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    CHAPTERTHREE

    INTRODUCTIONTOOCEANOGRAPHY

    3.1 OCEANBOTTOMCHARACTERISTICSLearningObjectives

    Namethefivemajoroceanicprovincesandtherelieffeaturesassociatedwiththeoceanbottom.

    Describethefivemajoroceanicprovincesandtherelieffeaturesassociatedwiththeoceanbottom.

    Nameanddescribethevarioustypesofbottomsediments.

    Earth'stopographyhasadefiniteandimportanteffectontheelementsandcharacteristicsofits

    surroundingatmosphere. Thesamerelationshipexistsbetweentheoceanfloorandtheoceans. The

    irregularterrainoftheoceanflooraffectsthemovementofoceanwater,temperaturegradientsin

    areasofchanneling,navaloperations,andsubmarine/antisubmarine(ASW)tactics. Manyrelief

    featuresandbottomtypesareusedbysubmarinerstoconcealtheirsubmarinesanddecreasetheir

    probabilityofdetectionbysurfacesonar. Sonartransmissionsthatimpactthebottomareaffectedby

    thebottomtopographyandbottomtypes(sand,mud,etc.). Sonarperformancemaybeimprovedor

    hinderedbythebottom;therefore,submarinersusethebottomtotheirbestadvantage. Thesurface

    fleetmustalsobeawareofbottomrelieffeaturesandbottomtypesinordertoimprovethe

    effectivenessoftheirsearchsonar.

    3.1.1 BOTTOMTOPOGRAPHY

    Fromsealeveltothedeepestdepthsbeneaththesea,therearefivemajorbottomprovinces. These

    provincesincludethecontinentalshelf,thecontinentalslopeandrise,andthemidoceanridges. See

    Figure31.

    3.1.1.1 ContinentalShelf

    TheContinentalShelfisthefirstprovinceoffshore. Theaveragewidthoftheshelfisapproximately40

    miles,butinsomeplacessuchasthewestcoastofSouthAmerica,theshelfisnonexistent. Thewidest

    shelfisfoundalongtheglaciatedcoastofSiberia,whereitextendsoutwardfromshoreroughly800

    miles. Continentalshelvescompriseabout7.5percentofthetotaloceanbottom.

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    Thecontinentalshelfhasaverygradualslope. Itdeclinesatanaveragerateof2fathomspermile.

    Eventhoughtheaverageslopeoftheshelfisgradual,terraces,ridges,hills,depressions,anddeep

    canyonsarefoundwithinitsboundaries. Theshelfregionisatransitionzonebetweenfreshwater

    runofffromlandandthemoresalinewaterofthesea;consequently,itisanareaofgreatmixingwith

    generallyunstablewaterconditions. Currentsnormallyrunparalleltotheshoreinthisregion.

    3.1.1.2 ContinentalSlope

    Attheseawardedgeofthecontinentalshelf,theslopebecomesmuchsteeperandthedropoffisvery

    rapid. Onaverage,theslantratioisroughly20timesgreaterthanthatofthecontinentalshelf. The

    ratioisgreateroffmountainouscoaststhanoffwide,welldrainedplains.

    Thecontinentalsloperesemblesasteepcliffthathasbeenerodedbyheavyrains. Itsmoststriking

    featuresarethesubmarinecanyonsthatareprevalentalongtheslopeface. Thesecanyonsarethought

    tohavebeenformedbyturbiditycurrents,whicharedense,sedimentladencurrentsthatflowalongthe

    oceanfloor. Attheseawardendofthesecanyons,largeamountsofsedimentaredepositedandspread

    outinafanlikemannertoformthecontinentalrise.

    3.1.1.3 ContinentalRise

    Thecontinentalriseisfoundseawardofthecontinentalslope,inapproximately500fathomsofwater.

    Itismadeupofthicksedimentdepositsthatcoverirregularrelieffeatures. Thesedepositsslopegently

    seawardformingtheabyssalplainsofthedeepoceanbasins. Attheseawardedgeofthecontinental

    rise,thewaterdepthisabout1,500fathoms.

    Figure31.BottomTopography(Source:NAVMETOCCOM)

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    3.1.1.4 OceanBasins

    Theoceanbasinsaccountfor76percent

    oftheoceanfloor,withdepthsranging

    from1,500to3,000fathoms. Ocean

    basins,onaverage,haveveryslight

    inclinesofnomorethan1:90miles. For

    every90milesseawardthebottom

    slopesnomorethan1mile.

    Superimposedonthisveryflatplainare

    manyruggedrelieffeatures,suchas

    seamounts,guyots,atolls,sills,and

    trenches.

    3.1.1.4.1 Seamounts

    Seamountsaresubmerged,isolated,pinnacledmountainsrising3,000feetormoreabovetheseafloor.

    3.1.1.4.2 GuyotsorTablemounts

    Guyotsaresubmerged,isolated,flattoppedmountainsthatrise3,000feetormoreabovetheseafloor.

    3.1.1.4.3Atolls

    Atollsareseamountsorguyotsthathavebrokentheseasurface,andcoraldepositshavebuiltup

    aroundtherim. Thecoralformsareefaroundashallowbodyofwater. SeeFigure32.

    3.1.1.4.4 VolcanicIslands

    Theseislandscanoccurindividuallyandingroups. Approximately10,000volcanoesarelocatedacross

    theoceanfloorandtheyareespeciallyabundantinthewesternPacificbasin. TheHawaiianIslandsare

    probablythebestknownexampleofvolcanicislands.

    3.1.1.4.5 Sills

    Sillsareelevatedpartsoftheoceanfloorthatpartiallyseparateoceanbasins. Asillrestrictsthe

    movementofbottomwatermassesandresultsintheirpartial,andinsomecasesnearlytotal,isolation.

    Figure32. Atoll

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    3.1.1.4.6 Trenches

    Trenchesarelong,narrow,andrelativelysteepsideddepressionsandtheycomprisethedeepest

    portionsoftheoceans. TrenchesaremorenumerousinthePacificthaninanyotherocean. Pacific

    Oceantrenchesaverage2,500milesinlengthandareofgreaterdepththanallothertrenchesaround

    theworld. Forexample,theMarianaTrenchis35,600feetdeep;theTongaTrenchis35,430;andthe

    MindanaoTrenchis34,428feetdeep. Trenchesarenormallyfoundontheseawardsideofislandarcs,

    whicharelong,curvedchainsofoceanicislandsassociatedwithintensevolcanicandseismicactivity,

    whilerelativelyshallowseasexistonthecontinentalside.

    3.1.1.5 Ridges

    Ridgesarethelastoftheoceanprovinces. TheMidAtlanticRidgeextendssouthwardfromIceland

    acrosstheequatortonear55S,dividingthe Atlanticintoaneasternandwesternbasin. The Mid

    AtlanticRidgerisesfromadepthof2,500fathomsand iscontinuous atdepthsoflessthan1,500

    fathomsoverthe greaterpartofits length. Inseveralplaces,thisridgerisestoabovesea level

    toformislandssuchasthe Azoresand AscensionIslands.

    3.2 BOTTOMCOMPOSITION

    Theoceanbottomiscoveredbyvarioustypesofbottomsedimentsmixedwithdissolvedshellsand

    bonesofmarineorganisms. Sedimentdepositsarethinorabsentonthenewlyformedcrustofmid

    oceanridgesandarethickestontheoldercrustandnearcontinents. Thefourmajorclassificationsof

    sedimentsareterrigenous,pelagic,glacialmarine,andvolcanic.

    3.2.1 TERRIGENOUSSEDIMENTS

    Terrigenoussedimentsarelandderivedsiltsandclaysthatarecarriedtoseabyrivers. Windsalso

    carrydustandsandouttoseaanddepositthemonthesurface,wheretheyeventuallysinktothe

    bottom. Terrigenousdepositsaremostlyfoundintheregionofthecontinentalshelf.

    3.2.2

    PELAGICSEDIMENTS

    Thesesedimentsarealsoknownasoozebecauseoftheirappearance. Theyformindeepwaterandare

    mostcommonlycomposedofshellsandskeletalremainsofmarineplantsandanimals.

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    Identifythosepropertiesofseawaterthatdensityisdependenton.

    Recognizehowdensityaffectsseawaterstability.

    Recognizethedegreetowhichseawateriscompressible,andtheimportanceofthisproperty.

    Definespecificheatandrecognizetheeffectsalinityhasonthespecificheatofseawater.

    Defineviscosity.

    Recognizetheotherpropertiesofseawaterthatcontrolit.

    Recognizetheeffectstemperature,pressure,andsalinityhaveonthethermalexpansionofseawater.

    Identifyoneofthemajorrolesofthermalexpansioninthesea.

    Identifythepropertiesthatcontrolsoundvelocityintheocean.

    Recognizehoweachcontrolsthespeedanddirectionofasoundwave.

    Identifythethreelayersofthethreelayeredoceanmodel.

    Differentiatebetweenmechanicalandconvectivemixing.

    Definewatertypeandwatermass.

    Identifythepropertiesusedintheclassificationofwatertypesandwatermasses.

    Recognize

    the

    oceans'

    basic

    vertical

    structure

    with

    regard

    to

    latitudinal

    distribution.

    Recognizewatermasssourceregionsandhowtheyareformed.

    Recognizehowdeepoceancirculationdiffersfromsurfacecirculationandhowthecirculationpatternismaintained.

    JustastheairofoneregionofEarthcandifferinitsmakeupfromthatofanotherregion,socan

    seawater. Forexample,thewateraroundAntarcticadiffersfromthatofthemidlatitudesandthe

    tropics,andwaterfoundattheoceansurfacediffersfromthatfoundatornearthebottom. The

    differencesfoundinseawaterarerelatedtoseawaterproperties. Itistheseawaterpropertiesthatare

    usedtoclassifywatermasses. Inthislesson,wewillcovervariousseawaterproperties,thethreelayer

    oceanmodelasitrelatestothepropertyoftemperature,andthewatermassesoftheworld'soceans.

    3.3.1PROPERTIESOFSEAWATER

    Temperature,pressure,andsalinityarethethreemostimportantpropertiesofseawater,andthey

    determinetheotherphysicalpropertiesassociatedwithseawater. Thisdiffersfrompurewater,where

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    onlytemperatureandpressuredeterminethephysicalproperties. Wavemotionandthepresenceof

    smallsuspendedparticlesinseawaterarealsoimportantvariablesthataffectthepropertiesofseawater.

    Wavemotioncausesachangeintheprocessesofchemicaldiffusion,heatconduction,andtransferof

    momentumfromonelayertoanother. Wavemotionalsoincreasestheamountofsuspendedparticles,

    causingincreasedscatteringofincomingsolarradiationandahigherabsorptionrateinseawaterthana

    similarlayer(thickness)ofpurewater. Thevariablesofwavemotionandsuspendedparticles,although

    important,cannotbemeasured. Inadditiontotemperature,pressure,andsalinity,othercommon

    physicalpropertiesofseawaterarewatercolor,transparency,ice,andsoundvelocity.

    3.3.1.1Temperature

    Theocean,liketheatmosphere,isheatedthroughabsorptionoftheSun'sincomingradiation. Atall

    latitudes,theicefreeportionsofoceansreceiveasurplusofradiation. Someofthisheatisgivenupto

    theatmospherethroughthereleaseoflongwaveradiation,andsomeofitisretained. Becausethesea

    retainsaportionofthisheat,theseasurfacetemperatureisnormallyhigherthantheairtemperature.

    However,thisistrueonlywhenaverageconditionsareconsidered. Whethertheseasurfaceiswarmer

    orcolderthantheairaboveitatanyparticularmomentisdependentuponthelocality,theseasonof

    theyear,thecharacteroftheatmosphericcirculation,andthecharacteroftheoceancurrents.

    Theaveragetemperatureoftheoceanrangesfromabout 2Cto30C. Oceanwaterthatisnearly

    surroundedbylandmayhavehighertemperatures,buttheopensea,wherethewaterisfreetomove

    about,rarelyexceedstemperaturesabove30C. Here,theoceancurrentsdistributetheheatandtend

    toequalizethetemperature. Deepandbottomwatertemperaturesarealwayslow,varyingbetween

    4Cand1C.

