Lighthouse a65 - The Sonar Image

19.7Thesonarimage
Interpretingthebottomusingsonar
Itisimportanttounderstandhowtocorrectlyinterpret
thebottomstructurerepresentedon-screen.
Thebottomusuallyproducesastrongecho.
Thefollowingimagesshowhowdifferentbottom
conditionsarerepresentedon-screen:
D6855-3
1
2
3
ItemDescription
1
Ahardbottom(sand)producesathinline.
2
Asoftbottom(mudorseaweedcover)produces
awideline.
3Arockyorunevenbottomorawreckproduces
anirregularimagewithpeaksandtroughs.
Thedarklayersindicateagoodecho;thelighter
areasindicateweakerechoes.Thiscouldmeanthat
theupperlayerissoftandthereforeallowingsound
wavestopasstothemoresolidlayerbelow.
Itisalsopossiblethatthesoundwavesaremaking
twocompletetrips–hittingthebottom,bouncingoff
thevessel,thenreectingoffthebottomagain.This
canhappenifthewaterisshalloworthebottomis
hard.
Factorsinuencingthesonardisplay
Thequalityandaccuracyofthedisplaycanbe
inuencedbyanumberoffactorsincludingvessel
speed,depth,objectsize,backgroundnoiseand
transducerfrequency.
Vesselspeed
Theshapeofthetargetchangesalongwithyour
speed.Slowerspeedsreturnatter,morehorizontal
marks.Higherspeedscausethetargettothicken
andarchslightly,untilatfastspeedsthemark
resemblesadoubleverticalline.
Targetdepth
Thecloserthetargettothesurface,thelargerthe
markonscreen.
Waterdepth
Aswaterdepthincreasessignalstrengthdecreases,
resultinginalighteronscreenimageofthebottom.
Sizeofthetarget
Thelargerthetarget,thelargerthereturnonthe
shnderdisplay.Thesizeofashtargetisalso
dependentuponthesizeofthesh’sswimbladder
ratherthanitsoverallsize.Theswimbladdervaries
insizebetweendifferentbreedsofsh.
Clutter/Backgroundnoise
Theshnderpicturemaybeimpairedbyechoes
receivedfromoatingorsubmergeddebris,air
bubblesoreventhevessel'smovement.Thisis
knownas‘Noise’or‘Clutter’andiscontrolledbythe
SensitivitySettings.Thesystemcanautomatically
controlsomesettingsaccordingtodepthandwater
conditions.Youcanalsoadjustthesettingsmanually
ifrequired.
Transducerfrequency
Thesametargetwillappeardifferentlywhenusing
differenttransducerfrequencies.Thelowerthe
frequencythebroaderthemark.
Recoveringlostbottom
Iftheseabedoor(bottom)islostthenfollowthe
stepsbelowtorecoverthebottomdepth.
Fromtheshnderapplication:
1.Ensureyourvesselisinclearundisturbedwater.
2.IfrangeissettoManual,adjusttherangetothe
known,charteddepthofyourlocation.or
3.IfrangeissettoAutothenswitchtherange
tomanualandadjusttherangetotheknown,
charteddepthofyourlocation.
4.Oncebottomhasbeenregainedyoucanswitch
rangemodebacktoAuto.
Fishnderapplication
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