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CoastalAcoustic Tomography CoastalAcoustic Tomography ArataKaneko GraduateSchoolofEngineering,HiroshimaUniversity, Hiroshima,Japan
Xiao-HuaZhu
StateKeyLaboratoryofSatelliteOceanEnvironmentDynamics,SecondInstitute ofOceanography,MinistryofNaturalResources,P.R.China
CollegeofInformationScience&Engineering,OceanUniversityofChina, Qingdao,P.R.China
JuLin
Elsevier
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9.1
9.1.2ExperimentandMethods.............................................................................107
9.1.3DifferentialTravelTimes............................................................................109
9.1.4Inversion.......................................................................................................110
9.1.5MappingCurrentVelocityFields................................................................112
9.2 TokyoBay
9.2.1OceanographicState....................................................................................113
9.2.2ExperimentandMethods.............................................................................113
9.2.3DifferentialTravelTimes............................................................................116
9.2.4Inversion.......................................................................................................116
9.2.5MappingCurrentVelocityFields................................................................117
9.3 KanmonStrait
9.3.1OceanographicState....................................................................................118
9.3.2ExperimentandMethods.............................................................................119
9.3.3DifferentialTravelTimes............................................................................122
9.3.4Inversion.......................................................................................................123
9.3.5MappingCurrentVelocityFields................................................................123
9.4 ZhitouyangBay
9.4.1OceanographicState....................................................................................125
9.4.2ExperimentandMethods.............................................................................126
9.4.3DifferentialTravelTimes............................................................................127
9.4.4Inversion.......................................................................................................129
9.4.5MappingCurrentVelocityFields................................................................129
9.4.6TidalHarmonics...........................................................................................133
9.4.7RotationofTidalCurrentsWiththeTidalPhase.......................................135
9.5 QiongzhouStrait
9.5.1OceanographicState....................................................................................135
9.5.2ExperimentandMethods.............................................................................135
9.5.3Range-AverageCurrentandVolumeTransport.........................................138
9.5.4Inversion.......................................................................................................140
9.5.5MappingCurrentVelocityFields................................................................141
9.6 DalianBay
9.6.1OceanographicState....................................................................................143
9.6.2ExperimentandMethods.............................................................................143
9.6.3DifferentialTravelTimes............................................................................147
9.6.4Inversion.......................................................................................................147
9.6.5MappingCurrentVelocityFields................................................................147
9.6.6Validation.....................................................................................................150
9.7 BaliStrait(June2016)
9.7.1OceanographicState....................................................................................151
9.7.2ExperimentandMethods.............................................................................151
9.7.3Range-AverageCurrents..............................................................................155
9.7.4North-EastCurrents.....................................................................................156
9.7.5Along-StraitVolumeTransportandEnergyBalance.................................157
9.7.6Inversion.......................................................................................................157
9.7.7MappingCurrentVelocityFields................................................................159
9.7.8Specialtyofthe3-hOscillation...................................................................161
9.8 HiroshimaBay
9.8.1OceanographicState....................................................................................162
9.8.6MappingReconstructedTemperatureFields...............................................167
9.8.7CoastalUpwellingandDiurnalInternalTides............................................168
9.8.8SeaSurfaceDepressionAssociatedWithUpwelling.................................171
9.8.9UpwellingVelocityandMixingRate.........................................................172
10.1.9ConcludingRemarks..................................................................................185 10.2 LuzonStrait
10.2.1OceanographicState..................................................................................185
10.2.2SiteandExperiment...................................................................................185
10.2.5IdentifyingArrivalPeaksintheReceivedData.......................................189 10.2.6ProfilingtheSoundSpeedDeviation........................................................191 10.2.7RetrievingthePeriodicPhenomena..........................................................193
11.1 NekosetoChannel
11.1.1ModelandMethods...................................................................................195
11.1.2Mapping2DCurrentFields.......................................................................197
11.1.3Validation...................................................................................................198
11.2 KanmonStrait 11.2.1ModelandMethod.....................................................................................199