    Theannualseasurfacetemperaturevariationinanyregiondependsuponthevariationofincoming

    radiation,thecharacteroftheoceancurrents,andthecharacteroftheatmosphericcirculation. The

    annualrangeofsurfacetemperatureismuchgreateroveroceansoftheNorthernHemispherethan

    thoseoftheSouthernHemisphere. Thiswiderrangeoftemperaturesappearstobeassociatedwiththe

    large

    difference

    in

    land/sea

    distribution

    between

    the

    Northern

    and

    Southern

    Hemispheres,

    and

    the

    characteroftheprevailingwinds,particularlythecoldwindsblowingfromthecontinents. Themore

    consistentannualtemperaturesintheSouthernHemispherearerelatedtothefairlyequivalentaccess

    toincomingsolarradiationduetotheabsenceoflargelandmassessouthof45S. Here,theprevailing

    windstravelalmostentirelyoverwaterwhichbringsaboutafargreaterdegreeofconsistencyand

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    moderationintheannualseasurfacetemperaturepatterns,andamuchsmallerannualtemperature

    rangewhencomparedtotheNorthernHemisphere.

    Watertemperaturesneartheequatorexperienceasemiannualvariationthatcorrespondstothetwice

    yearlypassageoftheSun'smostdirectraysacrosstheequator.

    Seasurfacetemperatureschangefromdaytonightjustlikethoseoftheatmosphere,buttoamuch

    lesserdegree. Thediurnalvariationofseasurfacetemperaturesintheopenoceanis,onaverage,only

    0.2Cto0.3C. Thegreatestdiurnalvariationtakesplaceinthetropics,withlesservariationathigher

    latitudes. Therangeofdiurnalvariationisdependentontheamountofcloudinessandthedirection

    andspeedofthewind. Thetropicshaveconsiderablylesswindspeedandcloudcoveragethan

    locationsinhigherlatitudes;thereforethereisagreaterdiurnalvariationinseasurfacetemperatures.

    Theannualvariationoftemperatureinsubsurfacelayersdependsonseveraladditionalfactors:

    Thevariationintheamountofheatthatisdirectlyabsorbedatdifferentdepths.

    Theeffectofheatconduction.

    Thevariationincurrentsrelatedtolateraldisplacement.

    Theeffectofverticalmotion.

    Diurnaltemperaturevariationsinsubsurfacelayersarelargelyunknown. Whatwedoknowisthatthey

    areextremelysmall.

    3.3.1.2VerticalTemperatureStructure

    Thebasicverticaltemperaturestructureoftheoceaninitssimplestformisbestdescribedusingthe

    threelayeredoceanmodelwhichwillbediscussedlaterinsection3.3.2. Generally,thereislittle

    temperaturechangewithdepththroughanupperormixedlayer,asharptemperaturedecrease

    throughamainthermoclinelayer,andareturntogenerallyconstanttemperaturethroughadeepwater

    coldlayer. AvisualexampleofthiscanbeseeninFigure33insection3.3.2.

    3.3.1.3Pressure

    Pressurebeneaththeseasurfaceismeasuredindecibars. Thepressureexertedby1meterofseawater

    verynearlyequals1decibar(1/10ofabar)or100,000dynespersquarecentimeter. Greaterdepths

    equalgreaterpressure,andsincepressureintheoceanisessentiallyafunctionofdepth,thenumerical

    valueofpressureindecibarsapproximatestheoceandepthinmeters. Therefore,pressurevaluesrange

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    fromzeroatthesurfacetoover10,000decibarsinthedeepestpartsoftheoceans. Thepressureis

    createdbytheweightoftheseawaterabove. Theweightperunitvolumeofseawater,inturn,varies

    withthetemperatureandsalinity. Inacolumnofwateratconstantdepth,thepressureincreasesas

    temperaturesdecrease,orassalinityvaluesincrease,orboth.

    3.3.1.4Salinity

    Thetermsalinityisrelatedoftentotheamountofsaltinthewater. Inoceanography,salinityis

    definedas"thetotalamountofdissolvedsolidsinseawater."Salinityismeasuredinpartsperthousand

    byweight,andissymbolized(thepermillesymbol). Themeasurementgivesusthegramsof

    dissolvedmaterialperkilogramofseawater. Thesalinityvaluesofoceanwaterrangebetween33

    and37,withanaverageof35.

    Intheopenocean,surfacesalinityisdecreasedbyprecipitation,increasedbyevaporation,andchanged

    bytheverticalmixingandinflowofadjacentwater. Nearshore,salinityisgenerallyreducedbyriver

    dischargeandfreshwaterrunofffromland. Inthecolderwatersthatfreezeandthaw,salinitygenerally

    increasesduringperiodsoficeformationanddecreasesduringperiodsoficemelt.

    Latitudinally,surfacesalinityvariesinasimilarmannerinalloceans. Maximumsalinityvaluesoccur

    between20and23northandsouth,whereasminimumsalinityvaluesoccurneartheequatorand

    towardhigherlatitudes. Thecontrollingfactorinaveragesurfacesalinitydistributionisthelatitudinal

    differencesinevaporationandprecipitation. Exceptionstothisstatementdooccur,andlocalvariations

    shouldbeexpected,especiallynearthemouthofthelargerriversystemsandintheAtlanticcoastal

    wateroftheUnitedStates,Labrador,Spain,andScandinavia. Atlatitudespolewardof40northand

    south,whereprecipitationgenerallyexceedsevaporation,salinityvaluestendtoincreasewithdepth.

    Usuallyduringsummer,thesepositivesalinitygradientsareaccompaniedbystrongnegative

    temperaturegradientsandresultinverystablewater,especiallyinthecoastalregions. Thesestrong,

    shallowsalinity(andtemperature)gradientspersistthroughthesummer.

    ThebestknownregionofstronghorizontalsalinitygradientsistheGrandBanksregion,wherewarm,

    salineGulfStreamwatermixeswiththecolder,lesssalinewateroftheLabradorCurrent. Here,water

    withasalinityvalueaslowas32maypossiblyoverrideorlieadjacenttowaterhavingasalinityvalue

    greaterthan36. AsimilarsituationprevailsinthePacificOcean,wheretheKuroshioandOyashio

    currentsmix.

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    3.3.1.5Density

    Thedensityofseawaterisalsodependentontemperature,pressure,andsalinity. Ataconstant

    temperatureandpressure,densityvarieswithsalinity. Atemperatureof32Fandanatmospheric

    pressureof1,013.2mbareconsideredstandardfordensitydetermination. Atothertemperaturesand

    pressurestheeffectsofthermalexpansionandcompressibilityareusedtodeterminedensity. The

    densityataparticularpressureaffectsthebuoyancyofvariousobjects,notablysubmarines. Densityis

    definedasmassperunitvolume,andisexpressedingramspercubiccentimeter. Thegreatestchanges

    indensityofseawateroccuratthesurface. Here,densityisdecreasedby:

    Precipitation

    Runofffromland

    Meltingofice

    Heating

    Whenthesurfacewaterbecomeslessdense,ittendstofloatontopofthemoredensewater. Thereis

    littletendencyforthewatertomix;therefore,theconditionisstable. Thedensityofsurfacewateris

    increasedbyevaporation,theformationofseaice,andcooling. Ifthesurfacewaterbecomesdenser

    thanthewaterbelow,itsinkstoalevelhavingthesamedensity. Here,itincreasesthethicknessofthe

    layerandtendstospreadout. Asthemoredensewatersinks,thelessdensewaterrises,anda

    convectivecirculationisestablished. Thecirculationcontinuesuntilthedensitybecomesuniformfrom

    thesurfacetoadepthatwhichagreaterdensityoccurs. Ifthesurfacewaterbecomessufficientlydense,

    itsinksallthewaytothebottom. Ifthisoccursinanareawherehorizontalflowisunobstructed,the

    waterthathasdescendedspreadstootherregions,creatingadensebottomlayer. Sincethegreatest

    increaseindensityoccursinpolarregions,wheretheairiscoldandgreatquantitiesoficeform,thecold,

    densepolarwatersinkstothebottomandthenspreadstolowerlatitudes. Thisprocesshascontinued

    forsuchalongperiodoftimethattheentireoceanflooriscoveredwiththisdensepolarwater. This

    explainsthelayerofcoldwateratgreatdepthsintheocean

    3.3.1.6Compressibility

    Seawaterisnearlyincompressible. Thecompressibilityofseawaterchangesslightlywithchangesin

    temperatureorsalinity. Theeffectofcompressionistoforcethemoleculesofthesubstancecloser

    together,causingthesubstancetobecomedenser. Eventhoughthecompressibilityofseawaterislow,

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    thetotaleffectisconsiderablebecauseoftheamountofwaterinvolved. Ifcompressibilitywerezero,

    sealevelwouldbeabout90feethigherthanitisnow.

    3.3.1.7SpecificHeat

    Inoceanography,specificheatisthenumberofcaloriesneededtoraisethetemperatureof1gramof

    seawater1C. Thespecificheatofseawaterdecreasesslightlyassalinityincreases,whileconversely,

    thespecificheatofseawaterincreasesassalinitydecreases. Thatbeingsaid,theratioofspecificheatto

    seawaterataconstantpressureandconstantvolumehasadirectrelationshiptothespeedofsoundin

    water.

    3.3.1.8Viscosity

    Viscosityisaliquidsabilitytoresistflow. Seawaterisslightlymoreviscousthanfreshwater,andthe

    levelofresistanceiscontrolledbyitsthermalexpansion. Liquidsexpandandcontractwhen

    temperaturechangestakeplace;somemorethanothers. Theresistancerateofseawaterisnot

    uniform, viscosityincreaseswhensalinityincreasesorthewatertemperaturedecreases. However,the

    effectofdecreasingtemperatureisgreaterthanthatofincreasingsalinity. Becauseoftheeffectof

    temperatureonviscosity,anincompressibleobjectmightsinkatafasterrateinwarmsurfacewater

    thanincoldersubsurfacewater. Formostcompressibleobjects,viscosityeffectsmaybemorethan

    offsetbythecompressibilityoftheobject. Inrealitythisisaverysimpleexplanationtoacomplex

    problem,sincetheactualrelationshipsexistingintheoceanareconsiderablymorecomplicatedthan

    portrayedhere.

    3.3.1.9ExpansionofSeaWater

    Seawaterhasahighercoefficientofexpansionthanthatoffreshwater. Withinthesea,thecoefficient

    ofthermalexpansionisaffectedbytemperature,pressure,andsalinity. Thecoefficientofthermal

    expansionisgreaterinhighsalinitywater;greaterinwarmwaterthanincold(undersimilarsalinity

    conditions);anditincreaseswithincreasingdepthunderconstanttemperatureandsalinityconditions.

    Ofcourse,constancyisnotatrademarkofanyoftheseproperties;theyareallquitevariable. Inturn,

    thethermalexpansionthattakesplaceintheseavariesandisdifficulttoassess. Amajorroleof

    thermalexpansionisintheformationofice. Purewaterismostdenseat4C. Thermalexpansiontakes

    placewhenwaterwarmsabove4C,butwateralsoexpandsevenmorewhenitcoolsbelow4Candas

    itfreezes. Whenexpansiontakesplace,thevolumeisincreasedresultingindecreaseddensity. Ifwater

    failedtoexpandduringthefreezingprocess,thedensityoficewouldbesuchthatitwouldsinktothe

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    bottomuponformingandinthecoldofwinter,freshwaterlakeswouldeventuallybecomesolidblocks

    ofice. Comesummer,onlytheupperfewfeetoficewouldmelt,leavingtheremainingicebeneaththe

    meltedwater.

    3.3.1.10Sound

    Velocity

    Velocitytakesintoaccountbothspeedanddirection. Thespeedofsoundinseawaterisgovernedby

    temperature,pressure,andsalinity. Anincreaseintemperatureincreasesthespeedofsoundinwater,

    whileadecreaseintemperaturedecreasesthespeedofsound. Thesamerelationshipappliesto

    pressureandsalinity. Anincreaseinpressurecausesanincreaseinsoundspeed,asdoesanincreasein

    salinity,andviceversa. Sincepressureisafunctionofdepthinthesea,ifweweretodiscounttheeffect

    oftemperatureandsalinity,soundwouldtravelfasterattheoceanbottomthanitdoesatthesurface.

    However,wecannotdiscounteitheroftheseothertwovariables,especiallytemperature. Temperature

    isthemostimportantpropertycontrollingthespeedofsoundinwater. Asfarasdirectionisconcerned,

    soundwavestravelinstraightlinesonlyinamediuminwhichthespeediseverywhereconstant. For

    thistooccurinseawater,thetemperature,pressure,andsalinityvalueswouldhavetobeunchanging.

