Simrad EK15 - Page 282

SimradEK15
Figure1Pulsedurationprinciples
(CD010217-005)
C
1
2
A
B
D
1
2
A
B
Apulsedurationof1mscovers
1,5metersinthewater,and
thiscorrespondstoatarget
separationofapproximately
75cm.Thisisatypicalpulse
durationthatyoumaywelluse
downto250to300meters.If
youworkindeeperwatersuse
alongerpulseduration,ifitis
shallower,useashorterpulse
duration.Tryoutdifferent
values,andseekoutthepulse
durationthatprovidesyouwith
theclearestechopresentation
withminimumnoise,butwith
maximumshdetectionand
separation.
Theleftvesselusesalong
pulseduration(C).Asyoucansee,thiscausestheechoesfromthetwoshes(A)and
(B)tomerge.Therightvesselusesashorterpulseduration,andthetwosheswill
thenappearastwoseparateechoesontheechogram.Thus,shortpulseswillprovide
thebestresolutionandseparationofindividualshes,buttheechosounderismore
sensitivetonoise.
Thespeedofsoundinwaterisapproximately1500m/s.Thelengthofa1mSsound
pulsewillthusbeapproximately1,5meter.Withtheechosounderyoucanthenadjust
thesoundpulsefrom7,5cm(0.05mS)to24m(16mS)dependingoftheoperational
frequency.Thisisanimportantfactorfortheappearanceofsingleshes.
•Whentheverticaldistancebetweentoshes,orthedistancebetweenashandthe
bottom,ismorethanthedistancecoveredbyahalfpulseduration,theechoeswillbe
presentedastwoseparateechoes.Theshabovethebottomwillbeidentied.
•Ifthedistancebetweentwoindividualshes,orthedistancebetweenasinglesh
andthebottom,islessthanthedistancecoveredbyahalfpulseduration,theecho
willbepresentedasoneecho.Theechofromtheshclosetothebottomwillbe
mergedwiththebottomecho.
Alloperationalfrequencieshavedifferentpulsedurations.Thedifferencebetweenfor
examplea50kHzanda38kHztransducerishowevernotlarge:
•A50kHztransducercanbeusedwithpulsedurationsfrom0,12mSto2mS
•A38kHztransducercanbeusedwithpulsedurationsfrom0,26mSto4mS.
Basically,boththesefrequencieswillprovideyouwiththesamedetectionability.A
50kHztransducermayprovidebetterresolutioninshallowwaters,whilethe38kHz
transducermayprovidelongerrangeondeeperwaters.Onthe38kHztransducerthe
shortestpulsedurationis0,26mS.Thisresultsina40cmsoundpulseanda20cm
shseparation.
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