Magnetotelluric Survey Underway in North Dakota · survey of Earth’s crust and upper mantle. As...
Transcript of Magnetotelluric Survey Underway in North Dakota · survey of Earth’s crust and upper mantle. As...
EarthScope is back inNorthDakota! Deep-earth sounding sta-tions of the National Science Foundation’s (NSF) EarthScopeproject’s Magnetotelluric Array (MT Array) are currently beingdeployedacrossNorthDakotabyresearchersfromtheNationalGeoelectromagneticFacilityatOregonStateUniversityinanap-proximate40milegrid(array)inordertoperformageophysicalsurveyofEarth’scrustanduppermantle.Asmanyas38MTArraystationscouldbeinstalledbythesummerof2018(fig.1).Eachstation collects induced electromagnetic field data fromwithintheearthforaboutthreeweeks. Theworkbeingconducted inNorthDakotaispartofalargerNorthernGreatPlains(NGP)studythatincludeslocationsinSouthDakota,Nebraska,andnorthernKansas(EarthScope,2017).
MTsurveyingisoneofthefewgeophysicalmethodsthatcanbeusedtoexamineEarth'scrustdowntothedepthsofthemantle(Park,2004),whichisabout30milesinNorthDakota.Conduct-ingthistypeofgeophysicalinvestigationacrossNorthDakotawillhelptounraveltheunderlyingarchitectureoftherocks,geotec-tonicstructures,andancientcontinentalboundariesthatmaybe
foundatdepthssignificantlygreaterthanthoseoftheWillistonBasin,NorthDakota’slargeststructuralgeologicfeature(fig.2).
Again,itisimportanttonotethatthecurrentconfigurationoftheMTArray(i.e.stationspacingandsensorfrequencyselection)per-mitsgeophysicalresolutionofdepthsgreaterthanaboutsixmilesanddownintotheuppermantle.InordertoinvestigatewithintheshallowerdepthsoftheWillistonBasinatighterstationarraygrid,coupledwithhigherfrequencyinstruments,wouldbeneeded.
TheMTArrayprojectiscomplimentarytothelargerEarthScopeUSArrayproject,adecade-longcontinentalgeophysicalsurveyoftheU.S.fromwesttotheeast(andAlaska)withtheultimategoalof surveying and imaging the underlying geologic and tectonicstructureoftheentirecountryusingseismologicalandothergeo-physicalmethodslikemagnetotellurics.
TheseismologicalcomponentoftheEarthScopeprojecthasjustreached full deployment in Alaska and will remain operationaltherethrough2019.EarthScope’sseismometersbegantheirtra-verseofNorthDakotanearlyadecadeagoin2008,withthein-
Fred J. Anderson
Magnetotelluric Survey Underway inNorth Dakota
Figure 1. PlannedlocationsofMTArraysoundingstationsbeingdeployedacrossNorthDakotaasacomponentoftheNationalScienceFoundationsponsoredEarthScopeinvestigationofthegeomagneticandgeoelectricpropertiesofEarth’scrustacrosstheU.S.
Background Image Credit: Lyndon Anderson – prairiejournal.com
Figure 2. Schematic cross-section segment of Earthabove the mantle beneaththe Williston Basin in NorthDakota as modeled from re-cent EarthScope continentalseismological investigationshighlights the depth range“window” that the Earth-Scope MT Array can investi-gate and the potential geo-tectonic structures present at thesedepths(afterAnderson,2016; Nelson et al., 1993),THO=Trans-HudsonOrogen.
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stallationofbroadbandseismicmonitoringstationsonasimilarstylegridinthewesternpartofthestateandendedin2012withtheplannedremovaloftheseismometerslocatedintheRedRiverValley(Anderson,2010).
EarthScopescientistsareconductingtheNGPMTstudyinordertofillthegapandtietogetherpreviouslycompletedMTArraysur-veys,withsimilarstationspacings,acrossthewesternandeasternU.S.(EarthScope,2017).ForNorthDakota,theNGPstudyisex-pectedtoprovideseveralnewinsightsintodeep-earthstructurebeneaththestateleadingtonewinterpreta-tions of geotectonic boundaries, continental assem-blage,andthemineralogicalandpetrologicalcharacterofEarthfarbelowthedepthsoftheWillistonBasin.