    Changesinany,orallofthesevariablesdoesoccurwhich,inturnaffectsthespeedofsoundwavesand

    thedirectionsuponwhichtheytravel. Soundwavesarebent(refracted)inthedirectionofslower

    soundvelocities. Thedegreeofrefractionisproportionaltothevelocitygradient,orthechangein

    soundvelocitywithdistance. Ifthevelocitygradientweresuchthatsoundspeedincreasedrapidlywith

    depth,

    sound

    waves

    would

    refract

    sharply

    upward

    toward

    the

    slower

    sound

    velocities

    at

    the

    surface.

    Ontheotherhand,ifthevelocitygradientweresuchthatsoundspeeddecreasedrapidlywithdepth,

    soundwaveswouldrefractsharplydownwardtowardtheslowersoundspeedsattheoceanbottom.

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    3.3.2THETHREELAYEREDOCEAN

    Aconvenientmethodofvisualizingtheseaistodivideit

    intolayersinmuchthesamewaythatwedothe

    atmosphere. Usingbathythermographinformation

    (temperatureversusdepthprofiles),asshowninFigure

    33,theoceansdisplayabasicthreelayeredstructure:

    themixedlayer,mainthermocline,anddeepwaterlayer.

    3.3.2.1MixedLayer

    Themixedlayeristheupperlayerofthethreelayered

    oceanmodel. Themixedlayerconsistsofnearlyuniform,

    orisothermalrelativelywarmertemperatureswith

    depth,inmiddlelatitudes,andextendsfromthesurface

    toamaximumdepthofabout450meters,or1,500feet. Thislayergetsitsnamefromthemixing

    processesthatbringaboutitsfairlyconstantwarmtemperatures. Thetwomixingprocessesare

    classifiedasmechanicalandconvective.

    3.3.2.1.1MechanicalMixing

    Thismixingprocessiscausedbywaveactionand/orsurfacestormsstirringupthewater. Warmer

    surfacewaterisdrivendownward,whereitmixeswithcoldersubsurfacewater. Eventually,alayerof

    waterwithafairlyconstant,orisothermal,temperatureisproduced. Thisprocessismoreimportantin

    summerthaninwinter,becausesurfacewatersaremuchwarmerandlessdensethansubsurface

    waters,therebyproducingastablewatercolumn. Themechanicalmixingprocessismorerapidand

    irregularthantheconvectivemixingprocess.

    3.3.2.1.2ConvectiveMixing

    Thisprocessoccursasaresultofchangesinwaterstability. Whensurfacewatersbecomedenserthan

    subsurfacewaters,anunstableconditionexists. Suchconditionscanoccurwhenthereisanincreasein

    surfacesalinityduetoevaporation,theformationofice,orbyadecreaseinthesurfacewater

    temperature. Atemperaturedecreaseof.01Corasalinityincreaseof0.01,issufficienttoinitiate

    theconvectivemixingprocess. Intheformercase,forexample,acoldpolarorarcticairmassmoving

    overwarmwatercoolsthesurfacewaterbeforeitcancoolthesubsurfacewater. Asthesurfacewaters

    Figure

    3

    3.

    Three

    layered

    ocean

    mode.

    (Source: PDC)

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    Figure34. VerticalTemperatureProfileinSummer.SourcePDC

    coolandbecomecolderthanthesubsurfacewaters,theybecomedenserandsink. Asthecolder

    surfacewatersinks,thewarmerandlessdensesubsurfacewaterrisestothesurfacetoreplaceit. This

    processcontinuesuntilthewateristhoroughlymixed,thedensitydifferenceeliminated,andthewater

    columnstabilized. Eventhoughwindsandtheresultantwaveactionaregenerallystrongerduring

    winter,convectivemixing,causedbythecolderwinterairtemperatures,producesadeepermixedlayer

    thancanbeattainedbymechanicalmixing. Itisforthisreasonthatconvectivemixingisconsideredthe

    moreimportantofthetwo,andthepredominantprocessofwinter.

    Theconvectiveprocessisstrongestinnorthernwaterswhereverticaltemperatureandsalinitygradients

    arenotextremeandsurfacewatersundergoahighdegreeofcooling. Convectivemixingattributedto

    salinitychangesismostnoticeableintheMediterraneanandRedSeas,whereevaporationfarexceeds

    precipitation.

    Wehavelookedatbothprocessesindividually;however,thetwoprocessescan,andoftendo,take

    placesimultaneously. Whenthisoccurs,themixedlayernormallyattainsagreaterdepththanwouldbe

    attainedbyeitherprocessindividually.

    3.3.2.2MainThermocline

    Themainthermoclineisthecentrallayerofthethreelayeredoceanmodel. Themainthermoclineis

    foundatthebaseofthemixedlayerandis

    markedbyarapiddecreaseofwater

    temperaturewithdepth. Athighlatitudes

    thereisnomarkedchangeinwater

    temperaturewiththeseasons,whileinthe

    midlatitudes,aseasonalthermocline

    developswiththeapproachofsummer(See

    Figure34). Thisseasonalthermocline

    comesaboutfromthegradualwarmingof

    the

    surface

    waters.

    The

    warming

    takes

    place

    intheupperfewhundredfeetofthesurface,

    andresultsintheseasonalthermocline

    becomingsuperimposedonthemain

    thermocline. Figure34illustratesthedevelopmentoftheseasonalthermoclineinthemidlatitudes.

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    Themidlatitudesummerthermoclineismorepronouncedthanthethermoclineofspringorautumn.

    Bathythermographtracesofthesummerthermoclineshowthatitaffectsamuchbroaderrangeof

    depththanatanyothertimeofyear. Theseasonalthermoclineisroughly35metersthick(90to125

    metersdeep). Note,also,thatawintertemperatureprofilewillshownoseasonalthermocline(See

    Figure35). Comespring,thesurfacewateriswarmedandaseasonalthermoclinedevelops. Inlow

    latitudes,smallseasonaltemperaturechangesmakeitdifficulttodistinguishbetweentheseasonaland

    thepermanentthermoclines.

    3.3.2.3DeepWaterLayer

    Thedeepwaterlayeristhebottomlayerof

    water,whichinthemiddlelatitudesexists

    below1,200meters. Thislayerischaracterized

    byfairlyconstantcoldtemperatures,generally

    lessthan4C. Tobetterunderstandthebasic

    verticaltemperaturedistribution,lookonce

    againatfigure35. Athighlatitudesinwinter,

    thewateriscoldfromtoptobottom. The

    verticaltemperatureprofileisessentially

    isothermal(nochangeintemperaturewith

    depth). Inlowlatitudes,themixedlayer

    extendstoadepthofabout300feet. Here,

    themainthermoclineisencounteredandthe

    temperaturedropsabout8Cmorethanit

    doesinthemidlatitudes. Thissharperdropisduetothehighersurfacetemperatureinthelower

    latitudes. Thethermoclineextendstoanaverageof2,100feet,wherethedeeplayerisencountered.

    3.3.3WATERTYPESANDMASSESS

    Theconceptofvisualizingwatermassesaswedoairmassesispossiblebecausebotharebasedonthe

    physicalpropertiesthatgointotheirmakeup. Thepropertiesoftemperatureandsalinityareusedto

    classifybothwatertypesandwatermasses.

    Awatertypehasasinglevalueofsalinityandasinglevalueoftemperatureassociatedwithit,For

    example,RedSeawaterisawatertypecharacterizedbyatemperatureof9Candasalinityof35.5.

    Figure35. VerticalTemperatureProfileinWinter.(SourcePDC)

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    Awatermasstakesintoaccountarangeoftemperaturesandsalinitiesandmaybeconsideredtobe

    madeupofacombinationoftwoormorewatertypes. Forexample,NorthAtlanticCentralWater(a

    watermass)ischaracterizedbyarangeoftemperatures(4Cto17C)andsalinity(35.1to36.2).

    3.3.3.1Water

    Mass

    Formation

    Thevastmajorityofwatermassesareformedatthesurfaceoftheseainmiddleandhighlatitudes.

    Cold,highlydensesurfacewatersinksuntilitreachesalevelhavingthesameconstantdensity. Here,it

    spreadsouthorizontally. Themannerinwhichitspreadsoutdependsonitsdensityinrelationtothe

    densityofthesurroundingwater. Thisistrueofnearlyallwatermasses,exceptthoseoflowlatitudes

    inparticular,theequatorialwatermassesoftheIndianandPacificOceans. Thesewatermassesare

    formedbythemixingofsubsurfacewaters.

    3.3.3.2Distribution

    Inlowandmiddlelatitudestheverticalarrangementofwaterissuchthatwecandistinguishasurface

    layer,upperwater(centralandequatorial),intermediatewater,deepwater,andinsomelocalities,

    bottomwater. Inhighlatitudes,thelayeredstructureallbutdisappearsbecausethesurfacewateris

    similartothewateratornearthebottom.

    3.3.3.2.1SurfaceLayer

    Thesurfacelayerisnotclassifiedaswatertypeorwatermass,becauseitspropertiesvarywidelyfrom

    oneareatoanother,dependingonoceancurrentvariations,ratesofevaporationorprecipitation,and

    variousseasonalchanges,especiallyinthemiddlelatitudes. Inlowandmiddlelatitudesitisfound

    abovecentraland/orequatorialwatertodepthsof100to200meters. Thesurfacelayerisseparated

    fromdeeperwaterbyatransitionlayer(themainthermocline). Beneaththesurfacelayer,we

    encounterthewatertypesandwatermasses. Likeairmasses,thewatertypesandwatermasseshave

    sourceregionsinwhichtheyform.

    3.3.3.2.2CentralWaterMasses

    Centralwaterisnormallyfoundinrelativelylowlatitudesalthoughitssourceregionisintheregionof

    thesubtropicalconvergence(betweenthe35thand40thparallelsineachhemisphere). Convergences

    areregionsintheoceanwheresurfacewatersarebroughttogetherbythecurrents. Inthewestern

    NorthAtlanticOcean,aregionofsubtropicalconvergenceexistswheretheGulfStreammeetsthe

    colder,denserLabradorCurrent. Convergencesaremarkedbyrapidlyrisingseasurfacetemperatures.

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    Centralwaterisnotusuallydiscernibleatthesurfaceandisgenerallyrelativelyshallow. Itsgreatest

    thicknessisobservedalongitswesternboundaries. InthewesternNorthAtlanticintheregionofthe

    SargassoSea,thethicknessmayreach900meters. Variationsinheatingandcooling,evaporationand

    precipitation,oceancirculationpatterns,andmixingprocessesallcontributetothesalinityvaluesof

    centralwaterbeingeitherquitesimilarorconsiderablydifferent. Forexample,centralwaterofthe

    SouthAtlanticOcean,theIndianOcean,andthewesternSouthPacificOceanallhavesimilarsalinity

    values,whilethesalinityvaluesofNorthAtlanticcentralwaterareconsiderablyhigherthanthecentral

    wateroftheNorth Pacific Ocean. CentralwateroftheNorthandSouthAtlanticoceansisnot

    separatedbyequatorialwaterlikethecentralwateroftheNorthandSouthPacificoceans. Instead,the

    centralwateroftheNorthandSouthAtlanticcometogetherandmix,formingaregionoftransition

    consistingofintermediateproperties.

    3.3.3.2.3EquatorialWaterMasses

    EquatorialwaterisfoundinthePacificandintheIndianOcean. InthePacificitisthoughttooriginate

    onthesouthernsideoftheequator. Therearetworeasonsforthis:Itspropertiesaresimilartothoseof

    thewatermassesoftheSouthPacific,anditssalinityvaluesarehigherthanthoseofthewatermasses

    foundintheNorthPacificOcean.

    EquatorialwaterisalsofoundinthenorthernpartoftheIndianOcean. Here,itshighersalinitiesare

    probablyduetoitsmixingwiththewatersoftheRedSea. However,thisconclusionhasnotbeen

    substantiated. Equatorialwater,likecentralwater,isnotdiscernibleatthesurface,becausethe

    temperatureandsalinityvaluesusedtoisolateitcannotbeclearlyascertainedintheupper100to200

    meters.

    3.3.3.2.4IntermediateWater

    Intermediatewaterisfoundbelowcentralwaterinalloceans. TheseincludeAntarcticintermediate

    water,Arcticintermediatewater,Mediterraneanwater,andRedSeawater.

