AirborneGeophysicalMethods
8-1.ScopeofAirborneInvestigations
Airbornegeophysicalprocedureshavehadanimportantimpactonthemineralrecoveryindustry.Severaladvancementsingeophysicalequipment,typesofairborneplatformsandglobalpositioningsystems(GPS)haveprovidedapplicationtoparticularengineeringandenvi-ronmentalproblems.Ingeneral,airborneplatformswillnotoftenbesufficientlydetailedandeconomicfortheselattertwotopicareas.Airbornemethodsmaybequitereasonableforsomespecificprojectsoflargeareawithtargetsofampleanomalystrength.
a.Scale.Scaleisanimportantconsiderationforairborneprocedurestobecost-effective.Sizeablecostsareassociatedwiththeplatformtoflythegeophysicalequipment.Towingageophysical“bird,”flightpathsurveying,andmoresophisticatedequipmentareusuallynecessaryforairbornemeasurements.Forsmallareasites,wheresurficialgeophysicsmaybeapplied,thegreatcostadditionandthereductionofavailablemethodswouldnormallyeliminateairbornegeophysics.Theless-enedfieldstrengthandthebroadeningoftheanomalousshapeasdistanceincreasesforpotentialfieldmethods(gravityandmagnetics)normallyarecountertothegreaterdetailrequirementsforengineeringandenviron-mentalstudies.Whenairbornemethodsareappropriate,lowerandslowerflyingplatformswillbemorebeneficialforengineeringandenvironmentaluses.Helicopterswillnormallyprovidemorecoverage,duetotheirslowerflightspeedsandpotentialfortighterflightpaths,thanfixedwingtransport.Helicoptermayalsobeabletoflyatloweraltitudes,providingbettermeasurementquality.b.Dimensions.Thedimensionsofasitemaybesogreatthatsomeairbornegeophysicalreconnaissancemaybeprudentpriortootherstudies.Alargesitewithpoten-tialradiometriccontaminationwouldbeaparticularprob-lemtypewithbeneficialairbornegeophysicalapproaches.Moderatelysizedsiteswherethesurfaceishazardous(orextremelyexpensive)forpersonnelentry,orisunavail-ableforpersonnelaccesstothesite,maybeassessedtosomeextentbyairbornegeophysicalmethods.
c.Purpose.Astudy’spurposewillbeanaspecttoresolvethemostappropriategeophysicalmeans.Geo-logiccharacterizationobjectivesforasitewithsignificantrockvariationsmaybemorelikelytoemployairbornemeasuresthanasitewithobjectivestodelineatelarge
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organicplumes.Theanomalouscontrastoftheobjectiveisthekeyinresolvingwhetherageophysicalprocedureisworthyofagivenpurpose.
8-2.AirborneGeophysicalMeasures
Thethreechiefairborneproceduresaremagnetic,electro-magnetic,andradioactivemethods.Airbornemagnetome-try(aeromag)isquitecommonandaeromagmapsareavailableformostoftheUnitedStates.Aeromagmap-pingisveryusefulformineralexplorationandgeologicalstudiesofmagneticrocks.Airborneelectromagneticmethodsareusedinboththefrequencyandtimedomains.OrebodyexplorationisthechiefuseofairborneEMmethods.Airborneradioactivemeasurementsofgammaraysmaybeusedforuraniumandthoriumexploration.
a.Magnetics.
(1)Engineeringandenvironmentalsurveyscouldhaveavaryingpurposeinwhichaeromagwouldprovideusefulinformation.Twoalternativetargetsmightbeconsidered:sitecharacterizationoflocalewithmagneticrocksorferrousman-madeobjects.Thedifferingpur-posesandtargetswouldnecessitatedifferingflightparam-etersfortheaeromagsurveys.Aeromagcanbeflownforregionalgeologicstructure,whichmayaidsitecharacter-ization.Fewsiteswouldexistwithmassive,buriedfer-rousobjectsinaregionoflittlemagneticmineralization.Searchesforburiedferrousobjectswouldrequirelow-altitudeflightsinsearchoflocalanomaliesrelativetoaeromagforstructuralinvestigations.
(2)Thereareseveralpossibleusesofaeromagflights.
(a)Onehypotheticalsituationtoillustratethepoten-tialofaeromagusemightbethesearchforburiedsteeltransmissioncasingoveralargesite.Thehorizontalsteelpipewouldproduceasmall,broadanomalyneartheaxisofthepipe.Inthecaseofdeterminingtherouteofa1-m-diam,horizontalsteelpipeinasparselypopulatedregion,amagnetometerslungfromahelicoptermightbeabletofollowtheanomalyroute(thepipeline’sunknownpath)bysurveyinitiationataknownoriginandheadingofthepipeline.ThesearchwouldcrossperpendiculartothelastobservedazimuthfollowingtheanomalyusingaGPSlocation.
