Simrad EK15 - Wave Propagation

SimradEK15
Wavepropagation
Thevelocityofsoundwavepropagationintheseavariesslightlywithtemperature,
salinityandpressure.Thevelocityvariesbetween1440and1520m/sinshallowsea
water,whileavelocityaround1480m/scanbeexpectedat1000mdepth.Inshallow
freshwaterthevelocityisapproximately1430m/s.
AgoodaveragevaluetobeusedintheEnvironmentdialogboxis1470m/s.
Wavepropagationfromaat
bottom
(CD010217C)
(CD010217G)
TheEK15transmitshighenergysoundwavepulses
intothesea.Aatbottomreectsthetransmitted
waveasifitwasamirror.Thepropagatingenergy
isspreadoveralargerandlargerareaasittravels
downtothebottomandupagain.Theenergyis
spreadoverafourtimeslargerareaeverytimethe
traveldistancedoubles.
Alargeschoolofshreectssoundwavessimilarly.
Thistypeofspreadingisreferredtoassquare-law
or20logTVG(TimeV aryingGain)spreading.
Thesituationisslightlydifferentwhenobserving
theechoesfromindividualsh.Thetransmitted
waveundergoessquare-lawspreadingwhentravellingfromthesurfaceanddowntothe
sh.Theswimbladderoftheshscattersasmallfractionofthearrivingenergyinall
directions.Travellingfromtheshandbacktowardsthesurfacethescatteredwave
undergoesanothersquare-lawspreading.Thecombinedeffectisreferredtoasquad-law
or40logTVGspreading.
Wavepropagationfromash
(CD010217H)
Intheechosounder’sEchogramdialog20logTVG
spreadingisreferredtoasSchoolGainandBottom
Gain,while40logTVGspreadingisreferredto
asFishGain.
Propagationlossesduetoabsorptionaremuch
higherinseawaterthaninfreshwater.Absorption
alsoincreaseswithfrequency.At38kHzthe
absorptionis0.5dB/kminfreshwaterand10dB/km
inseawater.At200kHztheabsorptionis10dB/km
infreshwaterand50dB/kminsaltwater.Theecho
soundermustknowwhichwatertypeispresentin
ordertocompensatefortheselossescorrectly.
ThedB(decibel)unithaslongtraditionsinunderwateracousticsandothereldsin
physics.Itisalogarithmicmeasurefortheratiobetweentwoquantities.
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