    3.3.3.2.4.1AntarcticIntermediateWater

    AntarcticintermediatewaterencirclestheAntarcticcontinentandisthemostwidespreadofallthe

    intermediatewatermasses. ItformsinthevicinityoftheAntarcticconvergence,whereitsinks. Asit

    sinks,itflowsnorthandmixeswiththewatermassesthatlieimmediatelyaboveandbelowit. Inthe

    Atlantic,theabsenceofequatorialwaterallowsAntarcticintermediatewatertoflowacrosstheequator

    andreachroughly20Nto35Nlatitude. IntheSouthPacificandIndianoceans,whereequatorialwater

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    doesexist,Antarcticintermediatewaterfailstoreachtheequator. Itspreadsnorthtoabout10S

    latitude. OneofthecharacteristicsofAntarcticintermediatewaterisitslowsalinity(34.1to34.6

    ). Incomparisontothewateraroundit,itdisplaysthelowestsalinityvalues.

    3.3.3.2.4.2Arctic

    Intermediate

    Water

    ArcticintermediatewaterandsubArcticwateraresimilar;however,intheNorthAtlanticOcean,Arctic

    intermediatewaterformsonlyinsmallquantities,andinarelativelysmallareaeastoftheGrandBanks

    ofNewfoundland. IntheNorthPacific,Arcticintermediatewaterformsduringwinteratthe

    convergenceformedbytheOyashiocurrentandtheKuroshioExtension. Itexistsbetweenlatitude20N

    and43N,exceptoffthewestcoastofNorthAmerica. Here,subArcticwaterextendstolowerlatitudes,

    andthenorthernboundaryoftheintermediatewaterispushedmuchfarthersouth.

    3.3.3.2.4.3Mediterranean

    Water

    ThiswatermassisformedbytheinteractionofdenseMediterraneanSeawaterwithwatersofthe

    adjacentNorthAtlanticOcean. ThemoredenseMediterraneanwaterflowsoutthroughtheStraitof

    Gibraltarandsinkstoadepthofabout1,000meters,whereitmixeswiththewateratthisdepth.

    3.3.3.2.4.4RedSeaWater

    ThiswatertypeisfoundoverlargepartsoftheequatorialandwesternregionsoftheIndianOcean.

    Largequantitiesofwarm,highlysalinewaterfromtheRedSeaflowintotheIndianOcean,whereits

    mixeswithAntarcticintermediatewatertoformtheRedSeawatermass. ThespreadingofRedSea

    waterisnotaswelldefinedasMediterraneanwater.

    3.3.3.2.5AntarcticCircumpolarorSubAntarcticWater

    Thiswatermassisthoughttoformthroughacombinationofmixingandverticalcirculationintheregion

    betweenthesubtropicalandAntarcticconvergence. Here,largequantitiesofAntarcticintermediate

    waterandAntarcticbottomwatermixwithNorthAtlanticdeepwatertoformAntarcticcircumpolar

    water. Thephysicalpropertiesofthiswatermassarequiteconservative,andasitsnameimplies,it

    extendscompletelyaroundtheAntarcticcontinentandtheSouthPole. BecauseAntarcticcircumpolar

    waterformsinthedeeperwatersoftheAntarcticOcean,itisoftenreferredtoassubAntarcticwater.

    3.3.3.2.6SubArcticWaterMasses

    SubArcticwaterismuchlikeAntarcticcircumpolarorsubAntarcticwater;however,thereare

    differences. Thedifferencesareattributedtothelandandseadistributioninthetwohemispheres. In

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    theSouthernHemisphere,theAntarcticconvergenceextendsaroundthecontinentofAntarctica,butin

    theNorthernHemisphere,theArcticconvergenceisfoundonlyinthewesternportionsofoceans.

    However,evenintheseareastheconvergenceisnotalwayswelldefined. IntheNorthAtlanticOcean,

    subArcticwatercoversarelativelysmallarea,anditpossessesahighersalinitythansurroundingwaters.

    Ontheotherhand,thesubArcticwateroftheNorthPacificismuchmoreextensive,anditssalinity

    valuesarelowerthansurroundingwaters.

    3.3.3.2.7DeepandBottomWaterMasses

    Inthedeepoceanbasinsbelowintermediatewater,highdensitydeepandbottomwaterexists. These

    watermassesforminbothhemispheres. IntheSouthernHemisphere,Antarcticbottomwaterforms

    neartheAntarcticcontinent,whileintheNorthernHemisphere,Arcticdeepandbottomwaterformsin

    northwesternLabradorBasinandinasmallareaoffthesoutheastcoastofGreenland. Thesewater

    massesformatthesurface,sink,andspreadouttofillthedeepoceanbasins. Deepandbottomwaters

    aredetectableinareasfarremovedfromtheirsourceregions. Moreinformationonthespreadingof

    deepandbottomwaterispresentedinthefollowingdiscussionondeepoceancirculation.

    3.3.3.2.8DeepOceanCirculation

    Methodsdevisedtodeterminedeepoceancirculationhavemetwithvaryingsuccess,butallpointtoa

    quitecomplexpatternofsubsurfacecurrents. Thedeepoceancurrentsdifferfromsurfacecurrentsin

    thattheyare:

    Densitydriven.

    Muchslower.

    Moveinapredominantlynorthsouthdirection.

    Crosstheequator.

    Thedeepoceancirculationisoftenreferredtoasathermohalinecirculation,becausethecirculationis

    controlledbydifferencesintemperatureandsalinity. Varyingcombinationsoftemperatureandsalinity

    producewaterofvaryingdensities,anditisthesedensitydifferencesthatproducedeepocean

    circulations.

    Sincethemajorityoftheworld'swatermassesareformedatthesurface,ourdiscussionofthedeep

    oceancirculationmuststartthere. Wewillmovethroughthecirculatorypattern,beginningandending

    withthesurfacewatersaroundAntarctica. AsthehighdensitysurfacewateraroundAntarcticasinks,it

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    mixeswiththewarmer,moresalinecircumpolarwatertoformAntarcticbottomwater. Because

    Antarcticbottomwateristhedensestwaterfoundintheocean,itsinkstotheoceanfloorandspreads,

    orflows,northwardintothedeepoceanbasinsoftheAtlantic,Pacific,andIndianOceans. Thiswater

    masshasbeentrackedasfarnorthasthe35thparalleloftheNorthernHemisphere.

    InthesubArcticregionsoftheNorthernHemisphere,thesametypeofprocessoccurs. Thecold,dense

    surfacewatersinksandformsNorthAtlanticdeepandbottomwater. Thiswatermassspreads

    southwardandisincontactwiththebottom,exceptwhereitencountersAntarcticbottomwater.

    BeinglessdensethanAntarcticbottomwater,itisfoundaboveAntarcticbottomwaterwhereverthe

    twocoexist.

    TheNorthAtlanticdeepandbottomwatereventuallymakesitswaybacktotheAntarcticOcean,where

    itmixeswithintermediatewatermassesandAntarcticbottomwatertoformAntarcticcircumpolar

    water. Here,thecyclebeginsagainasthecold,densesurfacewaterofAntarcticasinksandmixeswith

    thecircumpolarwater. Abovethedeepandbottomwaters,theintermediatewatermassesalsoshowa

    basicequatorwardmovement. Antarcticintermediatewateractuallycrossestheequatorandmovesas

    farnorthas20to35N. ItsNorthernHemispherecounterpart,Arcticintermediatewater,movessouth

    butdoesnotcrosstheequator. MediterraneanandRedSeawaterbothcrosstheequator,andhave

    beenidentifiedfarintotheSouthernHemisphere. TheCentralandEquatorialwateroflowandmiddle

    latitudesmovepolewardintheirrespectivehemispheres,whileinhighlatitudesthenearsurfacewaters

    move

    toward

    the

    equator.

    The

    Atlantic

    circulation

    is

    considered

    much

    more

    vigorous

    than

    that

    of

    the

    Pacific,becausesurfacedensitycontrastsaremuchgreater. However,evenwiththegreatersurface

    densitycontrasts,thecirculationisSLOWVERYSLOW. Thedeepseacurrentsassociatedwiththe

    deepoceancirculationflowatarateofafewcentimeterspersecondorless. Ifwewereabletofree

    floatabottleatadesignateddepth,thisrateofspeedwouldequatetothebottlemovinglessthan2

    degreesoflatitude(120nauticalmiles)inayear,or0.0137knots.

    Wecansaythedeepoceancirculationconsistsprimarilyof(1)equatorwardflowingsubsurfacewater

    whichmovesatanextremelyslowrateofspeedand(2)themuchfasterpolewardflowingsurfacewater.

    3.4 HYDROACOUSTICS

    LearningObjectives

    Identifythevariouspropertiesofsoundwaves.

    Defineenergylossorspreadinglossasitpertainstosoundwaves.

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    DefineDopplerEffect.

    RecognizehowtheDopplerEffectaffectsthepitchandfrequencyofsound.

    Definesoundvelocity.

    Describetheeffectoftemperature,pressure,andsalinityonsound.

    Explainwhysoundpropagatesalongmoreorlesscurvedpaths.

    Describethefivebasicsoundraypatternsandtheirattendanttemperatureandsoundvelocityprofiles.

    Differentiatebetweenactiveandpassivesonar;definethemodesofactivesonarsearch.

    Describethepropagationpathsusedwitheachmodeofactivesonar.

    Defineanddifferentiatebetweentheelementsusedintheactiveandpassivesonarequations.

    Hydroacousticsisthestudyofsoundinwater. InthecaseoftheNavy,itisthestudyofsoundenergyin

    seawater.

    TheNavy'sgreatestinterestinhydroacousticsisrelatedtosubmarineandantisubmarinewarfareor

    morepreciselytheeffectofseawateronsonar. Certainpropertiesofseawatercontrolsoundasit

    propagatesthroughthewater. Theireffectmayaidorhindersonaroperations.

    3.4.1 PROPERTIESOFSOUND

    Soundin

    the

    world

    of

    oceanography

    has

    asignificant

    meaning.

    It

    is

    necessary

    to

    be

    familiar

    with

    some

    ofthefundamentalconceptsconcerningthepropertiesofsound.

    3.4.1.1 SoundProduction

    Soundisthephysicalcauseofhearing.

    Beforesoundcanbeproduced,three

    basicelementsmustbepresent:asound

    source,amedium,andadetector.

    (RefertoFigure36)

    3.4.1.1.1 Source

    Anyobjectthatvibratesordisturbsthe

    mediumarounditmaybecomeasoundFigure36 PropertiesofSound. (Source:PDC)

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    source. Thesoundsourceistheinitialrequirementintheproductionofsound.

    3.4.1.1.2 Medium

    Themediumistheelementthatcarriessound. Airactsasamediumintheatmosphere. Particlesinthe

    aircarrythesoundsthatyouheareveryday. Noisescanalsobeheardunderwaterwhereparticlesinthe

    watercarrysound.

    Themediumisthecontrollerofsound. Itcontrolshowfarandhowfastsoundtravels. Soundtravels

    faster,farther,andwithmoreeasethroughmediumsofhighelasticityanddensity. Solidsarebetter

    transmittersofsoundthaneitherliquidsorgases.

    3.4.1.1.3 Detector

    Adetectoractsasareceiverofsound. Thedetectorallowsustotellwhethersoundhasbeenproduced.Soundtravelsinwavesthatmoveradially(360)fromtheirsource,andonlyasmallpartofawaves

    energyreachesadetector. Therefore,detectorsoftencontainamplifierstoboostasignalsenergy

    permittingreceptionofweaksignals.

    3.4.1.2 SpeedofSound

    Thespeedofsoundinairisapproximately331.5m/secat0C. Soundspeeddecreasesatlower

    temperaturesandincreasesathighertemperatures. Soundspeedincreasesatarateof

    approximately3.2m/secforevery1Cincreaseintemperature.

    Thespeedofsoundinwaterisabout4timesgreaterthanthespeedofsoundinair. Seawateris

    denserthanfreshwater;therefore,atthesametemperature,thespeedofsoundinseawaterwillbe

    slightlygreaterthanthespeedofsoundinfreshwater.

    Insteel,soundspeedisabout15timesgreaterthaninair. Soundtravelsatapproximately5,200

    m/secthroughathinsteelrod.

    3.4.1.3

    Sound

    Waves

    Soundtravelsintheformofwaves. Soundwavesarebroughtaboutbyvibrationswithinamedium.

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    3.4.1.3.1 Wavelength

    Thelengthofasoundwaveisthedistancebetweenanytwosuccessivecompressionsorrarefractions.

    Onecompletewavelengthiscalledacycle. Wavelengthsvarydependingonthenumberofcyclesper

    secondproducedbythesoundsource.

    3.4.1.3.2 Frequency

    Thenumberofcyclespersecondisameasureofasoundsfrequency,thehigherthefrequency,the

    shorterthewavelengths. Theoppositealsoapplies,thelongerthewavelengththelowerthefrequency.

