Remote Sensing and Electromagnetic Methods for Groundwater Exploration in the Cameroon Coastal Plain

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sensing and electromagnetic methods in the hard- rock formations of the Cameroon coastal plain (Central Africa):

implications for water borehole location

André Firmin Bon

^a^,^∗

, Auguste Ombolo

^a

, Patrick Mboa Biboum

^a

, Jacques Mahop Moutlen

^b

, Guillaume Ewodo Mboudou

^a

aNational Advanced School of Engineering of Maroua, The University of Maroua. P.O.Box 46, Maroua, Cameroon

bOﬃce of study, control and expertise in engineering (BEC INGEREX), Littoral, Cameroon

a rt i c l e i nf o

Article history:

Received 13 March 2022 Revised 18 May 2022 Accepted 5 July 2022

Editor: DR B Gyampoh Key words:

Remote sensing Electromagnetic surveys Hydrogeological features Hard-rock aquifer Cameroon coastal plain

a b s t ra c t

Sustainable accesstogroundwater inruralareassuchas the coastalplainof Cameroon (Sub-SaharanAfrica)isanongoingchallengeduetothelackofknowledgeonhydrogeolog- icalfeaturesfavorabletotheidentificationofpotentialgroundwatertargets.Inthisstudy, remote sensingimageryandelectromagneticsurveys wereused toassessthehydrogeo- logicalpotentialoftheNgweilocalityhard-rock.Automaticextractionoflineamentsfrom Landsat-8imagesindicated200lineamentswithatotallengthof293kmcorresponding to0.6 kmoflineament persquarekm, suggestingamedian groundwaterpotential.The maindirectionsareNNW-SSE,N-SandNNE-SSWwhilethesecondarydirectionsareNW- SE;NE-SW andE-W. Surfacegeophysicalinvestigationsshowed anelectromagneticfield of0 to0.57mV.Thesesurfaceandboreholeinvestigationsconfirmedbasement fractur- ingand spatialvariationsinweatheringprofiles whoseaquiferstructureand yields(4.4 m³/honaverage)aresimilartowhatiscommonlyobservedinthehardrockaquifer.The correlation betweenboreholeproductivityandthenearest lineamentsuggeststhatthese lineamentsarenot(atleastnotall)surfacetracesofregionaldiscontinuitiesthatmayact asmajorgroundwaterflowpaths.ThisimpliesthattheproductivityoftheNgweiaquifers appearstobedependentonthefissured/fracturedhorizon,soexplorationshouldbebased onacontextualratherthangeneralapproach.

Analysisoftheconsistencyoftheresultswiththeﬁelddataallowedtoproposeapre- liminary electromagneticlogmodel and alsoshowsthevalueofusingthisapproachto assess the availability of potential groundwaterzones in hardrock areas. However, the methodsusedshouldbesupportedbyfurtherinvestigationstoallowabetterunderstand- ingofthehydrogeologicalcharacteristicsofthispartoftheCentralAfricanbasementso thatnooneisleftbehindintheachievementoftheSDGs.

ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/)

∗Corresponding author.

E-mail address: [email protected] (A.F. Bon) .

https://doi.org/10.1016/j.sciaf.2022.e01272

2468-2276/© 2022 Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

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A.F. Bon, A. Ombolo, P.M. Biboum et al. Scientiﬁc African 17 (2022) e01272 Introduction

Hard rocks(HR)(plutonic andmetamorphic rocks,withtheexception ofmarbles,asthey canbe karstiﬁed)constitute the basementofall continentswithabout35%oftheworld’sHR providedbyAfrica [1,2].Theiraquifers(0-100mdepth) are ofparticularimportanceinthe tropicalandsubtropicalregions,both dueto theirwidespread extentandaccessibility andbecause thereis oftennoother readily availablewater supply,especiallyfor thelarge ruralpopulation(235 million) living onthese geologicalenvironments [3–5].Unlike other typesofaquifers (e.g.,volcanic andsedimentary formations), hard rocksaquifers (HRA)are heterogeneousandhavelowprimary porosity andhydraulic conductivity,requiringspeciﬁc knowledge andtechniques ifgroundwater isto be extractedandmanagedeffectivelyespecially in ruralareaswherethe developmentoftheseaquifersasareliablesourceofwatersupplynotoriouslycomplicated[2,3,6–9].

In recentyears,manyconceptual-hydrogeological models havebeen developedforHRA. However, intermsof geome- tryandstructure,two centralconceptsareconsideredrelevantbyalargemajorityofhydrogeologists[1,10]:theconcepts ofhydraulic propertiesdueto weathering(anewconcept)andthose duetotectonic fracturingrevealedby lineaments(a previous concept). The recent concept [1,2,11,12] revealsthat the typical weatheringprofile of HR includes, from top to bottom: (i)asaprolitelayer(consistingofthefineandcoarsesaprolite)withamainlycapacitivefunction(when saturated withwater),surmounted,insomesplace,bytheironorbauxiticduricrust;(ii)afissuredlayer(ofvariablethickness)mainly transmissivebutalsocapacitiveandshowingfractures,inmajority,subhorizontalinthegranitoidswhosedensitydecreases withdepth;andfinally(iii)thefreshbasement,cutbyanetworkofmajorfractures,verydeep(>100m),withakilometer extension,islocallypermeable.Itsstoragecapacityislimitedonalocalandregionalscale[1,12,13].Thisprofilecanbesplit intotwo categories:theprofilefromsingle-phaseweatheringwherelayersarehorizontal[11]andtheprofilefrommulti- phase weatheringwithstratiform layers,the mostrecentofwhichmayfollow thetopography[12].Instead, theprevious concept indicates, withoutanyevidenceofthe genesis ofthefissuredlayerin tectonicallystableterrain [1],that thehy- draulicpropertiesofHRAderivemainlyfromtectonicoriginsandlithostaticdecompression[3,14–18].Thegeometryofthe aquiferiscontrolledbysandy-claylevelsandfracturesofvaryingdirections.