11.3
13.1.6YearlyVariationsoftheObservedCurrent andTemperature........................................................................................256
13.1.7ResidualCurrentCalculatedFromUpslopePointMethod......................257
13.2 BayWithMultiinternalModes
13.2.1SpecificFeatures........................................................................................259
13.2.2ExperimentandMethods...........................................................................260
13.2.3Range-AverageSoundSpeed....................................................................264
13.2.4SpectralAnalyses.......................................................................................265
13.2.5PropagationofInternal-ModeWaves........................................................267
13.3 BayWithResonantInternalModes
13.4 StraitWithInternalSolitaryWaves
13.4.1Background................................................................................................275
13.4.2ExperimentalSiteandMethods.................................................................276
13.4.3TravelTimesandRange-AverageTemperaturesfortheLargest ArrivalPeak...............................................................................................277
13.4.4DistanceCorrection...................................................................................281
13.4.5SoundTransmissionDataWithMultiarrivalPeaks..................................282
13.4.6RaySimulationandInversion...................................................................282
13.4.7ProfilingTemperatures..............................................................................285
13.4.8ConcludingRemarks..................................................................................285
13.5 RiverWithTidalBores
13.5.1SpecificFeatures........................................................................................287
13.5.2ExperimentandMethods...........................................................................289
13.5.3Cross-RiverSurveysbyShipboardAcousticDopplerCurrent Profiler........................................................................................................292
13.5.4Cross-RiverSurveysbyCoastalAcousticTomography...........................294
13.5.5RiverDischarges........................................................................................294
13.5.6ConcludingRemarks..................................................................................298
13.6 LargeCircularTankWithOmnidirectionalWavesandCurrents
13.6.1FloWaveCircularTank.............................................................................299
13.6.2SimulatingFlowFields..............................................................................301
13.6.3ExperimentandMethods...........................................................................302
13.6.4IdentifyingMultiarrivalPeaks...................................................................303
13.6.5MappingtheTwo-DimensionalCurrentVelocityFields..........................305
13.6.6RemainingIssues.......................................................................................306
Preface Coastalacoustictomography(CAT)ismotivatedbyhowsubsurfaceflowmotionintheseacanbe visualizedashasbeenperformedinmodernfluiddynamics.CATisaninnovativetechnologywhich hasbeendevelopedasacoastal-seaapplicationofdeepsea orientedoceanacoustictomography (OAT).Instrumentation,inversemethod,anddataassimilation,whichhavebeendevelopedinthe OATstudy,areapplicabletoCATstudiesaswell.TheOAT-relatedscienceisnicelyorganizedin themonographoflateprofessorWalterMunk,publishedin1995,althoughtemperaturemeasurementsareemphasizedratherthancurrentvelocitymeasurements.However,CATstudiesbasedon theshallow-seasoundpropagationwithmultireflectionsofacousticwavesatthesurfaceandbottom requireexperiencesandstrategiesquitedifferentfromOATstudiesthatuseadeep-seasoundchannel.Furthermore,theprocessofsoundpropagationdependsstronglyonspecializedthermalstructuresinindividualcoastalseasandmaybesignificantlyinfluencedbyambiguityofbottom topographies.TheoperationaluseofCATmaybeacriticalissueintheworldoceanographiccommunitythathasnotoolstomapsimultaneouslyrapidlyvaryingcoastalenvironments.
ThisbookcoversallsubjectsnecessaryforunderstandingtheprincipleofCAT,operatingCAT systems,analyzingtheCATdata,andmappingcurrentvelocityandtemperature/salinityfields. Also,dataassimilationstillplaysanimportantroleinthisbookasafantasticpartnerofCATto predictcoastal-seaenvironmentalvariationsasinthedailyforecastofregionalweathersbyradars. TheaimofthisbookistoimprovethepresentstatethatCAThasbeenlessprevailedintheworld oceanographiccommunityinspiteofitsownspecialty.Thebookisplannedsuitableforthelecture noteintheseniorundergraduateandmastercourselevelsinnotonlyinphysicaloceanographyand oceanfluiddynamicsbutalsoacousticaloceanographyandoceanacoustics.Thisbookalsotargets coastalenvironmentalistsandpolicymakerswhohavestrongconcernintheecologicaleffectsof globalwarmingoncoastalseasbyprovidingapossibilityoflong-termmonitoringofcoastalthermalenvironments.
AllmaterialsusedinthisbookareprovidedmainlyfromthePhDthesesandjournalpapersthat GangYuan,HongZheng,Jae-HunPark,ToshimichiIto,Xiao-HuaZhu,HaruhikoYamaoka, KeisukeYamaguchi,ZhouYuan,JuLin,FadliSyamsudin,Hong-QuangNguyen,Yudi Adityawarman,NaokazuTaniguchi,Chuan-ZhengZhang,andMinmoChenpublishedwiththefirst authorsofthisbookasPhDcandidatesinthelast25years.ThisbookwouldnothavebeencompletedwithoutcooperativeworkswiththemduringeachPhDcourse.Mostapplicationsdiscussin thisbookregardingtheChineseSeasarecontributedbynoticeableworksperformedbyZe-Nang ZhuduringthePhDworkswiththesecondauthorandinthefollowingstudies.Thedataof Indonesianseas,whichmakeapplicationfieldsofthisbooksofruitful,wouldnotbeacquiredwithoutmemorablecontributionbyFadliSyamsudin.AsupporttotheSetoInlandSeaandthestudies byHidemiMutsudawascriticalforsuccess.ThecontributionofGuangmingLitotheFloWave experiment,andTongshengShenandGuangmingLitotheLombokStraitexperimentis pronounced.
CATsystemwasfirstdesignedbyHongZhengandadvancedtoauser-friendlylevelbyNoriaki GohdabyusingeasilyaccessibleCPUboard,operationsystem,andrelatedprograms.Especially,his 30-yearcontributiononsystemdesignanddevelopmentofCAT/mirror-typeCATintheHUgroup wouldbeadmiredbyallofus.Finally,weappreciateChuan-ZhengZhang,MinmoChen,Ze-Nan Zhu,GuangmingLi,andAruniDinanHanifaforreadingthemanuscriptandreplottingfigures.
