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1、FundamentalsofthecapacitiveresistivitytechniqueOliverKuras,DavidBeamish,PhilipI.MeldrumandRichardD.OgilvyBritishGeologicalSurvey,KingsleyDunhamCentre,Keyworth,NottinghamNG125GG,UnitedKingdom(June17,2005)ABSTRACTCapacitiveresistivity(CR)isanemerginggeophysicaltechniquedesignedtoex
2、-tendthescopeoftheconventionalmethodologyofDCresistivitytoenvironmentswheregalvaniccouplingisnotoriouslydifficulttoachieve,forexampleacrossengi-neeredstructures(roads,pavements),hardrock,drysoilorfrozenground.Concep-tually,CRisbasedonafour-pointarraycapacitivelycoupledtotheground.U
3、ndercertainconditionscapacitivemeasurementsofresistivityareequivalenttothoseob-tainedwiththeDCtechnique,thusmakingDCinterpretationschemesapplicabletoCRdata.Thecouplingpropertiesofpracticalsensorrealizationsareshowntobeafunctionoftheirgeometricalarrangement.Separatebodiesoftheorya
4、reassociatedwithtwocomplementarybutdistinctsensortypes:thecapacitivelineantennaandtheplate-wirecombination.Theuseofplate-wirecombinationsresultsinlocalizedcoupling,which,inconjunctionwithaquasi-static(lowfrequency)formulationofthetransferimpedance,providesavalidemulationoftheDCme
5、asurementwithpointelectrodes.Aparametricstudyofthecomplexquasi-statictransferimpedancerevealstheexistenceofarestrictedrangeofpracticalparametersthatpermitssuccessfuloper-ationofCRinstrumentsatlowinductionnumbers.TheorypredictsthatemulationoftheDCmeasurementiscompromisediflow-indu
6、ction-numberoperationisnotmaintainedthroughoutasurveyarea,forexampleinazoneofhighconductivity.1Validationofthetheoryisachievedusingspeciallydesignedfield-scaleexperimentscarriedoutwithaCRinstrumentcapableofmeasuringthefullcomplextransferimpedance.Atintermediatedipoleseparations,th
7、eresultsareconsistentwiththepredictionsofquasi-statictheory.Deviationsobservedinthenearandfarseparationzonescanbeexplainedbygeometriceffectsorthebreakdownofquasi-staticcondi-tions,respectively.Finally,undersuitableconditionsthephase-sensitiveexpressionforapparentresistivityasafunc
8、tionofthecomplextransferimpedanceisshown
