigure3.53,notethattheanomalyhasacharacteristicsignature.Thepositivecoaxialresponse(theredlinefortheinphasecomponentandtheblueforthequadrature)ismirroredbyalowinthecoplanarresponse(maroonforinphaseandtealforquadrature).
Figures3.54and3.55illustratethesignaturesofasurficialconductorandofaconductorwhichcontainssignificantmagnetitecontent.Notethatthesurficialconductorisbroadandlacksthehigh
coaxial/lowcoplanarresponseoftheverticalsheetanomalyinfigure3.53.Themagnetiteresponseisnegativeintheinphasecomponent.
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Figure3.54:TypicalsignatureofanHEManomaliesduetonearsurface"surficial"material.Notethatthequadratureresponseofthecoaxial,(blue),andcoplanar,(teal),profilesarenearlyidenticalwhile
thereisnoinphaseresponseforeithercoilpairs.
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Figure3.55:TypicalHEMresponseofaconductorthatcontainsasignificantAMOUNT ofmagnetite.
Infigure3.55:Notethatboththecoaxialandcoplanarinphaseresponseisstronglynegativewhilethereislittleornoquadratureresponsefromeithercoilpair.
Whiletheprocessdescribedabovedoesproduceveryuseful INFORMATION abouttherelativeimportanceofvariousanomaliesintheEMdata,ithasseverelimitationsincluding:
Theassumptionofverticaldip:Iftheconductorisnonvertical,itsapparentdepthwillbeunderestimatedbuttheconductanceestimatewillnotbegreatlyaffected.Theassumptionofsemiinfinitesize:Thedepthestimatetendstobetoogreatandtheconductancetoolow.Theassumptionofnonconductivehostrockandoverburden:Iftheconductorisincontactwithaconductivehostrockoroverburden,thequadratureanomalywillbeenhancedandthedepthandtheconductancewillbothbeunderestimated.Iftheconductorunderlies,butdoesnotcontactthe
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conductiveoverburdenthedepthandconductancewillbothbeoverestimated.FailuretocorrectlyremovelocalEMbackground:IftheresidualeffectoflocalEMbackgroundhasshiftedtheassumedEMbaselevelusedintheCALCULATION ,thedepthestimateswillbetoolowandtheconductanceunderestimated.Presenceofmagnetite:suppressionoftheinphaseanomalybymagneticsusceptibilityinthecaseofconductorsinmagneticenvironmentswillleadtounderestimatingconductanceandwildlinetolinevariationsintheestimate.
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3.5aOtherInterpretationMethodsForreasonssimilartothosesuggestedinthesectiononmagneticinterpretation,detailedinterpretationofspecificanomaliesonamapisalmostalwaysdonebytheexplorationmanagerspersonnel.Manyofthesemethodsrelyontheapplicationofsophisticatedmodelingalgorithms.Figure3.56showsanexampleofanHEMmodeloftwoconductiveplatesinonesuchmodelingprogram.
Figure3.56:ACALCULATED theoreticalcoaxialinphaseelectromagneticresponseoftwodippingconductivedikes.
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Appendix1:TypicalElectrical PROPERTIES ofEarthMaterials.
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Rock,Mineral,etc. Conductivity(mohs/meter)Resistivity(ohmmeters)
Bornite 330 3x103Chalcocite 104 104Chalcopyrite 250 4x103Galena 500 2x103Graphite 103 103Marcasite 20 5x102Magnetite 17x1042x104 5x1056x103Pyrite 3 0.3Phrrhotite 104 104Sphalerite 102 102IgneousandMetamorphicRocks 107102 100107
Sediments 1055x102 20105Soils 1030.5 2103FreshWater 5x1030.1 10200SalineOverburden 0.15 0.21SaltWater 520 0.052SulphideOres 10210 0.1100GraniteBedsandSlates 1021 1100AlteredUltramafics 1030.8 1.25103Waterfilledfaults/shears 1031 1103
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SelectedBibliographyAirborneElectroMagneticSurveysGRANT F.S.andWest,G.F.,1965,InterpretationTheoryinAPPLIED Geophysics,McGrawHillBookCompany.
Fraser,DouglasC.,1976,ResistivityMappingwithanAirborneMulticoilElectromagnetic SYSTEM:Geophysics,vol.41,no.1(Fedbruary1976).
Fraser,DouglasC.,1979,TheMulticoilIIAirborneElectromagnetic SYSTEM :Geophysics,vol.44,no.8(August1979).
Paterson,NormanR.,1982,UseofAirborneE.M.(AEM)inExplorationforBedrockConductors,inMiningGeophysicsWorkshop.,PatersonGRANT andWatsonLimited.
Paterson,NormanR.,1982,ProspectingbyCombinedAEM/Magnetometer SURVEYS ,inMiningGeophysicsWorkshop.,PatersonGrantandWatsonLimited.
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