ArataKaneko,Xiao-HuaZhuandJuLin December2019
FUNDAMENTALKNOWLEDGE 1 1.1 OCEANACOUSTICTOMOGRAPHY Oceanacoustictomography(OAT)wasproposedbyWalterMunkofScrippsInstitutionof Oceanographyin1970s(MunkandWunsch,1979).Informationfromseismic(AkiandRichards, 1980)andmedicaltomographyservedasaprecursortechnologytorealizeOAT.Especially, advancedinversionmethodshavebeenimportedfromseismictomography.ThefirstOATexperimentwascarriedoutin1981tomapthetemperaturestructuresofmesoscaleeddieswith 100 300kmscales,pinchedofffromtheGulfStreamanddriftedtotheBermudaSea(TheOcean TomographyGroup,1982).Inthisexperiment,underwatersoundchannel(USC)playedanessentialroleinlong-rangesoundtransmissionoverabout300km.Thethermalstructuresofmesoscale eddiesreconstructedbytheinverseanalysisofone-waytravel-timedatabetweenthearrayed sourcestoreceiverswerevalidatedwithconductivity temperature depth(CTD)andairborne expendablebathythermograpgh(AXBT)data,performedduringandbefore/aftersoundtransmission.Intheremainingperiodof1980s,intensivetomographyexperimentsweresucceededinthe NorthAtlanticOceanandtheMediterraneanSeaincollaborationoftheUSandEuropeanOAT groups.Time-varyingtemperaturefieldswithdeepwaterformationweretargetedintheexperiment andtheirthermalstructureswerefirstmeasuredundercarefulconsiderationofpositioningaccuracy suchthatthepositionofsubsurfaceacoustictransmittersonmooringlineswastrackedby triangular-arrayedtranspondersattheseafloor,increasingtheoperationcostofthemooringsystem. Sufficientpositioningaccuracyfortemperaturemeasurementmaybehopelesswithouttracking mooringlinemotion.
In1990stheJapaneseOATgroup,initiatedbyJapanAgencyforMarine-EarthScienceand TechnologyCenter,startedthetomographyexperimentsintheNorthPacificOceanincollaboration withtheUSandEuropeanOATgroups.Byusinga200Hzpressure-compensatedtypeacoustic transceiver,observationrangewasextendeduptoabout1000km.Eddy-embeddedthermalstructuresintheNorthEquatorialCurrentandtheKuroshioExtensionweresuccessfullyreconstructed bythestochasticinverse(Gauss Markov’smethod)accompaniedbythefunctionexpansionof sound-speeddeviationfields(Yuanetal.,1999).Incontrasttothermalstructures,thetomographic measurementofcurrentvelocitystructures,whichrequiresstrictclockaccuracyratherthanpositioningaccuracy,wasrarebecauseacontinuous,long-termoperationofatomicclockintheunderwaterwasdifficultowingtointolerablepowerconsumption.
Althoughtheinstrumentationandmethodologyfordeep-seaoperationandtheinversionmethodsforanalyzingdeep-seatomographydatawerewellestablisheduntiltheendofthe20thcentury, thedrivingforceonOATstudieswasremarkablyweakenedintheoceanographiccommunityin thefirstdecadeofthe21stcentury.Thisunexpectedsituationmaybemainlycausedbyheavy
instrumentationcostrequiredforalong-termoperationofOATmooringarraysinthedeepsea. Theheavyinstrumentationcostalsoprohibitedaworldwideoperationofdeep-seatomographysystemsbycommonoceanographers,whowerenotsupportedwithbigprojectfundsandnotspecializedinoceanacoustics.ThusoceanographerslostthefirstchanceofoperationaluseofOAT, whichmaybeacentralinstrumentinEulerianmethods.Intheseconddecadeofthe21stcentury, deep-seaOATexperimentrestartedintheFramStrait(Sagenetal.,2016;Dushawetal.,2016a,b) andCanaryBasin(Dushawetal.,2017)bythespecializedorganizationssupportedbyahugefund, targetingalong-termmonitoringofstraitthroughflowforwarminginthearcticseaandthetrackingofmeddieswanderingtheNorthAtlanticOceanaroundtheexit(theGibraltarStrait)ofthe MediterraneanSea,respectively.Inspiteofthepasttremendouseffort,whichhasbeendevotedto OAT,OATcouldnotreachanoperationalphaseunlikeotherstandardoceanographicinstruments characterizedwithacost-efficientperformance,suchasacousticDopplercurrentprofiler(ADCP) moorings,Argofloats,highfrequency(HF)oceanradars,andinvertedechosounders.
OATisanadvancedoceanographictechnology,whichcanmakeasimultaneousmapping(snapshot)oftime-varyingsubsurfacestructuresofcurrentvelocityandsoundspeed(mainlytemperature)withaspeedofabout1.5kms 1 usingunderwatersoundchannel.InOAT,observation domainissurroundedbymultiacousticstationsandreciprocalsoundtransmissionisperformed betweenthesestationpairs.Dataarethetraveltimes(time-of-flight),whichareobtainedalong acousticraypathsandconvertedtopath-averagecurrentvelocityandsoundspeedthroughstandard equations.Theaccuracyofobserveddataisremarkablyimprovedthroughapathintegralalongthe ray.ThesuperiorityofOATiswellunderstoodincomparisonwiththedataobtainedbyconventionalone-pointsensors,suchascurrentmeters,andtheprofilingdatabyADCP.Furthermore,an advantageofOATmooringsincomparisonwithconventionalcurrent-metermooringsispronouncedwithincreasingnumberofmooringlines.InthecaseofOATmooringarraywithseven acousticstations(Fig.1.1),thenumberofsound-transmissionlines(datanumber)is 7C2 5 21significantlygreaterthanthedatanumberof7forconventionalcurrent-metermoorings.Thereduced numberofmooringlinesrequiredforattainingthesamespatialresolutionasconventionalmoorings
FIGURE1.1
SketchoftheOATarraycomposedofsevenacousticstations.T1 T7aretheacousticstationsandthestraight linesconnectingthestationsarethesound-transmissionlinesprojectedtoahorizontalslice.Circlesarethe positionsofconventionalcurrent-metermooring(M1 M7)withthesamenumberastheOATarray. OAT,Ocean acoustictomography.