(b)Ashortlengthofvertical,steelwell-casingburiedbelowthesoilsurfacewouldlikelyproduceasharp,largemagneticanomaly.Anaeromagsearchforunknown,short(<20mlength)abandonedwellsmaynot
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beproductive,becausethediameteroftheanomalywouldbesoconfinedthatanairborneflightpathwouldbeunlikelytopasssufficientlyclosetothewellaxis.Theseparationdistancebetweeneconomicallyfeasibleaero-magflightpathswouldmostlikelybetoolargeforthewell-headsearchtobesuccessful.
(c)Searchesforlonglengths(>50m)ofburiedsteelcasingproducelarge,broadmagneticanomalies.Properlyplannedflightpathsofaeromagsurveyswouldbelikelytoencounterthecasing’smagneticsignature.(d)Thetargetanomaly,linearbutsmallinthefirstcase,enablesthemethodtobeusefulforthehorizontaltransmissionlines.Thesecondtargetofshort,abandonedwellcasinghasananomalyofnarrowdimensionsandisbiasedagainstaeromagdiscovery.Thelastcaseoflong,deepcasingsusuallyhasbroad,largeanomaliesandaero-magsurveysshouldbeeffective.
b.AirborneEM.
(1)ThebenefitofairborneEM(AEM)proceduresisthesearchforconductors;conductorsneednotbeferrousobjects.Further,airasamediumdoesnotattenuateanEMfield.Thesecondaryfieldfromthetarget,albeitsmall,islessaffectedbydistanceabovethesurfacethanaeromag.
(2)ThereareseveraldifferentAEMmethodsinusesincethe1960s.Telford,Geldart,andSheriff(1990)describetheVLFprocedureofAEMasfollows:“simple,cheapcomparedtootherairsurveys,andprovideslimiteddataforshallowdepths.”NooneAEMtechniquewouldbepreferredfordifferentproblems.
(3)AEMmightprovideabetterdefinitionofthetransmissionpipelineexampleabovethanaeromag.How-ever,AEMwouldbemuchpreferredifthepipelinewasnotaferrousmetal.Thetargetanomalyduetotheengi-neeringorenvironmentalproblemwouldsuggestwhetherAEMwasanappropriatetechnique.
c.Radioactivesearches.
(1)Airborneradioactivesearchesareobviouslylimitedbythetypeofsourcetarget.ThesemeasureswouldbeusedinfrequentlycomparedtoaeromagandAEMwork.
(2)Radioactivedetectorsmaybeflownforasearchofburiedradioactivewastecontainers.Asahypotheticalconsideration,anairborneexplorationfora1-m-diam,8-2
20-m-long,horizontalleadpipefilledwithlow-levelradioactivewastewouldbeapossiblesolution.
d.Complementaryairbornesurveys.
(1)Oncetheplatformforonetypeofsurveyisselectedanothermethodisoftenaddedasacomparatorforthedatataken.HempenandHatheway(1992)recom-mendthatcomplementarymethodsbeutilizedtoreduceambiguityandtolessenthenumberofsolutionmodelsforthemeasurements.Theincreasedcostofasecondmethodissmallcomparedtotheexpenseoftheairborneplatform,itsflightpathandthelabortoconductthesurvey.
(2)AeromagandAEMwouldbeveryappropriatecomplementstolocatethesteeltransmissionpipelineintheexampleabove,giventhatanairbornesurveywasselected.AirborneradioactivedetectionandAEMwouldbesupportiveofasearchfortheradioactivewaste-filled,leadpipecited;aeromagwouldnotbeacceptable,asaferrousmetalwasnotinvolved.8-3.Contracting
a.Airbornegeophysicalsurveysarespecializedpro-cedures.Thenature,scope,andcostofthesemethodsdictatesthatspecializedcontractorsshouldprovidetheseservices.Thecostofairbornegeophysicalcontractswillnormallybetensofthousandsofdollars.Theproductionbylengthorareaofairbornesurveyswillcompetewithsurficialgeophysicsbecauseofthelargedimensionscoveredbyairborneplatforms.
b.Selectionofairbornegeophysicalcontractorswillbeanalogoustoothergeophysicalservices.Theairbornegeophysicalcontractormusthaveexperience,equipment,anddocumentedresultsfrompriorairborneservices.Preferablythecitedworkshouldnotonlybeofthesamemethodology,butthepreviousservicesshouldresolveasimilarproblem.FlyingAEMformineralexplorationwillnotbesimilarinscopetoobtainingAEMforengi-neeringpurposes.Avoidcontractsforserviceswithoutinterpretation.Nomatterhowwellanothergeophysicistcanevaluateairbornegeophysics,themostprudentcon-tractwillpayforusefulinterpretedresults.Flyingspe-cializedequipmentwithcomplexdatastreamsdoesnotguaranteethatthemeasurementshaveanyapplicationtothepurposeforthework.Payforresults,notdata.c.HempenandHatheway(1992)suggestthatclientlistsberequestedofspecialtycontractors.Thecitedclientswouldthenbecontactedconcerningreliability,
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timeliness,accuracy,andcostexperiencewiththecontractor.Normally,airbornegeophysicswillbecon-ductedviaarchitect/engineerserviceswithproposal
submissionsandevaluationoftheproposals.Itmaybewisetohaveagovernmentgeophysicistontheproposalevaluationteam.
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