The workcarriedoutby [1]todemonstratethat thehydraulicconductivityofHRAisa consequenceof(paleo)weathering processes indicated, independentlyof the concepts, that the fractured/fissured reservoir from which theseaquifers derivetheirproductivity[13]hasingeneralahigherpermeability,althoughboreholeyieldsareusuallylimitedtolessthan 10m³/h[19].Forallconcepts,theheterogeneityoffracturinginfluencestheboreholeyieldandtheiroperationalframework requireswelladaptedtechniques.Forinstance,[1,2]indicatedthatthestrategyforsitingawaterboreholewillbedifferent in thegeneralcase(lookingforthe subhorizontalfractures ofthestratiform fissuredlayer thatwill efficientlybe tapped withverticalboreholes)orwherethislayerhasbeencompletelyremovedbyerosion(searchingforaverticalfissuredlayer located alongbothwallsofa subverticaldiscontinuity).ThismodelhasbeensuccessfullyappliedinvariousHRAinAfrica (includingIvoryCoast,SouthandEastAfrica,BurkinaFaso[7,10,20])andelsewhereintheworld(India,SouthKorea,France, [11,12]).Furthermore,topographyhasbeenshownbymanytobeaparameterinfluencingonboreholeyield,withthecom- monresultthatboreholeslocatedinvalleys(valleysareoftenassociatedwithregionalfracturefeatures)andflatareasshow generallyhigheryieldscomparedtoboreholeslocatedonslopesandhilltopsduetothepresenceofthickerweatheredma- terialsthatallowforgoodrechargeconditions[9,16].However,thiscommonpracticedoesnotaccountfortheeffectsofthe layered organization ofcuirassedpaleosurfaces.Indeed,[20] have shownthatHRA are theproduct oflong-termgeomor- phicevolutionofthelandscapethatoccurredthroughtectonically-controlledcyclesofdeepweatheringanderosion.Ironor bauxiticduricrustthatlocallyoverliesaprolite(acommonfeatureofdeeplyweatheredHRterrains)aregenerallyabsentat lower altitudesandthefissuredlayer isthuscloserto thegroundsurface(sometimesoutcrops)[2,7,12,21].Unfortunately, the resultsare often notsatisfactory (high failure rate)because theinfluence oftopography isoften overridden by addi- tionalfactors[9,19,22].Thispracticeisbasedonthelocationconditions(easieratthebottomoftheslope)and/orthecost ofdrilling(priceper meterdrilled,shallowerdrillingatloweraltitudes)oftheboreholeandnotonthestrategyproposed by [1,2] which,it should be recalled, require additionalknowledge and techniquessuch as lineamentanalysis when the stratiformfissuredlayerhasbeencompletelyremovedbyerosion.

Lineament analysisis a standard technique(in HR areas) that tectonic models rely on for locating fractures(perme- ablestructures,[14,18]andthensitingwater-boreholeswithoutanyprior/exhaustivehydrogeologicalstudy[23].Thiswould resultinhighratesofnegativeboreholesdueto thelackofsubstantialcorrelation betweentectoniclineamentsandfrac- tures [10,23]. Despitethe unresolvedrelationship between lineaments andsubsurfacepermeability, the useof lineament identificationingroundwaterexplorationremains animportantinitialguidefordrillingtarget selectionbytheprojectsu- pervisors duringdrillingcampaigns [9,10,15]. Theseare assumedto be thesurface representationofsubvertical fractures andaredetectablebysuitableprocessingofsatelliteimagesandgeophysicaltechniques[17,22–29].Amongallthegeophys- ical methods,electricalandelectromagnetictechniquesarethemostpopularingroundwaterexploration dueto theclose relationshipbetweenelectricalconductivityandsomehydrogeologicalpropertiesnamelytheporosity,mineralizationofthe groundwater anddegreeof watersaturation[25].Intermsofdata quality,predictive management andcost-effectiveness, someauthorshaveshownthatthecombinationofelectrical/electromagnetic(EM)methodsandremotesensingareeffective ingroundwaterexplorationworkinHRareasandmoreadvantageous(intermsofcostanddatainterpretation)forproject beneficiarycommunities[22–24,27,30]especiallyinruralareaswherehydrogeologicalstudiesaremoredeficient.According to [8],ruralAfricans remainone ofthemostmarginalisedpopulationsin termsofwatersupply,beingalmost fourtimes

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populationisestimatedatnearly15,000inhabitantsaccordingtoCentralBureaufortheStudyandCensusofthePopulation of Cameroon.Theproject servedasthebasis fora DesignEngineer thesis(see [33]), unfortunatelythistypeof projectis oftennotsustainableforthebeneﬁciary populationswhentheimplementationisdonerandomly. Beside,boreholeyieldis frequentlylow intheHRareasnamelyduringthedryperiodswherewatersuppliesdonot havelong-termsustainability.

