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18www.cadc.caFallWinter2013featureActivesonarisadevicethattransmitsacousticenergyintothewatercolumnandmeasurestheelapsedtimeoftheechoedreturns.Themostcommonsonarsystemsusedtoaugmentdiverinspectionsincludeechosounderalsocalledadepthfinderorfathometerside-scansonarandscanningsonaralsocalledmechanicallyscannedorradialsonarwithimagingandorprofilingcapability.Eachsystemhasitsapplicationstrengthsandinherentlimitations.Theechosounderthesimplestofsystemsisalmostexclusivelyusedtodeterminewaterdepth.Thesounderstransducerthesystemcomponentthattransmitsandreceivestheacousticenergyisalignedsotheacousticpulseprojectstowardtheseabed.Withinthesonarelectronicsisaclockthatmeasuresthetimefromtransmittoreceivingtheechoedreturn.Toderivedistancetheclockedtimeismultipliedbyanin-waterspeedofsoundapproximately1500m4921persecondthisgivesthetotalortwo-waydistancethepulsetravels.Sincethewaterdepthisofinterestthetwo-waydistanceishalvedandgraphicallypresenteddisplayedasanumericvalue.Theaccuracyofthedataisafunctionofthetransducerbeamangletheseabedslopeandortransducerpitchrollandapplyingthecorrectin-waterspeedofsoundusedtocalculatedistance.Themaximumrangeofanechosounderoranysonarisafunctionofitsfrequency.Thestrengthofthesystemissimplicityofoperationandeaseofdatainterpretation.Whenechosounderdataistime-taggedrecordedwithaccuratepositionandpitchandrollinformationhighlyaccuratetwo-andthree-dimensionalplotsandbottomdepthcontourscanbegenerated.Theweaknessofthissystemisitslimitedcoverageandbeingabletopositionthevessel-mountedtransduceraboveascourholeorpointofinterestespeciallyinhighcurrentorareaswithlimitedvesselaccess.side-canSide-scansubseaelectronicsareoftenmountedinahydrodynamically-shapedtowedinstrumentcalledatowfishthathastransducersoneithersideofitslongaxis.Thesoundenergytransmitsviathetransducersperpendiculartothetowdirection.Whensoundenergycollideswithtargetsinitspaththeseabedsomeisreflectedbacktothesonarwhereitisdetectedandprocessed.Theresultisaplain-viewrecordofthebottomwiththeimagebuiltbynarrowacousticpulsesscrollingsimilarlytotheline-by-lineimagescrollingfromaprinter.Side-scanisanunbeatablewideareasearchandseabed-mappingtool.Itsresolutionisfunctionofitsfrequencyanditstransducershorizontalbeamanglehighfrequencyandnarrowerhorizontalbeamanglesimproverecordclarity.Typicallypulledviaacablecontainingelectricalconductorsbehindthesurveyvesselside-scanproducesthebestimagewhentowedatafixedspeedinastraightlineandatadeterminedaltitudeabovebottomrangedependant.Whencompletingaside-scansearchdeployamarkerbuoywhenatargetofinterestappearsonthedisplay.Thisisfollowedbyasecondsearchlinerunonareciprocalheadingandreversedalignment.Whenthetargetreappearsdeployasecondmarkerbuoy.Assumingthetowcablelengthandshipsspeedarethesameonbothsearchlinesthetargetisclosetomidpointbetweenthetwomarkersandoffsetfromthevesselcoursebytheperpendicularmeasureddistanceofthetargetfromthetowfish.Alternativelytheoperatorcanelectronicallymarkatargetusinganintegratedsonarchartplotterwhenthetowfishpositionanditsheadingareknown.Measurementandgeographicaccuracybetweentwoside-scantargetsisafunctionofknowingthetowfishpositionthatisontheendofacablebehindthevesseltowfishheadingandapplyingthecorrectsoundspeed.Mostside-scansystemsoftwareincludesaslant-rangecorrectionfunctionwhichworkswhentheseabedisflat.Itdoesnotscaletheimagecorrectlyonaslopedbottom.Thestrengthsofside-scanaretheimageclarityandsurveycoveragespeed.Withsingle-beamside-scanwhereasingleIntheSpring2013editionofCADCMagazineweranthefirstpartofthistwo-partseriesonsonarindivinginspections.Readonfortheconclusiontothislookathowsonartechnologycanaffectdivingoperations.UsingSonarsforUnmaPartIIByMarkW.AthertonTypicalechosoundercoverageanddisplay.