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COMPARING VTEM TIME-DOMAIN EM AND ZTEM NATURAL FIELD AIRBORNE EM SURVEY RESULTS OVER THE MCARTHUR RIVER UNCONFORMITY URANIUM PROJECT PRESENTED AT SEG 2018 Jean M Legault* Geotech Ltd. Garnet Wood Cameco Corporation

Carlos Izarra Geotech Ltd. Clinton Keller Cameco Corporation

Alexander Prikhodko Geotech Ltd. Clare O’Dowd Cameco Corporation

SUMMARY Helicopter ZTEM natural field EM and VTEM helicoptertime-domain EM system results are compared over theMcArthurRiverProjectinnorthwesternSaskatchewan.TheZTEM survey datawere complicated by the fact that theywere obtained in low signalwinter conditions andwith amajorpowerlinepresent.TheZTEMresults from30 to90Hzweregenerallyofgoodqualitytowithin~1-1.5kmfromthe powerline; however the higher frequency (>180 Hz)tipperdataareclearlyaffectedbypowerlinenoiseasfaras5 km away. The ZTEM signatures agree well with thosefrom theVTEMMax system; however source depths from2D inversion indicate that the bedrock sources areshallowerthanexpected(<250m),possiblydueto limitedfrequency bandwidth or else an improper (low) aprioriresistivitymodel.3DZTEM inversion resultsappearmuchimproved over 2D. In nearly all cases, depths ofinvestigation estimates exceed 2 km for the ZTEMinversions. The VTEM Max system 30 Hz TDEM resultswere of a comparatively good quality, with powerlineaffecting thedata towithin less than<1kilometre.Depthstobasementrangefrom250-300mandimagedthelocationof the basement graphitic pelites. Depths of investigationfromRDI resistivity-depth imaging are estimated to reachand exceed 600 m in some areas. Both EM surveys andmagneticsassistedinunderstandingbasementgeology.

INTRODUCTION The McArthur River uranium mine is located in theAthabasca Basin in northern Saskatchewan, Canada(Figure1),andistheworld’slargesthighgradeuraniummine, co-owned by majority partner Cameco in joint-venturewithOrano.TheMcArthurRivermineregionhasbeenpreviouslystudiedwithairborneelectromagnetics,,with Fixed-wing and VTEM (Witherly et al., 2004)helicopter time-domainEM fromnearbyWestMcArthurprojectcomparedbyIrvineandWitherly(2008)andwithZTEM (Lo and Zang, 2008) natural field helicopter EMtestresultsbyWitherly(2009).

In late 2015, system availability prevented a plannedVTEM helicopter time-domain electromagnetic surveyfrombeingflownoveranareawithintheMcArthurRiverproject,situated14kmalongstriketothenortheastfromMcArthurRiverminesiteand15kmsouthwestofCigarLake mine. As a substitute, a ZTEM natural field

helicopterEMtestsurveywasflowninwinterconditionsin late December, 2015 to early January, 2016. As afurther complication, a major powerline extends acrossthe property. The property was subsequently reflownwith the VTEM Max system (Prikhodko et al., 2010) inSeptember,2016.

Figure 1: Location of McArthur River Project in Athabasca Basin, northwest Saskatchewan, over regional geology (yellow = Athabasca sandstone, other colours = Basement Domains; courtesy Cameco Corp.).

As result, these surveys provide a relatively uniqueopportunityto learnfromcomparisonsbetweenthetwodifferent airborne EM technologies directly over arelativelydeepunconformityuraniumsetting,andtoalsoassess the ability to acquire natural field ZTEM dataduringthelownaturalfieldsignalWinterSolstice,andinthepresenceofman-madeculture.GEOLOGY

The McArthur River project is situated in the south-eastern portion of the Athabasca Basin within thenortheast to east trendingChurchill Structural province,oftheCanadianShield.Theareaiscoveredby~550moflate Paleoproterozoic Athabasca Group sedimentaryrocks,whichunconformablyoverliePaleoproterozoicandArchean rocks of the Wollaston Lithostructural Domain(Figure1).TheWollastonDomain iscomprisedofmeta-sedimentary and granitoid gneisses. Themetasedimentary sequence includes graphitic and non-graphitic pelitic and semi-pelitic gneisses, felsic andquartz-feldspathic gneisses, metaquartzite andcalcsilicategneisses.MagneticArcheangranitoidgneissesflankthesemetasedimentaryrocks(McGilletal.,1993).

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AIRBORNE SURVEY RESULTS

ZTEMNaturalFieldEMSurvey

The2015ZTEMsurveyconsistedof1064 line-km flownalong200mspacedlinesovertheMcArthurRiversurveyblock. ZTEM lines were oriented N 165°E toperpendicularly cross-cut the geology. The EM andmagneticsensorsweresituatedat~85mand100mavg.elevations, respectively. The ZTEM survey results overtheMcArthurRiverstudyareaarepresentedinFigure2.