Toensurethesustainabledevelopmentandmanagementoftheregion’spotentialgroundwaterresources,itisimperativeto locatedrillingsitesandvalidatetheinformationobtainedwiththefieldrealitiesandothersimilarinformation.Thus,inthe contextofpreliminarystudiesandgiventhesocio-economicconditions,theuseofefficientandinexpensiveapproachesfor groundwater explorationcouldbepromising.Theobjectiveofthisworkwastocontribute, throughtheanalysisofremote sensingandelectromagneticdata,tothedeterminationofthehydrogeologicalfeatures likelytoallowtheidentificationof potentially favorablesitesforthelocation anddevelopmentofboreholesinthislocality.Thestudiesfocused ondetermin- ing thestructuralorganization ofthelineamentnetwork, thesubsurfaceelectrical characteristicsandtheimplications for groundwater exploration in thispart ofthe CentralAfricanbasement whereexploration and sustainablemanagement of groundwaterresourcesarefragile.

Materialandmethods

Geographicalandgeologicalsetting

The studyarea islocated intheLittoralregion (southernpartofCameroon)about90km fromtheAtlanticcoast and between latitudes 3°30’and 4°10’N andlongitudes 10°09’ and10°54’E (Fig.1). The climatic indications are those of the climaticstationofthecityofEdéacorrespondingtothehumidtropicaltype(equatorialmonsoon)withfourseasons(with rainfallpresentevenduringthedriestmonth)markedbyameanannualtemperatureof26°Candanaverageannualrainfall of2600mm.Thevegetationcorresponds tothedomainofthedenseombrophilousforest,whichisverysecondarizedand often degraded by human activities [34],particular by thecultivation ofoil palm andcocoa. This zone islocated in the coastalplainofCameroonwithsubmeridianridgesrisingtoabout500m(Fig.1).Thisplaincorresponds toapost-African surfaceandhasanundulating morphologysimilar tothatofthe southernCameroonian plateau(averagealtitude750m), whoselandformcorresponds tothedismantlingofanold ironduricrust undergoingmorehumidclimaticconditions[35]. Thehighaltitudeareasaredominatedbysmoothrockyhillswithlargeconvexslopes(80or100%)relayedbynarrowvalleys (1to2mwide)forthetributariesoftheSanagaRiverinthenorth(flowtowardstheNorthorNorth-West)andmarshyflat valleys (40to50mwide)forthetributariesoftheN’Djiba,asub-tributaryoftheNyongRiverinthesouth(flowtowards theSouthandSouth-East).Thesewide,flat,marshyvalleysareafeatureofthetributariesoftheNyongRiver,whichflows overthesouthernCameroonianplateaubeforeemptyingintotheAtlanticOcean.

Fromageologicalviewpoint,thestudyareaispartoftheNyonggroup(Fig.2)whereseveralpetrographicandstructural studies havebeencarriedout. Thesestudieshaveshownthat theNyonggroupinthe NWcornerofthe CongoCraton in CameroonrepresentsthosepartsoftheArcheancratonthatwereremobilizedduringthePaleoproterozoicEburnean/Trans- Amazonianorogeny[36–39].ThisGroupconsistsofasetofmetasedimentaryandmetaplutonicrockwhoseheterogeneityis alsoobservedattheNgweilocality(Fig.2b,[40]).Theweatheringrocksunderhumidtropicalconditionsresultedinlateritic weatheringproﬁleswhichfrombottomtotopconsist of[41]:i)aweatheringoralteriticset(isalteritesurmountedby the alloterite); ii)an intermediate orglebular set characterizedby the accumulationof metallic oxyhydroxidesandclay with totallyorpartiallytransformedlithorelictualorganization;andiii)asurfacesoftlevel(soil);whichisclayey-sandytosandy- clayey andmaybeabsentwherethenodularhorizonisdeveloped.However,thetopography,inlinkwiththearchitecture oftherock,determinesthedifferenttypesofsoils[41]:hydromorphicsoilsinthemarshylowlands;erosionhasputlateritic gravelsoutcroppingeverywhereontheslopes(nooverallironorbauxiticduricrustingduetothehumidclimaticconditions, a few residual blocks at the bottom of the slopes); the plateaus and summits have moderately deep and evolved soils (ferralliticsoils).Structurally,[38]showedthattherocksoftheNyongComplexunderwentabrupttectonicscharacterizedby fourdeformationphasesnamely:ductileD1(representedbyasub-E-WS₀/₁foliation)andD2(representedbyasub-vertical S₀/₁/₂ foliation,L2lineation inferred toD2 are parallel andorientedWSW–ENE to WNW–ESE),ductilobrittleD3 (F3 fold orientedN176/21°andC3SW-NE shear-zones)andpost-orogenic brittleD4 (NE-SW, NNE-SSW,ENE-WSW andE-W).The

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A.F. Bon, A. Ombolo, P.M. Biboum et al. Scientiﬁc African 17 (2022) e01272

Fig. 1. Map showing the location of the Ngwei District.

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Fig. 2. Geological sketch map of the southern Cameroon (A, after Toteu et al., 2006, modiﬁed) and Geological sketch map of the study zone (B, after Champetier De Ribes and Reyre, 1959, modiﬁed).

