Magnetotelluric monitoring of a fluid injection: Example ...
Magnetotelluric Survey Underway in North Dakota · survey of Earth’s crust and upper mantle. As...
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EarthScope is back in North Dakota! Deep-earth sounding sta- ons of the Naonal Science Foundaon’s (NSF) EarthScope project’s Magnetotelluric Array (MT Array) are currently being deployed across North Dakota by researchers from the Naonal Geoelectromagnec Facility at Oregon State University in an ap- proximate 40 mile grid (array) in order to perform a geophysical survey of Earth’s crust and upper mantle. As many as 38 MT Array staons could be installed by the summer of 2018 (fig. 1). Each staon collects induced electromagnec field data from within the earth for about three weeks. The work being conducted in North Dakota is part of a larger Northern Great Plains (NGP) study that includes locaons in South Dakota, Nebraska, and northern Kansas (EarthScope, 2017). MT surveying is one of the few geophysical methods that can be used to examine Earth's crust down to the depths of the mantle (Park, 2004), which is about 30 miles in North Dakota. Conduct- ing this type of geophysical invesgaon across North Dakota will help to unravel the underlying architecture of the rocks, geotec- tonic structures, and ancient connental boundaries that may be found at depths significantly greater than those of the Williston Basin, North Dakota’s largest structural geologic feature (fig. 2). Again, it is important to note that the current configuraon of the MT Array (i.e. staon spacing and sensor frequency selecon) per- mits geophysical resoluon of depths greater than about six miles and down into the upper mantle. In order to invesgate within the shallower depths of the Williston Basin a ghter staon array grid, coupled with higher frequency instruments, would be needed. The MT Array project is complimentary to the larger EarthScope US Array project, a decade-long connental geophysical survey of the U.S. from west to the east (and Alaska) with the ulmate goal of surveying and imaging the underlying geologic and tectonic structure of the enre country using seismological and other geo- physical methods like magnetotellurics. The seismological component of the EarthScope project has just reached full deployment in Alaska and will remain operaonal there through 2019. EarthScope’s seismometers began their tra- verse of North Dakota nearly a decade ago in 2008, with the in- Fred J. Anderson Magnetotelluric Survey Underway in North Dakota Figure 1. Planned locaons of MT Array sounding staons being deployed across North Dakota as a component of the Naonal Science Foundaon sponsored EarthScope invesgaon of the geomagnec and geoelectric properes of Earth’s crust across the U.S. Background Image Credit: Lyndon Anderson – prairiejournal.com Figure 2. Schemac cross- secon segment of Earth above the mantle beneath the Williston Basin in North Dakota as modeled from re- cent EarthScope connental seismological invesgaons highlights the depth range “window” that the Earth- Scope MT Array can inves- gate and the potenal geo- tectonic structures present at these depths (aſter Anderson, 2016; Nelson et al., 1993), THO = Trans-Hudson Orogen. 8 GEO NEWS
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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