becomesafurtheradvantagebecausetheshipboardworkimposedondeep-seamooringsisoneof theheaviestdutiesinobservationaloceanography.
Asforhorizontal-slicetomographyassketchedin Fig.1.1,acousticraysareusuallyapproximatedwithstraightlinesowingtonegligiblysmallhorizontalrefractionofsoundandhorizontal resolution ΔL iscalculatedbythefollowingformula:
where A istheareaoftomographydomainspannedbyraysofthenumberof M.Itprovidesonly anaverageresolution,althoughraydistributionisusuallynotuniform.InOAT,whichusesalongpathinformation,dataareacquiredalongtheraypaths.Thusnoinformationisobtainedinsubspacessurroundedbyeachray.Finehorizontalresolutionisattainedintheregionswithadense distributionofraysandaroundtheregions,whereraysmakecrossingpoints.Dataintheregions withnoncrossingofraysmaybeproducedbyaninterpolationbytheuseofinformationfromthe neighboringrays.
1.1.1 BREAKCORNER(PROJECTEDRAYSONAHORIZONTALSLICE) Raysonahorizontalslicearenotreal,butonesprojectedfromreal,refractedraysonavertical slice.Theyarewellapproximatedbystraightlinesbecausethegradientofsoundspeed,constructed mainlybytemperature,ismuchsmallerinthehorizontaldirectionthanintheverticalone.While theverticalstratificationoftemperatureisconstructedbysolarradiationabsorbeddownward fromthesurface,thehorizontalvariationofsolarradiationisnegligibleatoceanicscalesranging fromthesmalltomeso,whichOATmainlytargets.Thussoundrefractionisslightonthehorizontalsliceandtheprojectedraysarenearlystraight.
1.2 ADVANCEMENTBYCOASTALACOUSTICTOMOGRAPHY ForOAT,whichcanusetheUSC,acoustictransceiversarepreferabletolocatenearthechannel axisthatisconstructedatthebottomofmain(permanent)thermocline(Fig.1.2).BysettingacoustictransceiversattheUSCaxis,raysaredistributedoverawiderangeofdepthfromthesurfaceto axis,constructingdifferenttraveltimes,whichmakevertical-sliceinversionvalid.Baroclinicocean variabilityisgenerallystrongaroundthemainthermoclinewithalargegradientoftemperatureand significantlyweakenedatthebottomofthethermocline,correspondingtotheUSCaxis.OceancurrentsaregenerallyconfinedtotheupperoceanfromthesurfacetothemailthermoclineandweakenedneartheUSCaxisatthebottomofthemainthermocline.OATmooringswithanacoustic transceiverneartheUSCaxis,characterizedwithweakcurrent,providemorecost-effectiveand easieronboard-handlinginstrumentationthantheconventionalcurrent-metermoorings,which requiretolocatecurrentmetersinthepermanentthermoclinewithstrongoceancurrent.Intheshallowseawithasound-speedprofile,featuredwithanupperpartofUSC,near-bottommooringsare preferabletoconstructrayspassingvariousdepths(Fig.1.3).Thecorrespondingmooringworks
FIGURE1.2
SketchoftherefractedrayspropagatingaroundUSCbetweenacousticstationsT1andT2.Theverticalprofileof soundspeedisshownattheleftofthefigure.TheUSCaxisisindicatedwithablankarrownearthesound-speed profile.Themainthermoclineisalsopointedoutbyablankarrownearthetemperatureprofile. USC,Underwater soundchannel.
FIGURE1.3
SketchoftherefractedrayspropagatingbetweenacousticstationsT1andT2intheshallowseafeaturedwithan upperpartofUSC.Theverticalprofileofsoundspeedisshownattheleftofthefigure.Thenear-bottom mooringcomposedoftwoPHs,acoustictransducer(TR),AR,floatsandanchorareshownwithamagnified scale. AR,Acousticrelease; PHs,pressurehousings; USC,underwatersoundchannel.
alsobecomeeasyduetoanear-bottomdeploymentofthetomographysysteminaweakcurrent. Depthresolutionforvertical-slicetomographyisestimatedby
Seafloor
where H isthewaterdepthand M isthenumberofrayspropagatingdifferentdepthsinaverticalslice.Generally,thedepthoftheoceanis muchsmallerthantherangeofsoundpropagatinghorizontallyintheoceansothatitisdiffic ulttoresolverange-dependentinformationin thevertical-sliceinversion.This difficultyisstrengthenedintheshallowsea,becausethenumberofresolvedraysandcorrespondingarrivalp eaksismorerestrictedintheshallowseathan inthedeepsea.Inspiteofthelackofrange-dependentinformationonave rticalslice,thelayeredstructuresofhorizontallyaveragedcurrent velocityandsoundspeedarereconstructedby inverseanalysis,usingalimitednumberofray spassingdifferentdepths.Ontheotherhand, range-dependentinformationiseasilyobtainedb yraysdistributedonahorizontalsliceateach depthofthehorizontallayers.