NNE-SSWdirection,similartothatoftheSanagainthisdownstreamsector(N070Einitsupstreampart,Sanagafault,[42]), isthetectonicdirectionofthebasementintheNgweilocality.Moreover,theSanagaaccident,whichistheextensionofthe Bozoum-Ndélé accident (CentralAfricanRepublic,seeESM1)isthemostsouthern andthemostcontinuousoftheCentral Africanlineaments[42].ThisdirectionfromPoumaintheNEtoCampointheSWwasindicatedasamajortectoniccorridor intheKribi-Campoarea[38].ThisfaulthasbeenindicatedasacontinuationoftheKribi-CampoFault(KCF)whoseseismic effectssuggestatectonicallyactivezone[42].Thesefracturedrocksformpotentialaquiferswhoseeﬃcientexplorationcan allowforasafe,reliable,andeasilyaccessiblewatersupplyforthepopulationsinthisregionfacingdrinkingwatersupply problems.

Remotesensingdata

The analysisofcertain characteristicsoftheEarth’ssurface, suchassoiltone, textureandpattern,straightnessofriver coursesand vegetationpattern, whoseradiationis measured asspectral bands,allows todetermine thelineaments from satellite images [22,43]. Accordingto some authors[29,44–46], Landsatimagery dataare one ofthe mostinteresting in- formation sources on theEarth surface. In this study,the remote Sensing involved obtaining the Landsat-8 OLI imagery (L8) downloadedwith path186androw58 injanuary 2021on thehttps://earthexplorer.usgs.-gov/ sitebelongingtothe UnitedStatesGeological Survey(USGS).Asreportedby[45],geometriccorrectionoftheacquiredL8dataisoftennotnec- essary since they are alreadygeometrically corrected by USGSin the Universal Transfer Mercator (UTM) projection with theWGS-1984, Zone32Nmapdatum.Based onanumberofprocessingtechniqueslisted intheliterature[43,47–49],the contrast of theBlue, Green, Red, NIR, SWIR1and SWIR2band data, witha spatial resolutionof 30m, wasdigitally en- hanced. Following visual evaluation, theRed band datawas selectedfor thisstudyasit appeared to havegood contrast andbetterdefinitionofgeologicalfeatures(lineaments)comparedtotheother bands.RadiometriccalibrationandtheFast Line-of-SightAtmosphericAnalysisofSpectralHypercubes(FLAASH)atmosphericcorrectionmodelwereappliedusingENVI softwareasdescribedbysomeauthors[18,25,45,50].Imagenormalizationwasthenperformedonthisdatasetinthesame environment, andan additionalsub-setting wasdoneto extract thestudyarea (Fig. 3).The next stepwasthe extraction ofthelineamentswhichcanbedone manuallyorautomatically[44,51].Inthisstudy,automaticlineamentextractionwas performedbyfirstapplyingSobeldirectionalfilteringandthenlineamentdetection.Theautomaticextractionoflineaments wasperformedthroughthemoduleLineofPCIGeomaticasoftware.ThisprocessisbasedontheCannyfilterandincludes the steps thatare managedmainly byseveralparameters such as[26,45]:theRadius filter(RADI),gradient edgethresh-

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Fig. 3. Images of the study area showing contrast and brightness differences.

old (GTHR),thecurve lengththreshold(LTHR),angulardifferencethreshold(ATHR)andlinkingdistancethreshold(DTHR).

The extractedlineamentswereexported asaShpextensionﬁleandthenimportedintoArcgissoftwareforstatisticaldata processingandanalysis.TherosediagramwasgeneratedfromtheRockworksoftware.Inordertoevaluatethelineaments extractedautomaticallybytheLinemodule,areferencelineamentmapcreatedbasedonthemanual lineamentextraction ofthestudyarea isrequired[45].SincetheNgweidistrictdoesnothaveadetailedgeologicalmap,thereferencemapfor theevaluationwascreatedusingthegeologicalmapofsouthernCameroon(see[36]).Thisgeologicaldataassociatedwith thesurfacetopographyandtheelevationmapwasveryusefultovalidate thelineamentsobtainedautomatically,allowing thedetectionofareaswithabruptchangesindiscontinuitieswhenall thisinformationissuperimposed.Theshadedrelief mapofelevationwasgeneratedfromDigitalElevationModel(DEM)withthehillshademoduleinArcGISsoftware[26].

The geometric parametersof thelineaments,especially thedirections (usingthe directionaldiagram) andthelengths, were statisticallyanalyzed todescribe thestructure ofthe zone.The statisticaldistributionoflineament length datawas evaluated usingtheKolmogorov–Smirnovtest.Thestudyoffracturelengthdistributionwasdone usingthehistogram, cu- mulativefrequenciesandfrequencydensity.Thelineamentdensity(Dl)wascalculatedwiththeLineDensityToolintheGIS environmentusingequation(1)whilethefrequencydensity(Df)wascalculatedfromequation2.

Dl=TotalLineamentLength

(

^L

)

Studyarea

(

^A

)

⁽¹⁾

D f= f requency

classamplitude (2)

Geoelectricaldata

The geophysical prospection wasbased on the electromagnetic(E.M) methodwhich is not widely used in hydrogeo- logicalprospection ofcrystallinerocksinCameroon unlikeelectrical resistivity.The datawereobtainedusingPQWT-S300

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Fig. 4. Typical movement of M and N Electrode along a measured line.