    FrequenciesaremeasuredintheHertzsystem. 1hertz(Hz)isequalto1cyclepersecond. Frequencies

    of1000Hzormorearemeasuredinkilohertz(kHz). Theaveragehumanhearssoundsbetween20Hz

    and15kHz,whilesoundsbelow20Hzandabove15kHzarenormallybeyondthehumanrangeof

    hearing.

    3.4.1.3.3 Pitch

    Thepitchofasounddependsonthefrequencyofthesoundasreceivedatadetector. Thehumanear

    detectssoundsandclassifiesthembasedonthesoundquality. Somesoundsareharsh,whileothersare

    pleasant. Pitchisasubjectivequalitydependentonthereceiver.

    3.4.1.3.4 IntensityandLoudness

    Intensityandloudnessareoftenmistakenashavingthesamemeaning. Althoughrelated,theyarenot

    thesame. Intensityisameasureofasoundsenergy,whileloudnessistheeffectonthedetector. If

    soundintensityisincreased,theloudnessisincreasedbutnotindirectproportion. Todoublethe

    loudnessofsoundrequiresaboutatenfoldincreaseinthesoundsintensity.

    Soundintensityismeasuredindecibels(dB). Adecibelistheunitusedtoexpressrelativeintensity

    differencesbetweenacousticsounds. Decibellevelsareassignedbasedonasoundsintensity

    comparedtoanestablishedstandard. Somecommonintensitylevelsareasfollows:awhisper,10to20

    dB;heavystreettraffic,70to80dB;thunder,110dB.

    3.4.1.4 EnergyLoss

    Asasoundwavemovesawayfromitsource,itspreadsout. Theenergywithinthewavedecreasesas

    thewavespreadsthroughanincreasinglylargearea. Thewaveenergyperunitareadecreasesasthe

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    distancefromthesoundsourceincreases. Thislossofenergyduetodistanceisknownasspreading

    loss.

    3.4.1.5 DopplerEffect

    TheDopplerEffectistheapparentchangeinasoundduetomotion. Itisachangeinpitchwithouta

    frequencychangeoccurring. Thechangeinpitchisbroughtaboutbytherelativemotionofasound

    sourceandadetector. Forexample,wehearthewhistleofanapproachingtrain. Thefrequencyofthe

    whistledoesnotchangeasthetrainapproaches,butourearsdetectanincreaseinthepitch. The

    increaseinpitchiscausedbythecompressionofsoundwaves. Thetrainactstopushthesound

    wavestowardus. Thesoundwavesarriveatafasterratethantheywouldifthetrainwasnotmoving.

    Then,asthetraingoesby,thesoundwavesarriveatamuchslowerrate. Thetrainisnowpushingthe

    soundwavesawayfromus. Thesoundwavestotherearofthetrainspreadfartherapartasthetrain

    movesfartherawayfromourposition,andtheeffectisoneoflowerpitch.

    3.4.2 SOUNDPROPAGATIONINSEAWATER

    Thewordpropagateistheactofsomethingmovingthroughamedium. Inthisinstance,themedium

    iswaterandsoundmovesthroughit. Theseainfluencessoundinmanywaysasitmovesthroughthe

    water.

    3.4.2.1 SoundVelocity

    SoundVelocitytakesintoaccountthespeedanddirectionofsoundrays. Thedirectionorpaththat

    soundenergytakesasitmovesthroughthewaterisprimarilyafunctionofsoundspeed.

    3.4.2.1.1 SoundSpeed

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    Thespeedofsoundintheseaisafunctionofwatertemperature,pressure,andsalinity. Ofthesethree

    variables,temperatureisthemostimportant. Itistheprimarycontrollerofsoundspeedanddirection,

    intheupper300meters(1,000feet)ofseawater. Ingeneral,soundspeedincreases3.2m/secforevery

    1Cincreaseintemperature.

    Theeffectofpressureonsoundspeedisa

    functionofdepth. Pressureincreaseswith

    depthandsoundspeedincreaseswith

    higherpressure. Soundspeedincreases

    approximately1.7m/secper100metersof

    depth. Pressureisthedominantsound

    speedcontrollerbelow300meters,because

    below300meters,thetemperatureis

    relativelyconstant.

    Theeffectofsalinityonsoundspeedisslight

    intheopensea,becausesalinityvaluesare

    nearlyconstant. Theaffectofsalinityon

    soundspeedisgreatestwherethereisa

    significantinfluxoffreshwaterorwheresurfaceevaporationcreateshighsalinity. Aonepartper

    thousand

    (1)

    increase

    in

    salinity

    increases

    sound

    speed

    1.4

    m/sec.

    3.4.2.1.2 SoundVelocityProfile(SVP)

    Asoundvelocityprofileisagraphicrepresentationofspeedversusdepth. (Seefigure37)SVPsprovide

    surfacesoundspeed,depthofmaximumsoundspeed(soniclayerdepth),andlayerswheresound

    travelsgreatdistances(ductsandsoundchannels).

    3.4.2.1.3 SonicLayerDepth(SLD)

    Thesoniclayerdepthisthedepthofmaximumnearsurfacesoundspeedabovethedeepsound

    channelaxis. Anegativetemperaturegradient(temperaturedecreasingwithdepth),within

    certainlimits,compensatesforanincreasein soundspeedwithdepthdueto pressure;this

    resultsinaconstantsoundspeedwithdepth. The gradientlimitsper 30metersofdepthare as

    follows:

    0.1Cper30metersinwater4.4C

    Figure37. SoundVelocityProfile.(Source: PDC)

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    0.17Cper30 meters in water12.8C

    0.22Cper30 meters in water18.3C

    Temperaturegradients morenegative thanthoselistedaboveresultin decreasingsound

    speedswith

    depth.

    Gradients

    more

    positive

    than

    those

    listed

    above

    result

    in

    increasing

    soundspeedswithdepth.

    3.4.2.2 SoundPaths

    Assoundenergyleavesasoundsource,ittravelsinwaves. Thesoundwavesexpandastheymove

    awayfromthesource. Asoundwave'spathoftravelisdependentonitsspeedandanymatterinits

    path. Sound,likelight,isrefracted,reflected,andscattered. Thesurfaceorobjectstruckdetermines

    ifthesoundenergyisrefracted,reflected,scattered,orabsorbed.

    3.4.2.2.1 Refraction

    Asasoundwavemovesthroughthesea,ittravelsalongacurvedpath. Thepathiscurved,because

    soundspeedvariesalongthewavefront. Soundwavesbend(arerefracted)inthedirectionofthe

    slowersoundspeeds. Thisisthefundamentalprincipleofsonarrangepredictionandisderivedfrom

    Snell'slaw. Snell'slawstatesthatasoundraypropagatingthrougharegionwithonesoundspeedwill

    changedirection(berefracted)onenteringaregionhavingadifferentsoundspeed. Thedegreeof

    refractionisproportionaltothesoundspeedgradient. Refractionincreaseswithagreaterchangein

    speedoveragivendistanceordepth. Thegradientisafunctionofspeedversusdepthordistance.

    Forexample,inalayerofwaterwheresoundspeeddecreasesrapidlywithdepth,soundwavesbend

    sharplydownward.

    3.4.2.2.1.1 StraightRays

    Figure38illustrateshowsoundraystravelin

    straightlinesonlywherethespeedisconstant

    (isovelocity);nochangeinvelocitywithdepth.

    Straightsoundraysoccurwhenthetemperature

    profileisslightlynegative(adecreaseofabout1C

    per30metersofdepth). Longsonarrangesare

    possiblewhenthistypeofprofileexists.

    Straight Rays

    Figure38. StraightSoundRays.(Source:PDC)

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    3.4.2.2.1.2 RaysCurvedDownward

    Anegativetemperaturegradient

    (temperaturedecreasingwithdepth)

    producesanegative

    velocity

    gradient.

    The

    soundraysleavethesonarandarebent

    downward(asshowninFigure39),thereby

    limitingsonartoveryshortdetectionranges.

    Forexample,adecreaseintemperature

    of.56Cinthefirst10meterscausesthe

    soundbeamtomissashallowtargetatarangeof1km. Thisisacommonoccurrenceinthenear

    surfacelayer. Beyondtherangeofthedownwardbendingsoundrays,soundintensityisnegligible.

    Thisareaisknownasashadowzone.

    3.4.2.2.1.3 RaysCurvedUpward

    Apositivetemperaturegradient(temperature

    increasingwithdepth)producesapositivevelocity

    gradient. Thesoundraysleavethesonarandare

    bentupwardtowardtheseasurface.(Referto

    Figure310.)

    Longer

    ranges

    are

    attained

    with

    this

    typeofgradient,especiallyiftheseaisrelatively

    smooth. Astheraysbendupwardandstrikethe

    seasurface,theyarerepeatedlyreflectedbackintothelayerandfurtherrefractedupwardtowardthe

    surfaceaslongastheyremainintheareaof

    positivevelocitygradient.

    3.4.2.2.1.4 SplitbeamPattern

    Asdepicted

    in

    Figure

    311,

    asplit

    beam

    pattern

    occurswhenthetemperaturegradientinthenear

    surfacelayerisisothermal,andnegativebelow.

    Soundraysfromasonarsplitatthedepthofthe

    gradientchange. Partofthesoundraysare

    Rays CurvedDownward

    Figure39. SoundRaysCurvedDownward.(Source:PDC)

    Rays CurvedUpward

    Figure310.SoundRaysCurvedUpward.(Source:PDC)

    Split-BeamPattern

    Figure311.SplitbeamSoundRayPattern.(Source:PDC)

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    refractedupwardtowardthesurface,andpartarerefracteddownwardtowardthebottom. Atthe

    pointwheretherayssplit,ashadowzoneexists. Asubmarineoperatingatthesplitdepthimproves

    itschancesofavoidingdetection.

    3.4.2.2.1.5

    SoundChannel

    Asoundchanneloccurswhenanegativevelocitygradientexistsaboveanisovelocityorpositive

    velocitygradient. Thedepthwherethevelocitygradientchangesfromnegativetopositiveistheaxis

    ofthesoundchannel. Theaxisisthelevelofminimumsoundspeed. Thesoundraysonbothsidesof

    theaxistravelfasterthantheraysinthecenter. Sincesoundrefractstowardslowersoundspeeds,

    thefasterraysarecontinuallyrefractedtowardtheaxis.

    3.4.2.2.2 Reflection

    Soundwaves

    that

    strike

    solid

    surfaces

    have

    all

    or

    aportion

    of

    their

    energy

    redirected

    or

    absorbed.

    Reflectedsoundenergycanbegoodorbad. Thetypeorqualityofreflectedsoundisdependenton

    thesurfacefromwhichthesoundbounces. Bottomroughnesscanbeslightorgreat,andthe

    wavelengthcomponentofthereflectedsoundcanrangefrommicronstomiles. Asmoothrockocean

    bottomisperhapsthebestreflectorofsoundinthesea. Asmoothsandbottomalsoreflectssound

    veryeffectively. Theseasurface,ifitiscalm,isalsoagoodreflector. Soundwavesbounceoffsuch

    surfacesandloselittleoftheirenergy.

    3.4.2.2.3

    Scattering

    Anirregularhardsurfaceisnotagoodreflector. Thesoundwavesarereflectedinmanydifferent

    directionsandlosemostoftheirenergy. Thistypeofenergylossisknownasscattering.

    Soundenergyintheseaisscatteredbytheseasurface,seafloor,andsuspendedmatter. Becausethe

    seasurfaceisrarelysmooth,itismoreapttoscattersoundthantoreflectit. Aroughorrockybottom

    alsodispersesorscatterssoundenergy.

    3.4.2.2.4

    Reverberation

    Reverberationisnoiseorinterferenceatasonarreceiver,whichmakestargetdetectionverydifficult.

    Thisinterferenceiscausedbyscatteredsoundenergybeingreflectedbacktothesonarreceiver.

    Therearethreetypesofreverberation:surface,volume,andbottom.

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    3.4.2.2.4.1 SurfaceReverberation

    Surfacereverberationisaproductofsurfacewaveaction. Atshortranges,surfacescattering

    increaseswithwindspeedsbetween7and18knots. Above18knots,afurtherincreaseinthe

    surfacereverberationlevelispreventedbyasoundscreenofentrappedairbubbles. Theairbubbles

    formnearthesurfaceandarecausedbythewaveaction.