Shallow-seaacoustictomography,calledcoastalacoustictomography(CAT)inthisbook,has beenexecutedasacoastal-seaapplicationofOATandfeasibilityexperimentswerecarriedoutin theGibraltarStrait(Sendetal.,2002),FloridaStrait(DeferrariandNguyen,1985;Koetal.,1989; ChesterandMalanotte-Rizzoli,1991),HaruStrait(Elisseeffetal.,1999),andtheBarentsSea (Chiuetal.,1994),usingalimitednumberofacousticstations(threeinmaximum).Morecomprehensive,multistationCATexperimentswithacousticstationsgreaterthanfivewereinitiatedby HiroshimaUniversityAcousticTomographyGroupin1990s.Thereductionofinstrumentationcost andthesystemcompactnessthatmakesanonboard-handlingeasywerecentralstrategiestodesign theCATsystem.Observationrangesmayberestrictedtolessthan50kminthecoastalseasshallowerthan100m.Artificialcoastalconstructions,suchaswharfs,piers,jetties,pontoons,and breakwaters,havebeenusefulasatemporalplatformtosetCATsystemespeciallyinJapanese coastalseas,wheremooredobservationisstrictlyprohibitedbyfisheriesactivityandshippingtraffic.AnarrayofanchoredboatssurroundingatomographydomainhasalsobeenusedasashorttermplatformsuitableinthecoastalseasaroundChinaduetolessexpensiverentalcharges. However,along-termoperationofCATsystemwasthenextissuetobetackledbyanarrayof anchoredships,becausehumansupportwasneededonboardduringtheentireobservationperiod. Attractiveexperimentswerecarriedoutwithstationnumbersof4 11intheNekosetoStrait, TokyoBay,theKanmonStraitandHiroshimaBayofJapan,ZhitouyangBay,QiongzhouStrait, SanmenBayandDalianBayofChina,andtheBaliStraitandLombokStraitofIndonesia. Especially,theDalianBayexperimentinMarch2015wascarriedoutwith11CATstationsand detailedstructuresoftidalcurrentandresidualcurrentwerereconstructedbythestandardinverse method.Applicationtotheterrestrialwaterswasalsoattemptedusinghigherfrequencysoundupto 50kHz(Kawanisietal.,2010).
CurrentvelocitystructureswerecorrectlyreconstructedbysynchronizingtheclockofmultistationswithGPSclocksignals.GPSprovidesnotonlyclocksignalsbutalsopositioningdatain CATexperiment.However,thepositioningaccuracyofabout10m,providedbyone-pointGPS positioning,wasinsufficientformappingaccuratelytemperaturefieldsowingtothelimiteddistancesofsoundtransmission.Then,thepositioningaccuracywasimprovedbycombiningCAT traveltimewiththesoundspeeddeterminedfromCTDdataatapointinwaterlyingbetween acousticsourceandreceiver.Observationrange(station-to-stationdistance)wasrestrictedupto 50kmbythemultireflectionsoftransmissionsoundatthesurfaceandseafloor.Theambiguityof bottomtopographymaybeafactortoproducebiasesinthereconstructedcurrentvelocityand sound-speedstructures.
1.3 COASTAL-SEAENVIRONMENTALMONITORING Generally,fishingactivityandshippingtrafficareheavyinthecoastalseas.Suchsocietaldemands haveprovidedseriousdifficultiesinenvironmentmonitoringofcoastalseas,especiallyaroundEast Asiancountrieswithstrongconcerninfishresources.Mooredobservationhasbeenstrictlyregulated,whileshipboardobservationisusuallypermitted.ItisaprofoundadvantageofCATthat multiacousticstationssurroundingatomographydomaindonotdisturbfisheriesworksandshippingtraffic,becausereciprocalsoundtransmissioncanbeperformedamongmultiacousticstations locatedattheperipheryoffishinggroundoroutsideshiptrafficroutes.
Althoughstudiesonshallow-seasoundpropagationmaybestillchallengingissuesforacousticians,CATapplicationtothecoastalseashasthehighestpriorityonsocietaldemands.Thepresent knowledgeonshallow-seasoundpropagationprocessesisrestrictedandfurthermore,theambiguity ofbottomtopographyisaddedindifficulty.Thesefactorsmaymakethecurrentvelocityandtemperaturefields,reconstructedbytheinverseanalysisofCATdata,lessaccurate.Thedoubtfulnesson theaccuracyofCATdataisvanishedbyadoptinganintelligentmethod,inwhichCATdataarevalidatedincomparisonwithdatafromotherstandardoceanographicmethods,suchasADCPandCTD.