AutomaticMappingWaterDetectorforDrillingWaterWell.Theinstrumentiswidelyusedintherapidanalysisofgeologi- cal structurechangesindifferentterrainssuchasplains,hills,mountains,plateaus,andbasinstodeterminewelllocations, aquifers, andaquifer depth [30,see alsohttps://www.pqwtcs.com]. A totalnumber offive (5)cross-sectionswere estab- lished acrossfive projectvillages (Fig.1). Themeasurement tranverseswere perpendicularto themajor tectonicaccident direction in thearea, which is also the major directionof the Sanaga River (see section 2.1). Thesedifferent tranverses, T1toT5,correspondedtoNdjockloumbe,Makondo,Makek,NsongdongandMapubirespectively.TheEMmachine(PQWT) was setup ateach site. As described inthe manual, E.M electrode probeM and Nwere placed on the soil at points0 and10 malongthe transverse;theprobes were connectedtotheE.M.consoleby acablethat readstheelectromagnetic pulsegeneratedbythesepoints.TheelectrodesMandNaremoved1mtotherightandtotheside ofthemeasuredline, i.e.Mis movedto 1m,whileNismovedto 11m(Fig.4),the readingsaretaken, thiscontinuesuntilthewholelength is covered.The machine,bypushinga button,automaticallyprocessesthedataandgeneratesgraphsofcurvesandaprofile mapofallthemeasuredpoints(inmV),thedifferentcontrastsandtheprobabledepthofdetectionoftheusefulsubstance (water)canbe obtained.Inthisstudy,onepotential boreholelocation (ineach locality)wasselectedalong thehorizontal axis.Subsequently,ajointanalysisoftheboreholedataandlineamentswasdonetoassesstheimplicationsforgroundwater exploitationandthehydrogeologicalmodel.

Resultsanddiscusion Structuralcharacteristics

Analysisofthestructuralframemap(Fig.5a),whichrepresentsthelineamentsconsideredtobeoftectonicorigin,iden- tiﬁed 200 structuralelementscorresponding to 0.4lineaments per square km.The area appearsto havea homogeneous lineamentdistributionbutsomeareasweredevoidoflineaments.Theshapeandlengthshowacomplexityofthesegeolog- icalobjectsandcanthereforepresentawiderangeofhydraulicconductivitiesfrompermeabletoveryimpermeable[1,19].

The totallengthofthemapped lineamentsis292.96km,whichcorresponds to0.6kmoflineaments persquare kmsuggestingamediangroundwaterpotential[14,18,25].ThislineamentdensityisclosetotheclassicalorderofmagnitudeinHR areas(1to2kmoflineamentsper squarekm,[18]).Thelengthsofthelineamentscanbe groupedintothefollowingfive classes(Fig.6):0.4and0.9km(6.5%);1and1.5Km(64.0%);1.6and2.3km(25.5%);2.4and3Km(3.0%)andfinallyaclass whoselengthsaregreaterthan3.4km(1%).Thelineamentdensitymap(Fig.7)weregroupedintothreeclasses:0.00–0.23 Km/Km²,0.23-0.63 Km/Km² and0.23-1.73Km/Km² indicating low,mediumandhighlineamentdensityrespectively[25]. Thisdensitycontrastmayrevealdiscontinuitiesthatmaybemoreusefulforgroundwaterexplorationandlikelytogenerate higherboreholeyields.Aco-existingofproductiveandbarren/non-productivesectorscanalsobeobservedduetothisvery discontinuous distributionoflineament density[52].According to[22] and[25],the occurrenceoflineaments is directly proportional tothegroundwaterpotentialofan area,sincelineamentsrepresentzonesoffaulting andfracturing resulting inincreasedsecondaryporosityandpermeability.Themediumlineamentdensityappearstobemoredominantinthestudy area againsuggestinga mediumgroundwaterpotential.Adjustmenttestswithaconfidencelevelof95.0%(p<0.05)donot statisticallydescribethelength ofthe lineaments.Attemptingtomodelthedistributionofthesefracturesby apowerlaw showsthat onlyfracturesbetween2and2.4Kmare closetothetheoreticalpowerlawlinewitha correlationcoefficient R²=0.98(Fig.8).At bothendsofthe distribution,theexperimental valuesdeviatefromthepower law,possibilydueto effects relatedto oversamplingof smallfractures(<2 km)andalteration of thedistribution oflarge fractures. The value ofthe exponenta(10.52)ofthepowerlawthat indicatestheproportionofsmallfracturestolarge fracturesis moreover inconsistentwiththevaluesofafaultnetworkgenerallybetween1and3[15,18].

TherosediagramshowedthatthemajorlineamentdirectionswereintheNW-NEframeandinclude(Fig.5b):NNW-SSE, N-S,andNNE-SSW)whilethesecondarydirectionswereNW-SE;NE-SWandE-W.Thesedirectionscorrespondtothoseof

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Fig. 5. Lineament map of Ngwei District showing the tranverses and borehole loccation.

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Fig. 6. Lineaments’ length histogram.

the brittle phase (D4) determined by [37] in the Nyongcomplex (NyC), inparticular the NNE-SSW direction, which has been describedasthe structuraldirectionofthe area,andalsocorrespond tothoseofthe hydrographicnetwork (Sanaga andNyongrivers,seesectionGeographicalandgeologicalsetting).Accordingto[37],themultiplenatureofD4lithoclases shows that the NyC recorded various post-orogenic brittle tectonic phaseswith a horizontal shortening N176 sub-N-S a sub-vertical extension N084induced bystresses

σ

1 and

σ

3, respectively. TheN-S lineamenttrendpatterncoincides with thegeneraldevelopmentofthesub-vertical foliationintheNyCimplyingatectonicoriginofthelineaments.Nevertheless, the factthatthe NNE-SSWdirection(themajor geologicdirectioninthearea,[38])isnot themostdominantshowsthat itisvery unlikely(notall)that theidentiﬁedlineamentsareassociatedwithtectonicfracturing.Indeed,inrelationto the genesis ofthefractured/ﬁssuredhorizon, [1,2] indicatedthatduringweathering,swellingofferromagnesian minerals(e.g.

biotite) createstensioncracks;inaccordance withrockmechanics,theresulting fracturesareperpendicular tothe minor (subvertical)stressandthereforearesubhorizontal(paralleltothesofttopographycontemporaneouswithweathering)and leadtotheformationofthesubhorizontalgranitejoint.