    3.4.2.2.4.2 VolumeReverberation

    Volumereverberationiscausedbyscatterersorreflectorsinthewatersuchasfish,marineorganisms,

    suspendedsolids,andbubbles. Volumescatterersarenotuniformlydistributedindepth,buttendto

    beconcentratedinadiffuselayerknownasthedeepscatteringlayer.

    Thedeepscatteringlayerisfoundintropicalwatersatdepthsbetween100and400fathoms. The

    intensityof

    the

    scattering

    is

    afunction

    of

    sonar

    frequency

    and

    the

    density

    of

    the

    organisms

    in

    the

    layer. IntheNorthernHemisphere,themaximumvolumereverberationoccursinMarchandthe

    minimuminNovember.

    3.4.2.2.4.3 BottomReverberation

    Bottomcompositionandroughnessgovernthedegreeofreverberationthatcontributestothe

    maskingoftargetechoes. Intheory,theamountofbottomreverberationisdirectlyrelatedtothe

    roughnessandcompositionoftheseafloor. However,theproblemofbottomreverberationisabit

    morecomplicated.

    Scientists

    consider

    the

    ocean

    floor

    to

    be

    atwo

    dimensional

    volumetric

    scattering

    surface. Inotherwords,soundisnotonlyreflectedofftheseafloorbutalsofromformationsofrock

    beneaththeseafloor.

    3.4.2.2.4 Absorption

    Absorptionisaproblemattheoceanbottom. Whenthebottomiscomposedofsoftmud,sound

    energyisabsorbed. Absorptionalsooccursassoundpropagatesthroughthesea,andtheenergyis

    convertedtoheat.

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    3.4.2.2.5 Attenuation

    Attenuationistheenergylossthatoccursinpropagatedsoundwavesduetoscatteringand

    absorption. Itcanalsobedefinedastheconversionofthemechanicalenergyinasoundwavetoheat.

    3.4.3Active

    And

    Passive

    Sonar

    Sonar(SoundNavigationandRanging)wasoriginallydesignedtoassistsurfaceshipswithnavigation

    duringbadweather. Later,sonarwasemployedonsubmarinesforacousticlocationoftargets,and

    today,itisourprimarymeansoflocatingsubmarines. Therearetwotypesofsonarsearches:activeand

    passive. Activesonaremploysatransmittertosendoutsoundpulsesandareceivertorecordreturning

    echoes. Passivesonarlistensforsoundsgeneratedbyothershipsandsubmarines.

    3.4.3.1 ActiveSonar

    Activesonarsearchisclassifiedintotwomodes:shallowwatertransmissionsanddeepwater

    transmissions. Theoretically,theessentialdifferencebetweenshallow anddeepwatertransmissionsis

    theinterferenceeffectsproducedbythemultiplereflectionsofsoundinshallowwater.

    Shallowwaterisclassifiedaswaterlessthan100fathoms. Deepwaterisclassifiedaswater1,000

    fathomsordeeper. Waterbetween100and1,000fathomsdeepismostcommonovercontinental

    slopes. Itisnotconsideredoverlyimportantinactivesonaroperationsbecauseitexistsinsuchasmall

    portionoftheworld'soceans.

    3.4.3.1.1Shallow

    Water

    Transmissions

    Shallowwaterpropagationpathsareclassifiedaseitherdirectpathorsurfaceduct.

    3.4.3.1.1.1 DirectPath

    DirectPathisthesimplestmode. Directpathsoundpropagationoccurswherethereisanapproximate

    straightlinepathbetweenthesoundsourceandreceiver,withnoreflectionfromanyothersourceand

    onlyonechangeofdirectionduetorefraction.

    3.4.3.1.1.2Surface

    Duct

    Asurfaceductissimplyanearsurfacelayerthattrapssoundenergy. Surfaceductsexistintheoceanif

    oneofthefollowingconditionsaremetwithinthemixedlayer: (1) temperatureincreaseswithdepth,

    (2)temperaturethroughthemixedlayerisisothermal.

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    Incondition1,soundvelocityincreasesasthetemperatureincreasesproducingalayerwithapositive

    soundvelocitygradient. Incondition2,anisothermallayernearthesurface,pressurebecomesthe

    dominantfactorandsinceanincreaseinpressurecausesanincreaseinsoundspeed,alayerwitha

    positivesoundvelocitygradientisagainproduced. Thegreaterthedepthofaduct,themoresound

    speedincreaseswithdepthproducinganevengreaterdifferencebetweenthesurfacevelocityandthe

    velocityatdepth. Ductsthatextendtogreaterdepthstrapagreaternumberofsoundwavesandcan

    extenddetectionrangestoverylongdistances. Theefficiencyofanysurfaceduct,nomatterthedepth,

    ishighlydependantonthesmoothnessoftheseasurface. Waveactioncausesreverberationand

    scattering,bothwhichreducetheefficiencyofasurfaceduct.

    3.4.3.1.1.3 EnvironmentalControls

    Thesuccessofactivesonarsearchesinshallowwaterdependsagreatdealonenvironmentalfactors.

    Horizontalandverticaltemperaturegradients,waterdepth,andthephysicalcharacteristicsofthesea

    surfaceandbottomallimpactshallowwatertransmissions. Ofthesecontrols,waterdepthisthemost

    important.

    3.4.3.1.1.3.1 TemperatureGradient

    Variationsintheverticaltemperaturegradient,whichresultinsoundspeedvariations,areofutmost

    importancewheresoundispropagatedthroughasurfaceduct. Achangeingradientof.2Cper30

    meterscanbethedifferencebetweenanexcellentductwithgoodrangesandnoductandpoorranges.

    Horizontalvelocitygradientsintheoceanarenotasgreatasthoseinthevertical;however,theycan

    completelydestroyaductiftheyoccurbetweenthesoundsourceandthetarget.

    3.4.3.1.1.3.2 WaterDepth

    Waterdepthdeterminestherangeandangleatwhichsoundraysstrikethebottomandtosomeextent

    thetypesoftransmissionpathsthatoccur.

    Inshallowwater,asindeepwater,thesoundvelocityprofilecontrolsthedegreeofrefractionofsound

    rays. Foranexampleofhowsimilarprofilesaffectshallow anddeepwatertransmissions,considerthe

    following: Indeepwater,whereastrongnegativegradientexists,soundraysarerefracteddownward

    andresultinshadowzones. Ontheotherhand,inshallowwaterthedownwardrefractedraysreflect

    offthebottom,travelupward,andreflectofftheseasurface,andthentravelbacktowardthebottom.

    Thisprocesscontinuesuntiltheshadowzoneiscompletelysaturatedwithenergy,resultinginvastly

    improvedprobabilitiesofdetection.

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    3.4.3.1.1.3.3 BottomComposition

    Bottomcompositionandroughnesscontrol,toalargeextent,thereflectiveandabsorbentcapabilities

    ofthebottom. Shallowwaterbottomcompositionandtopographycontrolthereflectivecapabilitiesof

    thebottomandtheattenuationofsoundenergy. Shallowwatersedimentsarequitediversewithareas

    ofmud,sand,mudsand,gravel,rock,andcoralnotuncommonovershelfregions. Topographyand

    bottomcompositionalsocontrolthedegreeofreverberationthatcanmasktargetechoes.

    3.4.3.1.2 DeepWaterTransmissions

    Indeepwater,soundcantravelfromandtothesonarviasurfaceduct,convergencezone,bottom

    bounce,andsoundchanneltransmissionpaths.

    3.4.3.1.2.1 SurfaceDucts

    Surfaceductsoccurindeepwaterjustastheydoinshallowwater.

    3.4.3.1.2.2 SoundChannels

    Asoundchannelisformedwhenanegativevelocitygradientexistsaboveapositivevelocitygradient.

    Thethermalgradientnecessarytoproduceasoundchannelisnegativeoverisothermalornegativeover

    positive. Thesoundchannelaxisisfoundatthepointofminimumsoundvelocity,wherethesound

    velocityprofilechangesfromnegativetopositive. Asoundchanneltrapssoundraysandprovides

    extremelylongranges. Theverticaltemperatureprofilesthatproducesoundchannelscanbefoundin

    bothshallowanddeepwater.

    3.4.3.1.2.2.1 ShallowSoundChannels

    Shallowsoundchannelsarefoundinthenearsurfacelayer. Theyarerareandtransitory,butcanoccur

    inshallowwaterduetotheintermixingofwatersdifferingintemperatureand/orsalinity. Asthese

    watersintermingleandmixaccordingtotheirdensitycharacteristics,weakshortlivedsoundchannels

    result. Theseshallowsoundchannelsareseldomofsufficientextentorpersistencetobetactically

    usefulinunderseawarfareoperations.

    InthePacificOcean,shallowsoundchannelsaremostcommonintheareanorthof40Nbetween

    HawaiiandthecontinentalUnitedStates. IntheAtlantic,theyaremostfrequentlyobservedinthe

    vicinityoftheGulfStream.

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    3.4.3.1.2.2.2 DeepSoundChannels

    Deepsoundchannelsarefarmoreprevalentthantheirshallowcounterpart. Inthedeepocean,

    temperaturegenerallydecreaseswithdepththroughthemainthermocline. Thisproducesanegative

    velocitygradientandsoundraysrefractdownward.

    IntheAtlantic,suchgradientsexisttoadepthofapproximately700fathoms. Below700fathoms,the

    gradientbecomesisothermal,whileinthePacific,theisothermallayerbeginsaround500fathoms.

    Belowthesedepths,increasingpressurebecomesthedominantfactoraffectingsoundspeedanda

    positivevelocitygradientisproducedrefractingsoundraysupward. Thedeepsoundchannelaxisexists

    attheminimumdeepsoundspeedwherethevelocitygradientchangesfromnegative(throughthe

    mainthermocline)topositive(throughthedeeplayer). Thedepthofthedeepsoundchannelaxis

    variesfrom1,225metersinthemidlatitudestonearthesurfaceinthePolarRegions.

    Extremelylongsonarranges(ontheorderofthousandsofmiles)arepossiblewithinadeepsound

    channel.

    3.4.3.1.2.3 ConvergenceZone

    Thissoundtransmissionpathisbasedontheprinciplethatsoundenergyfromashallowsourcetravels

    downwardinthedeepoceanandisrefractedandupwardtowardthesurfaceatadepthwherethe

    deepwatersoundvelocityequalsthenearsurfacemaximumsoundvelocity. Thesoundraysare

    refractedandfocusedupwardandreflectoffthesurfaceabout30milesfromthesoundsource. The

    reflectedraysthentraveldownward,andthepatternrepeatsitself. Thesoundraysreappearinthe

    surfacelayeratsuccessiveintervalsofabout30milesandmaycontinueforseveralhundredmiles.

    Therearetwoconditionsnecessaryforconvergencezonetransmission: (1)Thesoundvelocityindeep

    watermustbeequaltoorgreaterthanthenearsurfacemaximumsoundvelocity(thisiscalledCritical

    Depth)and(2)waterdepthbelowthecriticaldepthmustbegreatenoughtopermittherefractedsound

    raystoconvergeinasmallareaatthesurface,thisiscalledDepthExcess.

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    3.4.3.1.2.4 BottomBounce

    Bottombouncetransmissionusessharplyangledraypathstoovercomevelocitygradientchanges. The

    soundenergyisdirecteddownwardatahighangleofincidencetothebottom(Figure312). With

    steeplyinclinedrays,transmission

    isrelativelyfreefromthermal

    effectsatthesurface,andthe

    majorpartofthesoundpathisin

    nearlystablewater. Thesound

    energyisaffectedtoalesser

    degreebyvelocitychangesthan

    themorehorizontalraypathsof

    other

    transmission

    modes.

    Longrangescanoccurinwater

    deeperthan1,000fathoms,dependingonthebottomslopeandbottomcomposition. Itisestimated

    that85%oftheoceanisdeeperthan1,000fathoms,andbottomslopesaregenerallylessthanorequal

    to1degree. Onthisbasis,relativelysteepanglescanbeusedforsinglebottomreflectiontoarangeof

    approximately20,000yards. Atshallowerdepths,multiplebouncepathsdevelopwhichproduce

    scatteringanditshighintensityenergyloss.