IncontrasttoOAT,whichrequireheavy,expensiveinstrumentationtomeasuremesoscaleto large-scaleoceanicphenomenawithscales100 1000kmindeepseas,CATcanbeexecutedwith easyhandlingonsmallboatsandcost-effectiveandeasyhandlinginstrumentationsencourageoperationbyanumberofcoastal-searesearchers,engineers,andenvironmentalists,supportedby common-sizedfunds.Variouscoastal-seaphenomena,suchascoastalcurrents,tidalcurrents,tidal vortices,residualcurrents,estuarycirculation,port/harborcirculationandonshore offshorewater exchange,inner outerbaywaterexchange,frontalcirculation,coastalupwelling,coastaljets,and thespreadingofriverplume,aretargetsofmappingbyCAT.
Thesimultaneousmappingoftime-varyingcurrentsinthecoastalseasisadifficulttargetfor shipboardobservationbecauseofrapidlyvaryingdeformationofcurrentfieldsduringtheshipboard survey.TherepeatADCPsurveysynchronizedwithtidalphasessometimesmakesitpossibleto measureresidualcurrent.However,asimultaneousmapping(snapshot)ofvortex-embeddedcurrent fieldsistoodifficulttobeperformedbyshipboardsurvey.CATmaybeauniquetechnologyfor mappingcoastal-seaenvironmentsvaryingrapidlywithtime.Real-timemonitoringofcoastal-sea environmentsisrealizedbyamultistationCATarray,locatedattheperipheryofbaysandports anddatatelemetryusingamobile-phonenetwork(Fig.1.4).Thepresentstateofcoastal-seaenvironmentsismeasuredbyCATarrayandthereal-timedataarebroadcastedtopublicsociety.By assimilatingtomographydataintoacoastalsea circulationmodel,thepredictionofcoastal-sea environmentvariationsisalsopossibleasdoneindailyweatherpredictionbyradar.
1.4 COASTAL-SEASOUNDPROPAGATION Soundpropagationinthecoastalseasisstronglyinfluencedbythestateofthermalstratification. ThusthemeasurementofcurrentandsoundspeedbyCATisdifficulttoexecutewithoutapriori informationofthermalstratification.Stratifiedcoastalseasareusuallyseeninthesummerseason atmidlatitudesandtropicalseasinallseasons.Inastratifiedsea,acousticraysdrawanupward refractedpattern,constructingmultibottomreflections.Anidealizedraypatternobtainedbetween acousticstationsT1andT2isshownin Fig.1.5.Underthisstate,raypathsaredistributed
FIGURE1.4
CAT-monitoringarraydistributedattheperipheryofabay.Real-timedataaretransferredtointegrateddata centerviaamobile-phonenetwork.Real(refracted)raysaredrawnontwoverticalslicesandallrefractedrays areprojectedwithstraightlinesontoahorizontalslice. CAT,Coastalacoustictomography.
FIGURE1.5
Idealizedsoundpropagationinastratifiedsea:(A)raypatternbetweenacousticsource(T1)andreceiver(T2), locatedneartheseafloor,and(B)raypatternbetweenT1andT2,locatedneartheseasurface.
constructingupperturningpointsatvariousdepthsfromthesurfacetoseafloorwhensource(T1) andreceiver(T2)arelocatedneartheseafloor.SoundisreceivedatT2,gatheringinformationat variousdepths.Itprovidesaniceconditionforvertical-slicetomography.Ontheotherhand,sound propagatesdrawingaconvergencezonewhenT1andT2arelocatednearthesurface.Shadow zoneswherenorayspassareconstructedinanumberofwideregionsbetweenT1andT2,soitis toodifficulttofindanappropriatereceiverpositionforsuccessfulCATexperiment.Soundpropagationsignalsarenotreceivedatanypointsinsidetheshadowzone.Forsimplicity,seaflooris hereassumedtobeflatwithoutlosingthegeneralityofsoundpropagation.
Anexampleoftherange-independentraysimulationusingasound-speedprofile,obtained betweenacousticstationsT1andT2inastratifiedsea,isshownin Fig.1.6.Atthiscase,two shadowzonesareconstructedintheupperlayer.However,transmitsignalscanbereceivedreciprocallyatT1andT2,becauseacousticenergyisconvergedtowardT1andT2onthenearshore platform.
Homogenizedcoastalseasappearinthewinteratmidlatitudesandinalltheseasonsathighlatitude.Furthermore,waterishomogenizedbystrongtidalcurrents,whichpassthroughnarrow straitsandchannels.Forthesecases,raypathsconstructdownwardrefractedpatterns,formingmultisurfacereflectionsandlowerturningpointsatvariousdepths.Anidealizedraypatternobtainedin ahomogenizedseabetweenacousticstationsT1andT2isshownin Fig.1.7.Inahomogenized sea,T1andT2shouldbelocatednearthesurfacetogatherinformationfromvariousdepths.In contrast,wideshadowzonesappearinmanyplaces,formingaconvergencezonewhenT1andT2 arelocatedneartheseafloor.Anexampleofrange-independentraysimulationusingasound-speed profile,obtainedinahomogenizedseabetweenacousticstationsT1andT2,isshownin Fig.1.8 Althoughshadowzonesareconstructedneartheseafloor,transmissionsignalscanbereceived
FIGURE1.6
Exampleoftherange-independentraysimulationusingasound-speedprofile,obtainedatasitebetweenacoustic stationsT1andT2inastratifiedsea.Threerayspassingdifferentdepthsaretraced,constructingshadowzonesat twoplacesintheupperlayer.