Geophysicalcharacteristics

Theelectromagneticsignaturedepictedonthesubsurfaceproﬁlemapsofthedifferentsites(Fig.9)indicatedaccording to [27]thepresence ofa heterogeneousformationwiththeintermittentpresence ofshallowaquiferbodies (bluecolored areas). Thesemapsalsoshoweda variationin thelocationsofweatheredzoneswithfractureswithin andbetweensites.

These zones corresponded [33] to location 2 m for transverse T1 (Fig. 9A); locations 3,14 and 20 m for transverse T2 (Fig.9B);locations4and8mfortransverseT3andT5(Fig.9CandE).TransverseT4hadweatheredareaswithinsignificant fractures(Fig.9D).Theselocationsexceeda penetrationdepthof70mbelowthesurfaceintersectingthepointmeasured horizontaltotheexistingtransverse.Theyrepresentpotentialtargets,ofvariableinterest,forwater-boreholesiting.However, boreholesuccessis dueto goodlocation oftheboreholesitingbecauseapoint alongthehorizontalaxiswithpotentially higher groundwater extractionsmay not be chosen [30]. Contraryto the other transverses, T5(Fig. 9E) showeda higher waterpotentialatthesurface(10mdepth)especiallybetweenlocations2and14(seeblueareacircledinred).Thisdepth mayindicatethesub-surfaceaquifersituatedwithintheupperweatheredpart.Theverticalelectromagneticsignatureglob- ally showedan upperzone (0-10m) withhighvalues(0.13 ± 0.14mV) compared to theintermediate zone withunclear boundaries(0.12±0.13mV)(Table1).ThistypeofverticalsignaturehasbeendemonstratedinHRAbyotherauthorsusing electricalresistivitytomography[10,17,26].Thesezoneswereassimilatedtothesaprolite,thefissuredhorizonandthefresh bedrock,respectively.Fromtoptobottom,theconfigurationoftheelectromagneticpattern(bluecolor)isgloballyelongated whateverthedepthindicatingthepresenceofsub-horizontalfaults.Thefracturesappeartobe(sub) horizontalsincethey subsurfaceprofilemapsdonotshow patternssimilar tothoseindicatingthepresenceofverticalfractures[30]orsuchas that correspondingtotheconceptualmodelofaverticaldiscontinuityinhardrock[13,19;seeESM2).Thesemapsmay alsoindicatethattheverticalfractures,ifpresent,arenotsufficientlywaterlogged.

These (sub)horizontal joints (or sheeting fractures) that appear to haveincreasing spacingwith depth havebeen described insome HRformations elsewhereintheworld(France,USA,India,other Africancountries)[1,3,6,14,22,53].Asre- ported inthe literature,in granites,subhorizontal fracturesappearto be morepermeablethan subvertical fractures(Kxy

≈ 10Kz[2]).[14]statedthathorizontal…sheetingfracturescanbea signiﬁcantsourceoffreshwater,canoftendominate

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Fig. 7. Lineament density map of the Ngwei District.

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Fig. 8. Distribution of fracture lengths: a) cumulative distribution and b) frequency density graph.

Fig. 9. Subsurface Proﬁle Map generated from the Electromagnetic Survey : A = T1 (Ndjockloumbe), B = T2 (Makondo), C = T3 (Makek), D = T4 (Nsongdong) and E = T5 (Mapubi). The dark red vertical bars on the horizontal axis are the locations where the boreholes were drilled. Values on the drilling points represent groundwater levels.

theﬂowsystem,andmayaccountforthelackofcorrelationbetweenwellyieldsandlineamentsinsomestudies.However, some authorshaveindicatedthattheabsenceofverticaldiscontinuitieslimitsdeepweatheringwithinthem;thismaynot locallyenhancehydrauliccontinuitybetweenthegroundwateraquifersandthesedeepstructures[2,13,16;19].

Implicationsforgroundwaterexploitationandhydrogeologicalmodel

Theimplicationsoftheresultsforgroundwaterexploitationandthehydrogeologicalmodelarebasedonthejointanal- ysis ofstructural/geophysicalcharacteristics andboreholedata (yieldandweatheringproﬁle) fromtheﬁve localities.The

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Fig. 9. Continued

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Fig. 9. Continued

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A.F. Bon, A. Ombolo, P.M. Biboum et al. Scientiﬁc African 17 (2022) e01272 Table 1

Synthesis of the values (mV) of the different layers whose limits were determined on the basis of the borehole logs. T1 (Ndjockloumbe), T2 (Makondo), T3 (Makek), T4 (Nsongdong) and T5 (Mapubi).