ZTEMisanairbornevariantoftheAFMAG(Ward,1959)naturalfieldEMtechniquewhereasinglevertical-dipoleair-core receiver coil is flown over the survey area in agrid pattern, similar to regional airborne EM surveys.Two orthogonal, air-core horizontal axis coils, placedclose to the survey site, measure the horizontal EMreferencefields.Datafromthetwosetsofcoils(receiverr& base r0) are used to obtain the TZX In-line and TZYCross-linetippers(Vozoff,1972)at6-7frequenciesinthe22 to 720 Hz band, according to the following formula(HolthamandOldenburg,2008):

HZ(r)=TZX(r,r0)HX(r0)+TZY(r,r0)HY(r0)

Figure2presentstheZTEMtipperresultsinplanshownas Total Divergence (DT) images that convert thecharacteristic tipper cross-overs into peak responsesusing a horizontal derivative approach for easierinterpretation/visualization (Lo et al., 2009). Shown arethe In-Phase DT’s from high too low (720 to 30 Hz)frequencies, for depth-comparison purposes, based onrelative EM skin depth (Vozoff, 1972; Spies, 1989). TheZTEM results appear to define complexly shapedconductive lineaments, related to basement graphiticpelite units below the Athabasca sandstone cover,particularly in the south-eastern survey area. Thesebecomebetterdefinedatprogressivelylowerfrequenciesand greater skin depths below the sandstone cover andintotheArcheanbasement.

Butalsoevidentisthevariableeffectandlateralextentofnoise on the ZTEM results from the major powerline,located to the northwest. The noise corridor is alsonotablywider(>10km)particularlyat frequencies>180Hz(Figure2ab),whereasitseffectislesswidespread(~3km) at frequencies <90 Hz (Figure 2cd). This ispresumablyduetolowersignal/noiseatthehigherendofthe frequency spectrum, exacerbated by weak naturalfield signal during the December-January period. Theseare nevertheless noteworthy examples of reasonablygood quality ZTEM data that can be obtained in mid-Wintersignalconditionsandatnorthernlatitudes.

Figure 2: ZTEM multi-frequency DT results at: a) 720 Hz, b) 180 Hz, c) 90 Hz and d) 30 Hz, showing effect & variable extent (dashed lines) of powerline noise, with outline of McArthur property boundary (grey polyline).

VTEMTime-DomainEMSurvey

The 2016 VTEM survey consisted of 543 line-km flownalong the200mspaced,northwest-southeast linesusedfor ZTEM, aswell as several northeast tie-lines. TheEMandmagnetic sensorheightswereat40mand65monaverageaboveground level, respectively.TheVTEMandmagnetic results at McArthur River are presented inFigure3.TheEMTimeConstant (Tau) inFigure3awasobtained from the Z-coil, off-time dB/dt response. The“slidingTau”methodthatwasusedcalculatestheslopeoftheEMdecay at the latest possible time-channels abovethechosenEMnoisethreshold level(A.Prikhodko,pers.comm.).

The VTEM images in Figure 3 appear to also define thesame complexly shaped basement graphitic conductorsthat were resolved in the ZTEM results. Similarly,comparing Figure 3abc, these become better defined atprogressively lower later decay times and greater skindepths (Spies, 1989). The Tau late-time decay constantimageinFigure3dbearsaverycloseresemblancetotheZTEMimagesinFigure2–albeitinsomeareastheVTEMconductorsareperhapsnotascontinuousorwelldefined,possibly an effect of greater depth of burial or greaterlate-timenoiseincloserproximitytothepowerline.

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Figure 3: VTEM multi-channel dBz/dt results: a) Early-time decay (ch10 = 55 usec), b) Mid-time (ch25 = 440 usec, c) Late-time (ch40 = 3.5 msec) and d) Late-time constant (Tau), showing effect & extent of powerline noise.

As seen previously, also evident is the powerline effectandlateralextentofnoiseontheVTEMresults fromthemajor powerline noise, to the northwest. However, thenoiseeffectisvisiblylessforVTEMrelativetoZTEMandthenoisecorridorisalsonotablynarrower(<2kmvs.>3km) which is testament to higher signal/noise fromactive-source systems relative to natural field systems.Clearly theVTEMresultsarebetterable tomapgeologyin the vicinity of the powerline than the ZTEM surveydata.

VTEM1DImagingandZTEM2D-3DInversion

RDI 1D resistivity depth imaging was used to rapidlyconvert the VTEM vertical component EM decay profiledata into equivalent apparent resistivity versus depthcross-sections. The RDI algorithm for resistivity-depthtransformation is based on the apparent resistivitytransform of Meju (1998) and the TEM response fromconductive half-space. The RDI transform is depthcalibrated based on forward plate modelling for AEMsystemconfigurations(A.Prikhodko,pers.comm.).