FIGURE1.7
Idealizedsoundpropagationinahomogenizedsea:(A)raypatternbetweenacousticstationsT1andT2,located neartheseasurface,and(B)raypatternbetweenT1andT2,locatedneartheseafloor.
FIGURE1.8
Exampleoftherange-independentraysimulationusingasound-speedprofile,obtainedinahomogenizedsea lyingbetweenacousticstationsT1andT2atdepth5m.Shadowzonesareconstructedattwoplacesnearthe seafloor(Adityawarmanetal.,2012b).
T1T2
FIGURE1.9
(A)BathymetricchartoftheLombokStrait.Thesound-transmissionlinesA BandC Dareindicatedwiththe blackandpinklines,respectively.ACTDpositionismarkedwithCTDatthecenterofthestrait.(B)and(C)are theresultsofrange-independentraysimulationbetweenthesource(A,C)andreceiver(B,D)locatedat5m abovethesiteswiththefloordepthsof450and250m,respectively.Thesound-speedprofileisshownattheleft ofthefigures(B)and(C).
reciprocallyatT1andT2locatedatdepth5m,convergingacousticenergytowardT1andT2on theshoreplatform.
OneexampleofshadowzonesisobtainedintheLombokStraitbetweenBaliandLombok IslandsinIndonesia,wheresignificantpartoftheIndonesianThroughflow(ITF)passesthrough (Fig.1.9A)(GordonandFine,1996).Range-independentraysimulationisperformedbetweenstationsAandBandstationsCandDusingoneCTDdataobtainedatthecentralpartofthestrait. Whenacousticsourceandreceiverarelocatedat5mabovetheseafloorwithdepth450m,no shadowzonesareconstructedinthenear-surfacelayerofthecentralpartwithamaximumfloor depthofabout1000m(Fig.1.9B).However,forthesource receiversystemlocatedat5mabove theseafloorwithdepth250m,awideshadowzoneisconstructedintheupper250mofthecentral region(Fig.1.9C).Asaresult,thesiteswithfloordepth450mareselectedasappropriateCAT stationstomonitortheITFtransportbecausethemajorstreamoftheITFisconfinedtotheupper 200maroundthepermanentthermocline.
ReciprocalsoundtransmissionbetweenacousticstationsT1andT2,placedatadistanceof 30.17kmoneithersideofthestrait,isheretakenintoconsiderationasatrickyexampleofsound transmission.T1andT2arelocatedatthebreakwateronbeacheswithagentleandasteepslope, respectively.Range-independentraysimulationisperformedusingaCTDdataobtainedonthe transmissionline(Fig.1.10).ThetransmissionlinecrossestheSetoInlandSea,Japan,fromT1at Honshu(thebiggestJapanesemainisland)onthenorthernsidetoT2atShikoku(thefourthbiggest Japanesemainisland)onthesouthernside.WaterwashomogenizedinMarch(Fig.1.10A),so soundreleasedsouthwardfromT1mainlytravelsintheupper20minthesurfacewaveguideand isreceivedpersistentlyatT2withdepth6m.Onotherhandthesurfacewaveguideissubmergedin adepthrangefrom10to25mowingtoseasonalwarming(Fig.1.10B).Accordingtotheray
Range-independentraysimulationforthereciprocalsoundtransmissionbetweenacousticstationsT1andT2 locatedoneithersideofthestrait:(A)March15dataand(B)May12data.ThecurvedredarrowoffT2 indicatesarayturningupwardinthemixedwaterabovethedeepchannel.Thesound-speedprofileispresented attheleftofthefigures(A)and(B)(Adityawarmanetal.,2011).
simulationforMay,soundpropagatinginthewaveguidehitsthesteepslopenearT2anddoesnot reachthesamestation.AsobservedintheCTDdata,waterismixedbystrongtidalcurrent,which occursabovethedeepchannelnearT2.Thuspropagatingsoundturnedupwardinthemixedwater andgotachancetoreachT2(seethe redarrow inthefigure).Thestrongtidalmixingoccurring offT2iscausedbythedeepchannel,whichhasbeenmaintainedasthemainshiptrafficrouteof theSetoInlandSea.
Specificshadowzonesareconstructedwhenreciprocalsoundtransmissionisperformedona slopingseafloorinnearshorebeachesandshelfseas.Inastratifiedseaashadowzoneisconstructedbelowthesurfacewithincreasingdepthtowardtheoffshore(Fig.1.11A).Raysreleased fromtheonshorestationT1aresubmergedfromthesurfaceaftertheyhitit.Ontheotherhanda shadowzoneinahomogenizedseaisconstructedalongtheseafloor,deepeningtowardtheoffshore (Fig.1.11B).RaysreleasedfromT1areconfinedtotheupperlayeraftertheyhittheseafloor.No transmissionsoundisreceivedattheoffshorestationT2whenitislocatedinsidetheshadowzone. AcousticenergyisconvergedtowardonshorestationT1forboththestratifiedandhomogenized seas.ThustheconvergingsoundissuccessfullyreceivedatT1,evenifitisplacedatanydepths.