T1 Depth (m) 0-20 20-70 > 70

Minimum 0.00 0.00 0.06

Maximum 0,45 0.42 0.42

Mean 0,21 0.20 0.18

Median 0.20 0.20 0.17

Standard deviation 0.10 0.09 0,08

T2 Depth (m) 0-5 5-70 > 70

Minimum 0.05 0.00 0.00

Maximum 0.32 0,23 0.18

Mean 0.17 0.11 0.11

Median 0.16 0.11 0.11

Standard deviation 0.06 0.06 0.04

T3 Depth (m) 0-17 17-31 > 31

Minimum 0.00 0.00 0.08

Maximum 0.53 0.49 0.57

Mean 0.26 0.25 0.29

Median 0.20 0.28 0.28

T4 Depth (m) 0-14 14-70 > 70

Minimum 0.000 0.009 0.012

Maximum 0.043 0.043 0.043

Mean 0.024 0.027 0.027

Median 0.026 0.026 0.029

Standard deviation 0.008 0,007 0,007

T5 Depth (m) 0-18 18-57 > 57

Minimum 0.000 0.000 0.003

Maximum 0.034 0.037 0.037

Mean 0.013 0.020 0.021

Median 0.014 0.022 0.022

numberofboreholes(ﬁve)islimitedbutarespatiallyrepresentativeofthestudyarea(Fig.5)andofthetwomainlitholo- giesencounteredinthearea(seeESM3,[33]).Thedistancesoftheseboreholesfromthenearest lineamentare229.26m;

28.4m;807.33m;613.6mand400.3mrespectivelyforNdjockloumbe;Makondo;Makek;NsongdongandMapubilocalities.

Accordingtosomeauthors,theradiusofinfluenceoflineamentsonboreholeproductivitycanbe300m[54],150m[9]and 60 m[14].Theyields ofthesedifferentboreholesare respectively2.85m³/h ;2.92m³/h;4.17m³/h;3.19 m³/hand3.22 m³/h;thecorrespondingdrilled-depthsare80matNdjockloumbe;90matMakondoand81mfortheotherthree.There appears tobe nodependencebetweenboreholes-yield andthedistanceto thenearestlineament sincetheboreholewith highyieldislocatedbetween400and800mfromthenearestlineamentsuggestingthatthehydraulicconductivityof(part of)thishardrockaquiferisduetoweatheringprocesses(productivitydependentonthefracturedhorizon)andnottotec- tonicfractures.Nevertheless,theMakekboreholewiththehighestyield(4.17m³/h)andradiusofinfluence(800m)appears to beassociatedwiththeNNE-SSW direction(parallelto T3,seeFig.5). Thisresultcorroboratesthoseof[54]whofound significantlyloweraverageyieldsforboreholeslocatedwithin50mofthelineamentscomparedtoboreholespositionedat adistancegreaterthan50m;theseaverageyieldswerealsohigheralongaspecificazimuthalsector. Itisrecognisedthat lineamentmappingisoftensubjectiveandprovideslittleinformationonthefracturetype,depth,openingorfillingpotential [23].However,theextenttowhichtheselineamentswouldinfluenceorberelatedtodrillingproductivityinthispartofthe NyongComplex hasnot yetbeenassessed.Itisanimportantstep,thisallowyoubetter understandingthehydrodynamic contextandbuildanaccuratemodel.

The analysisofthe05lithologsrevealedthat theweatheringprofiles havea verticalstructuretypicalinHRareas.This includes,fromtoptobottom(Fig.10A):(i)asaprolitelayer(sandyclaytoclayeysand)surmountedinplacesbyanodular level (iron crust), especially on strong or weak slopes, little marked talwegs. The absence of a typical iron crust could allow a generalizedrecharge[10].The thickness ofthislayer variesbetween0 (wherethe dislocated rockblocks appear or weathering horizon is truncated by erosion) and 18 m; (ii) a fissured layer whose cuttings are distinguished during drillingbyelementsofweatheredrockandfreshrock.Waterstrikesisoftenobservedduringdrillingwithadown-the-hole hammerbutthisrequiresspecialattentionfromthedrillers(inparticular,afterthefirstwaterarrivals).Inthisregard, the waterstrikes havebeenonly indicatedon twoboreholes (Fig.10), whichallows toestimate the thicknessofthefissured layer between13and39 m;and(iii)freshrock.However, the saprolitethickness, thewaterstrikes andthe saturatedor unsaturatedstateofthesaprolitesuggesttwoboreholecontexts[55]:

1) boreholesinanaquifercontextofthe« ﬁssuredwithweatheringcover» type,correspondingtotheMakek,Nsongdong andMapubienvironments;

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Fig. 10. Typical examples of the logs of Ngwei boreholes (A) and Electromagnetic log model (B).

Table 2

Borehole data and some data from wells in the vicinity of the boreholes.

Borehole Well

Yield (m ³/h) Depth (m) Static level (m) Weathered zone (m) Depth (m) Static level (m)

Ndjockloumbe (T1) 2.85 80 6.49 20 15 7.5

Makondo (T2) 2.92 90 6.49 5 15 6.7

Makek (T3) 4.17 81 7.69 18 12 7.0

Nsongdong (T4) 3.19 81 5.32 14

Mapubi (T5) 3.22 81 9.47 18

2) boreholesinanaquifercontextofthe« ﬁssuredwithoutweatheringcover» type,corresponding totheMakondoenvi- ronments.

The fact that the productivity ofthe Ngwei aquifers appears to be dependent on the fractured horizonimplies that explorationshouldbebasedonacontextualandnotageneralapproach.

Validationofresultsusingwellandboreholeyielddata

Validationisconsidered an essentialstep toverifytheaccuracyofthe groundwaterpotential modelproduced[25,29].