    The

    geologic

    composition

    of

    the

    ocean

    bottom

    has

    an

    extremely

    significant

    effect

    upon

    the

    final

    strengthofbottomreflectedsound. Dependingoncomposition,suchinterrelatedeffectsasreflection,

    absorption,scattering,attenuation,andreverberationcomeintoplay. Factorsthatincreasethesound

    reflectivityofthebottomare:(1)anincreaseinthecalciumcarbonatecontentofthesediments,(2)a

    decreaseinporoussedimentandcompaction,(3)anincreaseinthemeandiameterofsediment

    particles,(4)anincreaseinthedegreeofcementationorrigidity,(5)anincreaseinthetemperatureof

    thesediments. Energylossintobottomsedimentsdependsprimarilyuponbottomcomposition. The

    NavalOceanographicOffice,StennisSpaceCenterproducesbottomlosschartsandbottomtypelosses

    forvariousbottomtypesarediscussedinNavalMeteorologyandOceanographyCommandInstructions.

    3.4.3.1.2.5 ArcticandHalfChannelPropagation

    IntheArcticOceanregion,thelackofsolarheatingpreventstheformationofthemainthermocline

    evidentinthelowerlatitudeoceans. Apositivesoundspeedgradientextendsuptoshallowdepthsin

    thesummerandallthewaytotheiceboundaryinthewinter.

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    Inthesummer,inopenwater,athinsurfaceduct(normally 100feet)canoccur. Strongsalinity

    generatedpositivesoundspeedgradientscanoccurinthesurfaceregionduetomeltingiceorfresh

    waterrunofffromriversnearcoastalregions;therebyremovinganysolargeneratednegative gradients.

    Theinteractionofupwardrefractedenergywiththeundericesurfaceisdependantupontheroughness

    oftheice,whichservesasthemajorcauseofattenuation. Duetotheupwardrefractionoftheenergy

    andthedominanteffectoftheicecoveronattenuation,bottombounce,orinteractionwiththe

    seafloorisaminorsourceofpropagationlossintheArcticRegion.

    3.4.3.2 ActiveSonarEquationSE=SL+TSRDNL+DI2PL,

    WhereSE=Signalorechoexcess,SL=Sourcelevel,TS=Targetstrength,RD=Recognitiondifferential,

    NL=Noiselevel,DI=Directivityindex,2PL=Twowaypropagationloss.

    Whenreverberationdominates,theequationmaybewrittenSE=SL+TSRDRL2PL,whereSE=

    Signalexcess,SL=Sourcelevel,TS=Targetstrength,RD=Recognitiondifferential,RL=Reverberation

    level,and2PL=Twowaypropagationloss.

    3.4.3.2.1 SignalExcess(SE)

    Signalexcessistheamountofsoundenergyreceivedfromatargetoverandabovetheamount

    requiredtodetectit. Signalexcessisbasedonprobabilityconditions. Whenthesignalexcessiszero,

    theprobabilityoftargetdetectionisconsideredtoberoughly50%. Signalexcess,likealloftheother

    factorsoftheequation,isexpressedindecibels.

    3.4.3.2.2 SourceLevel(SL)

    Sourcelevelofthesonarprojectorpertainstotheintensityoftheradiatedsound,indecibels,relative

    toareferencedintensity. Sourceleveliscontrolledbythedesign,maintenance,andsonarmodeof

    operation.

    3.4.3.2.3Recognition

    Differential

    (RD)

    Recognitiondifferentialpertainstotheabilitytodifferentiatetargetnoisefrombackgroundnoise. Itis

    afunctionoftargetdesign,maintenance,atarget'smodeofoperation,andtheexperienceofthesonar

    operatortodetectatargetthroughthebackgroundnoise. Recognitiondifferentialwasoriginally

    definedasthesignaltobackgroundnoiseratiorequiredatthereceivertorecognizeatarget50%ofthe

    time. However,usingthe50%probabilityresultedintoomanysignalsbeingclassifiedastargetsthat

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    werenottargets. TheinordinateamountoffalsealarmsledtoamorespecificqualificationofRD.

    Today,RDcanapplytoaspecificprobabilityofdetectionandaspecifiedprobabilityofafalsealarm.

    3.4.3.2.4 TargetStrength(TS)

    Thetargetstrengthofareflectingobjectistheamountbywhichtheapparentintensityofsound

    scatteredbytheobjectexceedstheintensityoftheincidentsound. Thisvaluedependsonthesize,

    shape,construction,typeofmaterial,roughness,andaspectofatarget,aswellastheangle,frequency,

    andwaveformoftheincidentsoundenergy.

    3.4.3.2.5 NoiseLevel(NL)

    Noiselevelpertainstoambientnoiseandselfmadenoiseatthelocationofthesonar. Noiselevelisa

    functionoftheenvironmentandship'sspeed.

    3.4.3.2.6 PropagationLoss(PL)

    Propagationlossisthelossofsignalstrengthbetweenthesonarandthetarget. Intheactivesonar

    equation,PLisatwowaylossofenergysincesoundenergytravelsoutfromthetransmitterthen

    reflectsoffthetargetbacktothereceiver. Propagationlossinwaterdependsonthefollowingfactors:

    Spreadingofthesoundwavefront.Thefartherthesoundwavemovesfromthesource,thegreaterthesizeof

    thewavefrontandthespreadingofthesoundenergy.

    Conversionofthemechanicalenergyinasoundwavetoheat(attenuation).

    Scatteringduetosurface,bottom,andsuspendedparticulatereflections.

    Diffractionloss,whichistheLeakageofsoundenergyfromlayersoftrappedsound(ductsandsoundchannels)

    andleakageofenergyintoareaswhereitisabsorbedornotcapableofdetection(shadowzones).

    Multiplepathinterferencethatoccurswhenoneormoresoundpathschangewithtimeandintensity

    fluctuationsoccur.

    3.4.3.2.7 DirectivityIndex(DI)

    Directivityisafunctionofthereceiversmechanicalabilitytofocusalongacertainazimuthandisbased

    onthedimensionsofthesonarshydrophone(receiver)array,thenumberandspacingofthe

    hydrophones,andthefrequencyofthereceivedacousticenergy. Thesefunctionsenablethedirection

    ofareceivedsignaltobedetermined. Directivityalsoreducesnoisearrivingfromdirectionsotherthan

    thatofthetarget. Thedirectivityindexpertainstoasonar'sabilitytodiscriminateagainstnoise. Itis

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    definedasthesignaltonoiseratio(indecibels)attheterminalsofahydrophonearrayoradirectional

    hydrophone,relativetothesignaltonoiseratioofanondirectionalhydrophone. Thusdefined,DIis

    alwaysapositivequantityintheequation.

    3.4.3.2.8Reverberation

    Level

    (RL)

    Reverberationisobservedatthesonarreceiver. Thelevelofreverberationisafunctionofsourcelevel;

    range;andsurface,volume,andbottomreverberation. Whenactivesonarisreverberationlimited,RLis

    usedintheequationinplaceofNLandDI.

    3.4.3.3 PassiveSonarEquation

    Inpassivesonaroperations,thehydrophonesreceivesoundsgeneratedbyamultitudeofsoundsources.

    Individualsmustdifferentiatebetweensoundsgeneratedbythetargetandinterferingbackgroundnoise

    calledAmbientNoise. Thisprocessisbestdescribedinwhatisknownasthepassivesonarequation.

    Thepassiveformofthesonarequation,liketheactiveform,iswrittenusingseveraldifferentsymbolsto

    representtheequationparameters. Oneformoftheequationisasfollows:SE=SLRDNL+DIPL,

    whereSE=Signalexcess,SL=Sourcelevel,RD=Recognitiondifferential,NL=Noiselevel,DI=

    Directivityindex,andPL=Propagationloss. Notethatpropagationlossinthepassivesonarequationis

    onlyonewaysinceallsignals(sounds)arereceivedpassively.

    3.4.3.3.1 SignalExcess(SE)

    Signalexcesshasthesamemeaninginthepassiveequationthatitdoesintheactiveequation.

    3.4.3.3.2 SourceLevel(SL)

    Sourcelevelpertainstotargetradiatednoise. Itistheamountofsoundenergygeneratedbyatarget.

    Thelevelofenergyreachingthesonarreceiverdependsonthetypeoftargetanditsmodeofoperation.

    Sourcelevelisafunctionoffrequency,speed,depth,andtargetaspect. Thelatterreferstoatarget's

    orientationinrelationtothesonarreceiver.

    3.4.3.3.3 RecognitionDifferential(RD)

    RDhasthesamemeaningasintheactivesonarequation.

    3.4.3.3.4 NoiseLevel(NL)

    ThedefinitionforNLinthepassiveequationisthesameasintheactiveequation. Passivesonarsmay

    beselfnoiseorambientnoiselimited. Thesesonarscanusebroadbandtolistenforawiderangeof

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    frequenciesorcanbetunedtolistenforanarrowrangeoffrequencies,thusfilteringoutcertain

    background,orambientnoise.

    Theprimarygoalinunderwateracousticsistodistinguishspecificsoundsfromthetotalbackground

    noise.

    Selfnoiseisthatpartofthetotalbackgroundnoiseattributabletothesonarequipment,theplatform

    onwhichitismounted,orthenoisecausedbythemotionoftheplatform. Themajorclassesofself

    noisearemachinerynoise,propellernoise,andhydrodynamicnoise. Thelatterresultsfromtheflowof

    waterpasthydrophones,supports,andthehullstructureoftheplatform.

    Ambientnoiseisthatpartofthetotalnoisebackgroundnotduetosomeidentifiablelocalizedsource.

    Ambientnoiseiscreatedbyseveraldifferentsourcesincludingsurfaceshiptraffic,waveaction,

    precipitation,ice,andcertainformsofmarinelife.

    3.4.3.3.4.1 SurfaceShipTrafficNoise

    Atthelowerfrequencies,thedominantsourceofambientnoiseisthecumulativeeffectofshipsthat

    aretoofarawaytobeheardindividually. Theradiatednoisespectrumofmerchantshipspeaksat

    approximately60Hz,afrequencythatcorrespondstothemaximuminthecavitationfrequency

    spectrumoftypicalmerchantships.

    3.4.3.3.4.2 SeaStateNoise(WaveAction)

    Seastateisacriticalfactorinbothactiveandpassivedetection. Forshipboardsonarsystems,the

    locationofthesonardome,shipsspeed,course,andrelationtotheseaallhaveaneffect. Thelimiting

    situationforactivesonaroperationsisgenerally612feet(seastate4or5). Forpassivedetection,the

    noiselevelcreatedbywindwavesof10feetorgreaterwillresultinaminimumofunderseawarfare

    operationaleffectiveness. Ambientnoisegeneratedbywaveactionusuallyvariesinrangefrom300Hz

    to5kHz.

    3.4.3.3.4.3 Precipitation

    Rainandhailwillincreaseambientnoiselevelsatsomefrequencies. Significantnoiseisproducedby

    rainsquallsoverarangeoffrequenciesfrom500Hzto15kHz. Largestormscangeneratenoiseat

    frequenciesaslowas100Hzandcansubstantiallyaffectsonarconditionsataconsiderabledistance

    fromthestormcenter.

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    3.4.3.3.4.4 Ice

    InPolarRegions,seaiceinfluenceswaternoise

    levelsdependingonthestateoftheice,whether

    itisforming,coversthesurface,orisbreakingup.

    Noiselevelsaregenerallylowduringice

    formationbutcanbecomeextremelynoisyif

    entrappedaircausesdeformationandthe

    temporarybreakupoficeduringiceformation.

    3.4.3.3.4.5 MarineLife

    Alsoreferredtoasbiologics,marinelifemay

    contributesignificantlytoambientnoiseinmany

    areasoftheocean. Becauseofthehabits,distribution,andsoniccharacteristicsofthevarioussound

    producers,certainareasoftheoceanaremoreintensethanothers. Theeffectofbiologicalnoiseon

    overallnoiselevelsismorepronouncedinshallowcoastalwatersthanintheopensea,andmore

    pronouncedinthetropicsandintemperatezonesthanincolderwaters. Muchmoreinformationis

    availableintheNavalOceanographicOffices,FleetOceanographicandAcousticReferenceManual,RP33.

    3.4.3.3.5 DirectivityIndex(DI)

    DIhasthesamemeaningasintheactivesonarequation.

    3.4.3.3.6 PropagationLoss(PL)

    PLhasthesamemeaningasintheactivesonarequationexceptthatwithpassivesonar,theenergyloss

    isonewaysincethesourcetargetgeneratednoiseratherthanthesonar.

    3.5OCEANOGRAPHICSUPPORTPRODUCTS

    3.5.1 NAVALOCEANOGRAPHICOFFICE

    TheNavalOceanographicOffice(NAVOCEANO)providesoperationaloceanographicsupporttotheFleet

    throughtailoredanalysis,realtimedata,climatologicalproductsandoperationaloceanmodels.

