laboratory-studies-of-seismic-wave-propagation-in-

BulletinoftheSeismologicalSocietyofAmerica,Vol.87,No.4,pp.809-823,August1997LaboratoryStudiesofSeismicWavePropagationinInhomogeneousMediaUsingaLaserDopplerVibrometerbyOsamuNishizawa,TakashiSatoh,XinglinLei,andYasutoKuwaharaAbstractWeperformedphysicalmodelexperimentsbyutilizingalaserdopplervibrometer(LDV).BecauseLDVconvertsvelocityofvibrationtotheDopplershiftfrequency,itenablesveryprecisemeasurementsofultrasonicwaveswithoutanyresonatingelementthatconventionaltransducersusuallyinclude.Apiezoelectrictransducer(PZT)wasusedasasourceofelasticwaves,andthewaveformwasmeasuredinaverysmallareaofabout400pmindiameterbyfocusingthebeam.Wecaneasilyperformveryprecisemeasurementsofwavefieldinaphysicalmodel,andthusphysicalmodelexperimentsofwavepropagationcansimulaterealisticseismicfieldobservations.Formodelsofinhomogeneousmaterial,weusedthreegraniticrockswithdifferentgrainsizes:Westerlygranite(finegrained),Oshimagranite(mediumgrained),andInadagranite(coarsegrained).Largerockprisms,300×300×80to90ram,wereusedtopreventcontaminationbymultiplereflec-tionsfromthesideendsintheearlierportionofwaveforms.ThedirectPandSwavesandreflectedwaveswereidentifiedbytheirtraveltimes.Observationsweremadebylongin-line,circular,andsmall-aperturearrays.Whentherockgrainsizebecomescomparabletothewavelength,transmittedwavesarestronglyattenuatedbybackwardscattering,andalargeamountofwaveenergyistransferredtothecodaportion.Semblanceplotsinthetime-slownessplaneobtainedfromthesmall-aperturearraysuggestthatincoherentwavesbecomedominantasthegrainsizebecomeslargeandcomparabletothewavelength.IntroductionPropagationofseismicwavesininhomogeneousstruc-tureisanimportantsubjectforseismicexploration,predict-ingearthquakestrongmotions,andstudiesoftheEarth'sinterior.SeismicwavesarescatteredbyinhomogeneifiesintheEarth'sinteriororarereflectedfromstructuralbound-aries(Aki,1969;AkiandChouet,1975).Complicatedgeo-logicboundariesandrandomdistributionoffaultsandmeltsintheEarth'scrustproducerandomincoherentwavesinthelaterportionofseismograms.Randomincoherentwavesap-pearingafterPandSwavescontainmuchinformationonheterogeneifiesandstructureoftheEarth'sinterior(Sato,1984).Thus,studiesoftheserandomwavesarevaluableandimportant.Wavepropagationthroughcomplexsubsurfacestruc-turehasbeenstudiedusingnumericalmodelstointerpretdatafromreflectionseismology(articlescompiledbyKellyandMarfurt,1990)andtopredictearthquakestrongmotions(Anderson,1991;Aki1988).However,duetothecomplex-itiesoftheEarth'sinterior,numericalmodelingcannotbeperformedforrealisticEarthmodels,whichincludebothlarge-andsmall-scalerandomheterogeneitiesindensitiesandwavevelocities.Experimentsusinglaboratory-scalemodels(physicalmodels)provideanalternativewayofstudyingwavepropagationininhomogeneousmedia(Du-bendorffandMenke,1986;Loetal.,1988;Matsunami,1990,1991;Vinogradovetal.,1989,1992;Pyrak-Nolteetal.,1990a,1990b;HsuandShoenberg,1993;SchultzandToksrz,1995).Inphysicalmodels,apiezoelectrictransducer(PZT)isusedforreceivingelasticwaves,becauseofitshigheffi-ciencyforconvertingelastic-wavesignalstovoltagesignals.However,thecharacteristicresonanceofthePZTandthecontactconditionbetweenthePZTandthematerial'ssurfacestronglyaffecttheobservedwaveforms.Moreover,thesizeofthePZTisusuallycomparabletothewavelengthoftheelasticwave.Itisdifficulttoobtaintheactualvelocityanddisplacementsofamaterial'ssurfaceusingPZTvoltagesig-nals.Thus,experimentalresultssofarhavebeenqualitativeratherthanquantitative.Accordingly,anelastic-wavesensorthatconvertstheactualvibrationofthematerialtoanelectricsignal,andalsomeasuresinanareathatissmallcomparedtothewavelength,isessentialforphysicalmodelstudies.Boleretal.(1984)developedacapacitance-typetransducerformeasuringactualvibrationofamaterial'ssurfacewithin809810O.Nishizawa,T.Satoh,X.Lei,andY.Kuwaharaaverysmallarea.However,itwasonlyusedforlimitedpurposesbecauseofthedifficultyofoperation(Boler,1990).AlaserDopplervibrometer(LDV)enableseasyandac-curatemeasurementofthevelocityofultrasonicvibrations(Yamamotoetel.,1992).ItconvertstheDoppler-shiftfre-quencytovibrationalvelocityofthematerial'ssurface,andthelaserbeamsizeisabout400/zmindiameter.UsingtheLDVasanelastic-wavesensor,wecanquantitativelyanalyzefullwaveformsinphysicalmodels.Inthefollowing,weshowexamplesofmodelexperimentsforstudyingelastic-wavepropagationininhomogeneousmaterialsandpresentsomeimportantfeaturesofwaveformsthatweobservedinmaterialswithdifferentsizesofinhomogeneities.ExperimentAlightbeamreflectedfromamovingobjecthasaDopplershiftfD:fD=2V/2,(1)whereVand2denotethevelocityoftheobjectalongthebeamandthewavelengthoflight,respectively.Weem-ployedanLDVLV-1300(manufacturedbyOno-sokkiInc.,Japan),whichisusedformeasuringvibrationoffactoryproductssuchasvideoheads,diskheads,carbodies,etc.(Yamamotoetel.,1992).TheLDVisequippedwithaHe-Nelaserhavingthewavelengthof632.8nm.TheDoppler-shiftedfrequencyfoisconvertedtoavoltagesignalthatindicatesactualvibrations.Sincethevelocityisdirectlycon-vertedtothefrequencyshift,thereisneitherthemechanicalnortheelectricalcouplingbetweenthematerial'ssur