Themagnetotelluric(MT)methodisconsideredapas-sivesourcegeophysicalmethod,ratherthananactivesourcemethodlikeexplosivesorvibratory,inthatthesource energy used originates from naturally occurring externalsources,dominantlyspaceweatherphenome-na(suchascoronalmassejections)thatoriginatefromoursunandregionalelectricalstormactivity.Bothofthese phenomena induce geoelectric currents (andcreateourspectacularNorthernLights)thatgeneratemagnetic fields in the earthwhich can bemeasuredandusedtodeterminetheelectricalconductivitystruc-ture (or inversely the resistivity) of Earth’s rocks andfluidslayersatdepth. Ofcourse,the“magneto”por-tionofthetermmagnetotelluricreferstothemeasure-ment of the resultantmagneticfields createdby theearth-induced electrical currents, and "telluric" fromtheLatintellusmeaningearth.
Historically,thedevelopmentoftheMTmethodiscreditedtotwoEuropeanscientists:AndreyTikhonovofRussiaandLouisCagniardof France (OilfieldReview, 1990), alongwith Japanese scientist,Tsuneji Rikitake, who recognized its applicability to deep-earthexploration(ChaveandJones,2012). Deepgeologicalsurveyingusing theMT techniquewas conductedheavily inRussia in thelatterhalf of the twentieth century in the search forpreviouslyunexploredsedimentarybasins.Atthattime,thismethodwaspri-marilyemployedasareconnaissancetechnique(Dobrin,1976).
AtanMTArraystation,anelectricalwirelinearrayisconstructedwithtwoorthogonallyorientedhorizontalelectricaldipole lines(fig.3)alongwithashallowlyburiedmagnetometer.Themagne-tometeriswiredintoaNarodIntelligentMagnetotelluricSystem(NIMS)receiverpoweredbytwoburied12-VDCbatteries.Mea-suredelectrical andmagneticfielddata is collected and storedwithinthereceiverandisextracteduponremovalofthestation.Eachlegoftheelectricalwirelinearraydipolelinescanbeaslongasafootballfieldandarecommonlyarrangedin“L”or“+”shapedconfigurations(fig.4).
Commonly, the two induced horizontal electrical field compo-nents(mathematicallynotedasExandEy)aremeasuredandtheresultanttwohorizontalandoneverticalmagneticfieldcompo-nents(mathematicallynotedasHx,Hy,andHz)aremeasured.
The type of magnetometer installed at each of the MT Arraystations is a custom-designed triaxial fluxgate magnetometerwhich simultaneously measures an induced magnetic field inthree directions as opposed to using three separately orientedsinglemagnetometers(figs.5and6).Afluxgatemagnetometer
Figure 4. LayoutofMTArraystationNDD29,justnorthofPettibone,ascapturedfromtheNDGS’sPhantom4ProQuad-CopterDrone.Thereversed“L”shapedlayoutoftheelectricfielddipolelines,whichmeasuretheinducedelectricalfieldsarehighlightedinred.Thelocationofthemagnetometerthatmeasurestheresultantmagneticfieldsisdepictedintheforegroundatleft.Viewistothesouth.
Figure 3. AtbothendsoftheelectricaldipolelinesarePb-PbCl2gel-typeelectrodes(builtbyOSUresearchers)likethiswhichareburieddirectly into the ground in a shallow hole containing a kaolinitemud.Thekaolinite(aluminumsilicate)basedmudensuresastrongelectricalconnectionbetweentheearthandtheelectricaldipoleline.
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measures the difference between a known time-varyinginstrumented magnetic field and an external magnetic fieldimposeduponit.