    3.5.1.1 NavyLayeredOceanModel(NLOM)

    Globalcoverage

    Figure313. ExampleofaNavyLayeredOceanModel(Source:NavalOceanographicOffice)

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    1/16degreeresolution

    Seawardof200mdepth

    Sixverticallayers

    Forecastsfrontandeddypositions

    dailyfrom0to48hours

    Forecastslayeredseasurface

    temperature(SST)andseasurface

    height(SSH)

    AnexampleoftheproductispresentedinFigure313.

    3.5.1.2 GlobalNavyCoastalOceanModel(GNCOM)

    1/8degreeresolution

    42verticallayers

    Willprovideboundaryconditionsfor

    higherresolutionnests

    AssimilatesNLOMSSH

    UnderwentvalidationtestinginFall2003

    Forecasts3Dtemperature,salinityand

    currentstructurefrom0to96hours

    Anexampleoftheproductispresentedin

    Figure314.

    3.5.1.3 ShallowWaterAnalysisandForecast

    System(SWAFS)

    3Dcoastalcirculationmodel

    BasedonPrincetonOceanModel(POM)

    Resolutionvariesbyregion(1/2to24km)

    Assimilatesobservationsfromsatellite(SST,SSH)andinsitu(ExpendableBathythermograph

    (XBT);Conductivity,Temperature,andDepth(CTD);andprofilingfloat)

    Figure314. ExampleProductofGNCOM.(Source:NavalResearchLaboratory)

    Figure315. ExampleofaSWAFSProduct(Source: NavalOceanographicOffice)

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    Figure316. ExampleofMODASDataSetSource: Naval Oceano ra hic Office

    ForcedbytidesandFleetNumericalMeteorologyandOceanographyCenter

    (FLENUMMETOCCEN)windsandfluxes

    Providesdaily3Dforecastsofcurrents,tides,temperature,salinityfrom0to48hours

    AnexampleoftheproductispresentedinFigure315.

    3.5.1.4 ModularOceanDataAssimilationSystem(MODAS)

    Statisticalanalysismodelfor:

    Temperature

    Salinity

    Derivedquantities(soundspeed,etc.)

    Relocatable,variableresolution. UsesOptimumInterpolationschemestocombine:

    o SatelliteDerivedSeaSurfaceAltimetry

    o Griddedclimatology(temperature,salinity)

    o NearrealtimeXBT,CTD,floatandbuoydata

    Provides3Dtemperatureandsalinitygrids

    Usedforacousticpredictionmodels

    FoundationforMODAS,runatNMOCregionalcentersanddeployedNavyships.

    Providesinitializationfieldsfor3Dmodels

    AproductexampleispresentedinFigure316.

    3.5.1.5 2DTidalElevation/CirculationModels

    RMA2Riverineandestuarymodel

    ADCIRCCoastalcirculationmodel

    WQMAPEstuarineandcoastalcirculation

    model

    PCTidesCoastalandsmallbasintidalmodel

    Delft3DIntegratednearshorecirculation,wave

    andsurfmodelingsystem

    Relocatablemodelswithhighresolutiondomains

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    3.5.1.6 WaveModel(WAM)

    Areacoverage

    Globallyrelocatable

    Currently

    running

    many

    domains

    Variableresolution(1/4to1/12degree)

    Deepwaterwavemodel(>20m)

    Analysisandforecaststo48/72hours(twicedaily)

    SurfacewindforcingusingFNMOC'sNavyOperationalGlobalAtmosphericPredictionSystem

    (NOGAPS)andCoupledOcean/AtmosphereMesoscalePredictionSystem(COAMPS)models

    Producesgraphicsandgriddedsetofwaveparameters

    Predominantwavedirection

    ProductsInclude:

    o Significantwaveheight

    o Swelldirection,period,andheight

    o Windwaveheight

    o Averagewaveperiod

    3.5.1.7 SteadyStateSpectralWaveModel(STWAVE)

    Areacoverage:~25kmalongcoast

    Relocatable,variableresolution(100to400m)

    Shallowwatermodel(

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    o Peakwaveperiod

    o DeepwaterinputprovidedbyWAM

    3.5.2 GEOPHYSICSFLEETMISSIONPROGRAMLIBRARY(GFMPL)

    GFMPLisasoftwarelibraryestablishedbyCommander,NavalMeteorologyandOceanography

    Command(COMNAVMETOCCOM). GFMPLprovidesenvironmental,meteorological,electromagnetic,

    oceanographic,hazardavoidance,acousticandweaponsystemsupportsoftwareforfleetair,surface,

    amphibious,andantisubmarinewarfareoperationsandplanningpurposes.

    GFMPLutilizesinsituandhistoricalenvironmentaldatarunthroughspecificalgorithms. These

    algorithmsaresupplied,ifnecessary,withforce,threat,sensorandweaponcharacteristicstoprovide

    environmental,sensor,orweaponperformancepredictions.

    3.5.3 MODULAROCEANOGRAPHICDATAASSIMILATIONSYSTEM(MODAS)

    MODAS2.05hasaGraphicalUserInterfacethatenablesausertoedittheBTdata,runMODASon

    demand,andbuildgraphicalproducts. ItishostedonahighendUNIXworkstationattheRegional

    METOCCenters.

    UndertheMODASConceptofOperations,NAVOCEANOreceivesandprocessessatelliteandBTdata

    andgeneratesanoceandepictionusingMODASHeavy. TheresultinganalysesaresenttotheRegional

    METOCCenters.

    TheRegionalCentersupdatetheseanalysesusingtheMODAS2.05toolandrecentBTdatathatmight

    nothavereachedNAVOCEANO. Theseupdatedanalysesarethendeliveredtocustomersatsea.

    Finally,thereisMODASLite. Thisthirdversionacceptsafirstguessfield(eitherastaticclimatological

    fieldorapreviousanalysis)andupdatesitwithBTdata. MODASLiterunsonaPersonalComputer(PC)

    and,likeMODAS2.05,doesnotexploitDynamicClimatology.

    MODASLiteenablesthesecustomerstoupdatetheMODASfieldsagainusinganyonsceneBTs.

    MODASLiteisintegratedwiththePCInteractiveMultisensorAnalysisTraining(PCIMAT)system. PCIMATwasoriginallyanaidfortrainingtheacousticsoftheocean. However,ithasbeensowellaccepted

    thatithasbecomeaTDA.

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    3.5.4 PERSONALCOMPUTERBASEDINTERACTIVEMULTISENSORANALYSISTRAININGSYSTEM(PC

    IMAT)

    ThePersonalComputerBasedInteractiveMultisensorAnalysisTrainingSystem(PCIMAT)isthe

    premieroceanacousticanalysisandplanningtoolavailableonallASWplatforms,includingsurface

    ships,aircraft,andsubmarines. TheSpaceandNavalWarfareSystemsCommandoriginallydeveloped

    thissystemasatrainingtool,butithassinceevolvedintoatacticaldecisionaidforASWplatforms.

    Oceanconditionscanberetrievedfromaworldwide,historicaldatabasethatusesuptodatesatellite

    imagery. PCIMATalsoprovidesoceantemperatureestimates,whichaffectsensorperformance,aswell

    asavisualrepresentationofestimatedsoundpropagationpaths(i.e.,raytracing),whichenables

    MeteorologyandOceanographyprofessionalstoprovideactionablerecommendationstothe

    Warfighteronthemosteffectivesearchpatterns,sensorplacements,andsonaroperationalmodes

    basedonrealtimeenvironmentaldataintheoperationalarena. ThenewestversionofPCIMATaddsa

    missionplanningmodulethatanalyzestheacousticconditionsatauserdesignatedgeographiclocation,

    andprovidesagraphicalrepresentationofaplatformseffectivesearchrangesanditsvulnerabilityto

    counterdetectionbyanenemy.

    3.6 OCEANOGRAPHICOBSERVATIONS

    LearningObjectives

    Recognizeandexplainthenearshorecirculationsystem.

    Figure317. ExampleProductofPCIMAT. (Source:SPAWARSYSCOM)

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    Identifythemajoroceancurrentsandtheirlocations.

    Recognizethemajoroceancurrentsand

    theireffectsontheweather.

    Explaintheimportanceofseaconditions

    tonavaloperations.

    Definedurationlimitedseasandfetch

    limitedseas.

    Definewaveheight,wavelength,wave

    period,andwavedirection.

    Defineanddistinguishthedifference

    betweenseawavesandswellwaves.

    Describetheformationoficeonthesurfaceofthesea.

    Differentiatebetweenseaiceandlandice.

    3.6.1 COASTALANDNEARSHORECIRCULATIONS

    Twointerrelatedcurrentsystemsmayappearneartheshore. Theyarethecoastalcurrentsystemand

    the

    nearshore

    current

    system.

    The

    coastal

    current

    system

    is

    a

    relatively

    uniform

    drift

    that

    flows

    roughly

    paralleltoshore. Itcanbecomposedoftidal,winddriven,orlocaldensitydrivencurrents. The

    nearshorecurrentsystemismorecomplexandiscomposedofshorewardmovingwaterintheformof

    wavesatthesurface,areturnflowalongthebottominthesurfzone,nearshorecurrentsthatparallel

    thebeach,andripcurrents.

    3.6.1.1 LongshoreCurrents

    Longshoreorlittoralcurrentsoccurinthesurfzoneandarecausedbywavesapproachingthebeachat

    an

    angle

    (Figure

    3

    18).

    At

    times

    the

    current

    is

    almost

    imperceptible,

    but

    at

    other

    times,

    it

    can

    be

    quite

    strong. Longshorecurrentsincreaseinvelocitywithincreasingbreakerheight,increasingbreakercrest

    speed,increasinganglebetweenbreakercrestsandbottomcontours,anddecreasingwaveperiod. A

    steepbeachwillhaveastrongerlongshorecurrentthanamoregentlyslopingbeach.

    Figure318. LongshoreCurrent.Source: Naval Oceano ra hic Office

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    3.6.1.2 RipCurrents

    RipCurrentsareoftenerroneouslycalledriptides,buttheyarenotassociatedwithtides. Theyare

    causedbyreturnflowofwaterfromthebeach. Thecurrentresemblesasmalljetinthebreakerzone,

    whichfansoutbehindthebreakersandbecomequitediffuse. Thisstrongcurrentextendsfromthe

    surfacetothebottom. Thestrengthofripcurrentsisnotpredictable,butisdeterminedusingthesame

    factorsthatcontrollongshorecurrents. Ripcurrentsmayormaynotoccur,butwhentheydo,theycan

    beirregularlyspacedorspacedatlongorshortintervals. Theycommonlyformatthedowncurrentend

    ofabeachwhereaheadland(apointwherethelandjutsoutintothewater)deflectsthelongshore

    currentseaward.

    3.6.2 MAJOROCEANCURRENTS

    Themajoroceancurrentsareestablished,andmaintainedbythestressesexertedbytheprevailing

    winds. TheoceaniccirculationpatternroughlycorrespondstoEarthsatmosphericcirculationpattern.

    Inthemiddleandlowerlatitudes,theoceaniccirculationismainlyanticyclonic. Warmcurrents(Ex.

    GulfStreamandKuroshio)flowpolewardalongtheeasterncoastofcontinentsandcoldcurrents(Ex.

    CanaryandCalifornia)flowequatorwardalongthewesterncoastofcontinents. Thisistrueforboth

    Figure319Themajoroceancurrentsoftheworld.(Source:AmericanMeteorologicalSociety)

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    hemispheres.

    Athigherlatitudes,thewindflowisprincipallycyclonicandtheoceaniccirculationfollowsthispattern.

    Coldcurrentsflowequatorwardalongtheeastcoastofcontinentsandwarmcurrentsflowpoleward

    alongthewestcoastofcontinentsintheNorthernHemisphere. Inregionsofpronouncedmonsoonal

    flow,themonsoonwindscontrolthecurrentsandvarywiththeseasons. Irregularcoastlinescan

    causedeviationsinthegeneraldistributionofoceancurrents.

    Theoceaniccirculationpatternactstotransportheatfromonelatitudebelttoanotherinamanner

    similartotheheattransportedbytheprimarycirculationoftheatmosphere. Thecoldwatersofthe

    ArcticandAntarcticmoveequatorwardtowardwarmerwater,whilethewarmwatersofthelower

    latitudesmovepoleward. Theeffectonclimateisseeninthecomparatively,mildclimatethatexistsin

    theareaofnorthwestEurope. Figure319displaysgeoloc


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