Aspecificproblemofsoundpropagationoccurswhenlow-frequencysoundpropagatesina shallowwaveguideonabeach,boundedbetweentheseasurfaceandseafloor.Thereisaminimum frequencyofacousticwaves,calledfrequencycutoff,whichthefirst-modewavewithfrequencies lowerthanacriticalvaluecannotpropagateintheshallowwaveguide.Thefrequencycutoff
FIGURE1.10
FIGURE1.11
ReciprocalsoundtransmissionbetweenonshorestationT1andoffshorestationT2onaslopingbottom:(A) stratifiedseaand(B)homogenizedsea.T2islocatedintheshadowzoneforboththecases.Thepositionwhere rayhitsthesurfaceorseafloorisindicatedwithacircle.
phenomenamaybepronouncedinashallowwaveguideabovesoftsedimentlayersowingtosignificantenergyabsorptionintothelayers.
Inmidlatitudecoastalseas,thermalstratificationinsummerisreplacedbyhomogenizedwater inwinterduetosurfacecoolingcombinedwithstrengthenedsurfacewindstresses.Forthecoastal seaswithsuchenvironments,specificconsiderationisneededtomaintainthesuccessreceivingrate oftransmissionsoundatasatisfactorypercentage.Torealizeitanappropriatedepthofacoustic transceiverisatamiddlebetweenthesurfaceandseaflooriftransducerdepthsarefixedduringthe sound-transmissionexperiment.AnothermethodtoincreasethesuccessrateistoexecuteCTDsurveysforconfirmingthepresentsound-speedprofileatthetransitionoftheseasonandthetransceiverdepthsareadjustedtomatchtheupdatedsound-speedprofile.Anoptimumreceivingrateof soundtransmissionisusuallyattainedbetweentheacousticstations,locatedatthesamedepthfrom theseasurfaceabovetherelativelyflatseafloor.
INSTRUMENTATION 2 2.1 SYSTEMDESIGN Aland-basedcoastalacoustictomography(CAT)systemprovidesanewobservationtooltoenable oceanographicobservationfromtheshore(Zhengetal.,1997;Zhengetal.,1998).Thesystemis composedofaCATcontrollerandrechargeablebatteriesonland,andsubsurfacetransducerand cableinwater(Fig.2.1).Atomographydomainissurroundedbymultiland-basedCATsystems, andonlysoundtraversesthetomographydomainsurroundedbytheacousticstations.Solarpanels thatsupplementtherechargeablebatteriesareequippedasanoptionoftheCATsystemwhena long-termoperationisrequired.CATcontrollerisstoredinaplasticcontainer.Thusradiowaves fromglobalpositioningsystem(GPS)canpropagatedirectlytotheGPSreceiverstoredinsidethe containerwithoutopeningtheupperlidofthecontainer.InafieldoperationofCAT,thisprovides acriticaladvantagetoprotecttheinternalelectriccircuitsfromtheattackbysaltywindandmakes along-termstableoperationofCATsystempossible.
Ontheotherhandasubsurfacemoored-typeCATsystemneedspressurehousingsforstoring systemcontrollerandbatterycellsasseenin Fig.2.2.TwopressurehousingsconstructedwithaluminumtubesarepreparedforthemooredCATsystem.Twopressurehousingsareconnectedbya stainlessframewithatransduceronthetopoftheframe.Observationperiodsdependonbattery capacity,system’spowerconsumption,andtransmitinterval.Theremainingspaceofthepressure housingwithasystemcontrollerandanotherpressurehousingisreservedforincreasingthenumberofbatterycells.
Blockdiagramoftheland-basedCATsystemisshownin Fig.2.3.ThetimingofsoundtransmissionandreceptionandA/DconversioniscontrolledcoherentlybySH7145FmicroPC.All clocksareprovidedby1Hzand10MHzclocksignalsfromGPS.Forthemoored-typeCATsystem,GPSclocksarereplacedbychip-scaleatomicclock(CSAC).Acarrierphasemodulatedby pseudorandomsignals(M sequenceandGoldsequence),constructedbythesoftwareprogram,is transmittedatapredeterminedtimeintervalfromabroadbandtransducerafterpoweramplification. Acousticsignalscomingfromthecounterstationarereceivedbythesametransducer(called mono-staticsonar),band-passfiltered,andamplified.Bytakingaproductofthereceivedsignals withcosineandsinefunctions,theyarebranchedintotwochannels;in-phaseandquadraturecomponentsandlow-passfilteredtopickuponlysignalsrelatedtothe M sequencethroughcomplex demodulation.Afterthe10-bitA/Dconversion,thereceiveddataaresampledattwicethecarrier frequencyandstoredastwo-channelrawdatainthe2GBmicroSDmemorycard.Thereceived signalsarealsocross-correlatedbyone-periodreplicaof M sequence,usedinthetransmissionto increaseremarkablythesignal-to-noiseratio(SNR).Asaresult,amatchedfilterisconstructed throughthecorrelationwith M sequence.Thecorrelationcalculationcanbeperformedbythe
CoastalAcousticTomography.DOI: https://doi.org/10.1016/B978-0-12-818507-0.00002-0 © 2020ElsevierInc.Allrightsreserved.