Figure 4 represents the RDI apparent resistivity depthslices from -100 m below surface to -600 m, with anestimated maximum depth of investigation (DOI) ofapprox. 600 m within the McArthur River project area.The RDI depth-slices define the same complexly shapedbasementgraphiticconductorsthatwereresolvedintheVTEMandZTEMrawdata.However, therelativedepthsofconductivesourcesareobviouslybetterresolved,withtheshallow(100mdepth)results.Alsonoteworthyistheimaging of conductive units across the powerlinecorridor.However,asshown,thequalityoftheRDIimageprogressively degrades below 400 m, with the 600 mimage in Figure 4d revealing little about the basementgeology,duetoitsnearingthemaximumDOIrange.

Figure 4:VTEM resistivity-depth imaging (RDI) results at: a) -100 m depth, b) -200 m, c) -400 m, and d) -600 m, showing effect & extent of powerline noise.

At theonsetof theZTEMdataanalysis, two-dimensional(2D) inversions of the ZTEM data used Geotech’sproprietaryAv2dTopo(Legaultetal.,2012)thatisbasedon thecodeofdeLugaoandWannamaker (1996).).Theinversion utilized the in-line (TZX) component, in phaseandquadraturedata forthreetosix frequencies(30,45,90,+/-180,360,720Hz)dependingoneffect/influenceofthepowerline(Figure2).

The2DZTEMresistivitydepth-sliceswerefoundtodefinesimilarbasementgraphiticconductorsthatwereresolvedinthepreviousVTEMdBz/dtandRDIresultsandalsotheZTEMfieldresults.However,itwasfoundthatatshallowdepths,theZTEMinversionsrevealedresistivityfeaturesthatclearlyrelatetothebasementandnottheAthabascasandstone. This was interpreted to be due to lack ofsufficientlyhighfrequencies(duetoPLnoise)forproperresolutionorelse thechoiceofhalf-space resistivity (2kohm-m), which, if too low, would underestimate thesource depth (Legault et al., 2009). And at depth, theresistivitydepthslicesdidnotcontainasmuchdetailasrevealedintheZTEMDTdata,whichisofcourserelatedtothelimitsof2Dinversionsin3Dgeology.Asaresult,a3DZTEMinversionwasperformedtohelpresolvetheseissues.

Themulti-frequency (30-720Hz)ZTEMdata inbothTZXand TZY components were subsequently inverted usingtheUBCZTEM_MT3DinvcodeofHolthamandOldenburg(2008). Resistivity depth-slices for the 3D inversion(Figure 5), similar to those presented earlier, at depth(800 m) define similar basement graphitic conductorsthatwere resolved in the previous VTEM and 2D ZTEMresults. However, as shown in Figure 5ab, at shallowdepths (<200 m), the 3D ZTEM inversions reveal noindicationofconductorswithintheAthabascasandstonedepth intervals, unlike the previous 1D VTEM andparticularly2DZTEM.Furthermore,at400m(Figure5c)the 3D inversion results better reflect the resistive

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sandstone thicknesses that are known to progressivelyincreasefromapprox.250mto400mfromsoutheasttonorthwest (C. Keller, pers. comm.). The extent ofpowerlineinfluencealsoappearstobelessfor3Drelativetotheprevious2Dinversions.

Figure 5: ZTEM 2D resistivity inversion results at: a) -100 metre depth, b) -200 m, c) -400 m, and d) -800 m, showing estimated extent of powerline noise.As a testament to the results of the airborne surveysperformed at McArthur River study area, a geologicalinterpretationwascreatedshortlyafterthesurveys,andwasbasedontheaeromagneticsandtheZTEMandVTEMsurvey results. Figure 6 shows the resulting basementgeologymapattheunconformity,interpretedbyCamecofromairbornegeophysicsandlimiteddrilling.

Figure 6:McArthur River interpreted basement geology, below sandstone cover, based on airborne EM & magnetic results and limited drilling (courtesy Cameco Corp.).

CONCLUSIONS The lesson learned from the surveys was firstly thatreasonablygoodlowtomidfrequency(30-90Hz)ZTEMcan be obtained in winter conditions, even in thepresenceofpowerlines,buthigherfrequencies(180-720Hz) are more strongly impacted, due to weak primaryfield strengths in northern latitudes. Secondly, goodspatialcorrelationcouldbeobservedbetweentheZTEMandVTEMconductorsbutpoorerdepthcontrolfromtheZTEM models reinforces the fact that a reasonablyappropriate starting model is critical for accurateinversionresults.3DZTEMinversionsseemedtoprovidefar more reliable geology and depth control than 2D.Finally,we also learned that good imaging of the upperbasementgeologycouldbeobtainedatthesedepthsfromthe VTEMMax system using 1D imaging approach. Thecombined VTEM and ZTEM airborne EM andaeromagnetic results, combinedwith imaging,1D-2D-3Dinversionandlimiteddrilling,werehelpfulinproducingareasonably satisfactory geologic model of the McArthurRiverstudyarea. ACKNOWLEDGEMENTS The authors thank McArthur River project partnersCameco and Orano Canada for allowing us to presenttheseresults. REFERENCES DeLugao,P.P.,andP.E.Wannamaker,1996,Calculatingthe two-dimensional magnetotelluric Jacobian in finiteelements using reciprocity: Geophysical JournalInternational,127,806–810.

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