Simrad EK15 - Page 275

Echosoundertheory
Onemethodforminimizingthe“bottomslopes”phenomenonistouseatransducerwith
narrowbeam,ortoincreasethepulseduration.
AnotherwaytominimizethisphenomenonistoopentheBottomDetectordialogbox,and
thenchangethesettingforBottomBackstep.Thisfunctioncanalsobeputtootheruse.
→BottomDetectordialogboxonpage218
TheBottomBackstep
principle
-50dB
-70dB
-30dB
(CD010217-003)
AThepeakofthebottompulse
BDefaultbottombacksteplevel
CApproximatebottombacksteplevelforatsh
detection
DApproximatebottombacksteplevelforseagrass
detection
Thebottompulsebasicallyidentiesthebottomdepth
justpriortothepeakofthepulse(A).However,this
maynotbethetruebottom.Forexample,ifthebottom
pulseisgeneratedbyarockbottomunderathicklayer
ofmud,theactualdepthisslightlyshallower.Forthis
reason,theEK15isbydefaultsetuptogiveyouadepthreadingafewmilliseconds
beforethepeakofthepulse.Thisisdonebysettingthebottombackstepleveltoa
defaultvalueof-50dB(B).
Byfurtherdecreasingthelevel(makeitmorenegative)thebottomdetectorwillbecome
more“sensitive”,andthebottomwillbedetectedearlier.
Ontheechogramthewhitelinewillthen“climb”uptheslope.Makesurethatyoudo
notincreasethesensitivitytoomuch.Thiswillhaveaneffectontheshdetection
onaatbottom,andthebiomassvalueswillbewrong.Ourexperienceshowthatan
approximately-75dBbottombackstepvaluecanbeusedsafely.
Byincreasingthebottombackstepvalue(makeitmorepositive)thebottomwillbe
detectedlater,anditwillappeartobedeeper.
Tip
Ruleofthumb:
•Reducethebottombacksteplevel(makeitmorenegative)todetectthebottomearlier,
andthusincreasebottomdetection“sensitivity”.
•Increasethebottombacksteplevel(makeitmorepositive)todetectthebottomlater,
andthusincrease“penetration”.
351718/C
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