The resultsofthepresentstudywere comparedtosomeprevious studiescarriedoutinthevicinityorinsimilargeologi- cal environmentsinordertoassessthereliabilityofthestudy.Thedataconsideredwere theboreholedataespecially the well depths, saprolitelayer, yieldandgroundwater depthmeasured duringtheshortrainy season(March-April).Ground- waterlevelisanimportantparameterforunderstandingthehydrogeologicalconditionsofaregion.Itprovidesinformation ongroundwaterdynamics,groundwaterrecharge,borehole/wellrechargeconditionsandisusefulforgroundwatermanage- ment. Welldata (Table2) conﬁrm theexistence oftheshallowaquifer (saprolite)indicatedon thesurfaceproﬁlemaps; the groundwater level inthe wellsis between6.7and 7.5m.The borehole yieldvaries from2.85to 7.35 m³/h withan average of4.37 m³/h whilethe groundwaterlevel variesfrom5.32to9.47 mwithan average of7.09 m.Severalstudies have reportedgroundwaterpotential in HRareas [7,19,22,56,57]andshowedthat boreholes withyields above6.25m³/h

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aregenerallycharacteristicofhighgroundwaterpotentialzones,whilethosewithyieldsof6.25-3.13m³/hand<3.13m³/h are typicalofmedium andlowgroundwater potentialzones,respectively.Inrelationtothelineamentdensitymap,itwas observedthattheboreholesaregenerallylocatedinthemediumdensityzoneconfirmingthemediumgroundwaterpoten- tialofthestudyareaorthelowtomediumpotentialgenerallyobservedinHRgroundwaterandevenaquifersdevelopedon post-African erosionsurfacesnotablyinMalawi, ZimbabweandUganda[56].The presenceoftwoaquifertypes(borehole contexts)validatestheboreholeyielddataandimpliescontiguouszonesoflowandmediumproductivity.Suchastructure isconsistentwithwhatisknownaboutregionalhydrogeology[52]:dewateringoftheshallowaquiferandlocationofwater inthediscontinuousreservoirformedbythefracturedbasement;coexistenceofpotentiallyaquiferousfracturedsectorsand non-fracturedsectors. However, thepositionofthepiezometriclevel isgenerallylocatedbetweenthealterites(Fig. 10B) indicating thatthelatterarenotdewatered.ThischaracteristiciscommontoHRAdevelopedunderhumidconditionsand is favourableto the installationof type 1boreholes that canbe cased atthe alterites.Onthe other hand,dewatering of thealterites,characteristicofaridenvironments[10,13,52]hasbeenobservedintheMakondoarea(withalowerflowrate) implyingthatthisboreholeisfedbyafewlocalfissuresthataffectthealmostfreshbedrock.Thisimpliesthatthecontext oftheboreholeandthepositionofthepiezometriclevelshouldbetakenintoaccountforasustainableproductivityofthe boreholesinthearea.

Conclusion

Thisstudy,carriedoutinaruralareaofCameroon(CentralAfrica)wherehydrogeologicalstudiesarenonexistent,aimed atdeterminingthehydrogeologicalfeaturesfavorabletotheidentificationofpotentialgroundwatertargets.Tothisend,the application ofthe remote sensing approachand thegeophysical approach (Electromagnetic) were used. Automatic lineament extractionshowedthat thetectonic backgroundofthe studyareais characterizedby 200 lineamentswhose orien- tations (NNW-SSE,N-S, NNE-SSW,NW-SE;NE-SW andE-W)are globallycorrelatedwiththe tectonicregime observed in theNyongcomplex.Thetotallengthofthemappedlineaments is292.96km,whichcorrespondsto0.6kmoflineaments per square kmsuggestingamediangroundwaterpotential (based onthelineamentdensityandyields data).The levelof bedrock fracturing isalso visualized on the electromagnetic patternresults,which indicate areasthat mayserve asma- jor preferential groundwaterflowcorridors. Thesefracturesappearto begenerallysub-horizontalsuggestinggroundwater exploitation through verticaldrilling.The yields obtainedfromtheboreholes drilledin thepotentially favorablelocations seem to indicate a non-dependence betweenthe boreholeyield andthe distance to the nearest lineament.These yields andtheresultingweatheringprofileshowedcharacteristicscommontothosegenerallyobservedinhard-rockaquiferswith hydraulic propertieswhoseoriginismorerelatedtoweatheringprocesses.Based onthegeophysicalandboreholedata,a preliminaryconceptualmodeldescribingthevariationoftheEarth’selectromagneticfieldhasbeenproposed.Despitelim- ited boreholedata,theconsistencybetweentheresultsandfieldobservationsconfirmsthevalueofthisstudy(approach) whichcanbeconsideredasaninitialassessmentofthepotentialrelationshipbetweenlineamentsmappedbyLandsat8im- ageanalysis,electromagneticgeophysicsandboreholeproductivity,fromwhichfuturestudiescouldimprove.Thesestudies should bebasedon characterizationofthefracturedhorizon(the zonewheretheHRA owesits productivity)whosecon- trasting geophysicalsignaturehasbeenambiguouslycharacterized.Nevertheless,thepresentstudyhasshowntheinterest ofusingthecombinedremotesensingandelectromagneticmethodtoassesstheavailabilityofpotentialgroundwaterzones inareaswithsimilargeologicalandclimaticconditions.

Compliancewithethicalstandards Funding

Thisresearchreceivednospeciﬁcgrantfromanyfundingagencyinthepublic,commercial,ornot-for-proﬁtsectors.

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoconﬂictofinterest.

Acknowledgements

TheauthorsthanktheEditorandthetwoanonymousreviewersfortheirmanyhelpfulsuggestions.

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