ElectroMagnetic Airborne Surveys

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GeoExploLtda. GeophysicalAirborneSurveyElectroMagneticMethods(EM)SantiagoChile BacktoHomePageTel(562)3265116Email:surveys@newsense.com 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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