In addition to enhancing our understanding of the geologicpropertiesanddeep-earthgeologicstructurebeneaththestate,anadditionalbenefitofthisinvestigationwillbeanenhancedandcomprehensive continent-wide understanding of the potentialgeohazard effects of large-scale geomagnetically inducedcurrents. Thesegeomagneticallyinducedcurrentsaretriggered
bycoronalmassejectionsbuffetingEarth’smagneticfield,causingitto“rattle”backandforth.Thisrapidmovementofthemagneticfieldcreates inducedcurrentsthatcanflowthroughconductiveinfrastructure like rail lines, pipelines and power grids (NASA,2016). Recent work by Love and others (2016) has suggesteda high susceptibility to this type of geohazard in northwesternMinnesota and has shown the potential to disrupt and disableregional and national scale electrical power grids, such as the1989eventthatblackedoutaportionoftheQuebecpowergrid.
Atthetimeofthiswritingaboutone-thirdoftheMTArraystationsplanned forNorthDakota have completed their data collectionruns.
TheauthorwouldliketoacknowledgeandthankDr.AdamSchultz,Principal Investigator for the EarthScopeMagnetotelluric ArrayandDirectoroftheNationalGeoelectricFacilityatOregonStateUniversityforprovidinganoverviewoftheMTArrayproject.Alsothe EarthScopeMTArray geophysical field staff TomBonofiglioandAlexKoverofGreeneGeophysics for theircooperationandcoordinationofMTArraystationvisitsinNorthDakotathispastOctober.
ReferencesAnderson, F. J., 2010, EarthScope Transportable Seismic Array
OperationalinNorthDakota:GeoNews,v.37,no.1,p.23-25.
Anderson,F.J.,2016,DepthoftheEarth’sCrustinNorthDakota:North Dakota Geological Survey Miscellaneous Map 42,scale1:1,000,000.
Chave, A. D., and Jones, A. G., 2012, Introduction to theMagnetotelluricMethod,inTheMagnetotelluricMethod-TheoryandPractice:CambridgeUniversityPress,p.9.
Dobrin,M.B.,1976,IntroductiontoGeophysicalProspecting(3ded.):Mc-GrawHillBookCompany,p.599.
EarthScope, 2017, Magnetotelluric Array. http://www.usarray.org/researchers/obs/magnetotelluric (retrieved October13,2017)
Love, J. J., Pulkkinen, A., Bedrosian, P. A., Jonas, S., Kelbert A.,Rigler, J. E., Finn,C.A.,Balch,C.C.,Rutledge,R.,Waggel,R.M., Sabata, A. T., Kozyra, J. U., and Black, C. E., 2016,GeoelectrichazardmapsforthecontinentalUnitedStates:GeophysicalResearchLetters,AmericanGeophysicalUnion,v.43,no.18,p.9415-9424.
NASA, 2016, Geomagnetically Induced Currents: NationalAeronautics and Space Administration, GIC Infographic.https://www.nasa.gov/sites/default/files/thumbnails/image/gicinfographicfinal.jpg(retrievedOctober24,2017)
Nelson,K.D.,Baird,D. J.,Walter, J. J.,Hauck,M.,Brown, L.D.,Oliver,J.E.,Ahern,J.L.,Hajnal,Z.,Jones,A.G.,andSloss,L. L., 1993, Trans-Hudson Orogen and Williston Basin inMontana and North Dakota: New COCORP deep-profilingresults.
Oilfield Review, 1990, Focus-Magnetotelluric Prospecting:October,p.4-8.
Park,S.,2004,MagnetotelluricMethod: InstituteofGeophysicsandPlanetaryPhysics,UniversityofCalifornia,LosAngeles,PowerPointPresentation,27p.
Figure 5. Triaxialfluxgatemagnetometer installedatanMTArraystationjustsouthwestofMcClusky.Thistrulyuniquecustom-mademagnetometermeasuresthemagneticfieldsresultantfrominducedtelluriccurrentsinthreedirections.
Figure 6. Themagnetometerisburiedintothegroundwithinamois-ture barrier bag and is orientedwith respect tomagnetic north andproperlyleveled.
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