Echo Sounders and Speed Measurements

7/23/2019 Echo Sounders and Speed Measurements

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Echo Sounders and Speed

MeasurementsLarry V Gonzales Msc

Assistant Professor



• SOLAS 74 hapter V !e"ulation #$Para"raph %&'( %&'&# stipulates that allships of ')) "ross tonna"e and up*ardsand passen"er ships irrespecti+e of sizeshall ,e fitted *ith an echo soundin"de+ice( or other electronic means( tomeasure and display the a+aila,le depthof *ater&



Echo sounder • -t is an instrument used in locatin" the features o,tained from .athymetric na+i"ation&• A ,athymetric na+i"ation is defined as the art of esta,lishin" a "eo"raphic position on

the open sea ,y use of "eolo"ical features of the ocean floor& -n this system it consistof t*o ,asic components/ the transducer and the recorder& 0he former used on mostships is located at or near the 1eel( and contains ,oth a pro2ector for transmission of asound si"nal into the *ater and a hydrophone for the reception of the returnin"echoes& 0he latter is located *here re3uired( usually in the chart room&

• 0he si"nal transmitted ,y an echo sounder is electromechanical in nature& 0hetransmitter *hich is usually physically located in the recorder emits a pulsedcontinuous *a+e 56 electroma"netic si"nal that is translated into a sound pulse ,ythe transducer& 0he sound pulse train is radiated into the *ater in the shape of acone& 0he normal cone *idth is )8& 5hen a pulse stri1es any surface or ,oundarylayer of a re"ion *ithin *hich the sound propa"ation characteristics are different fromthose of the *ater into *hich the pulse *as transmitted( an echo is returned& 0hestren"th of the echo depends on the 3uality of the reflecti+e surface the pulse stri1es&0he returnin" echo is con+erted into an electroma"netic si"nal ,y the transducer andin the recorder the depth is o,tained ,y ta1in" into consideration the elapsed time,et*een the transmission of the pulse and the return of its echo and the transmissionspeed of sound in sea *ater 9o,,s( #$$:6&



• 0he echo sounder or fathometer consist oft*o ,asic components/ transducer and therecorder& 0he transducer used on mostships is located at or near the 1eel( andcontains ,oth a pro2ector for transmissionof a sound si"nal into the *ater and ahydrophone for reception of the returnin"echoes& 0he recorder is located *herere3uired( usually in the chart room&



omponents of an Echo Sounders

#& 0ransmitter %& 0ransducer

'& !ecei+er 4& -ndicator or recorder





















.asic Principles of Marine Echo Soundin"E3uipment

• 0he echo sounder transmits a sound pulse +ertically intothe *ater( and computes the depth ,y measurin" thetime inter+al from transmission of the sound si"nal untilthe return of its echo from the ,ottom& Most ne*er

models of echo sounders allo* the operator to displaythe depths thus measured in feet( fathoms( or meters(usin" any one of se+eral scales&

• 5hen usin" echo sounder depths( the na+i"ator mustal*ays remem,er that for most models the depthsrecorded are those from the position of the sonartransducer to the ,ottom& ;or actual *ater depths( thena+i"ator must add the transducer depth of the +essel toall readin"s& 9o,,s( #$$76



<oppler Speed lo"

• 0he au=illiary function of <SL as a depthfinder in shallo*er *ater& 0his feature of<SL on most models only operates toma=imum depth of a,out #())) feet&.eyond this limit( the na+i"ator must re+ertto the ships> echo sounder for depth

determinations 9o,,s(#$$76&



Limit of +elocity in the *ater • ;or the purposes of the depth computation echo sounders are cali,rated to

use a standard 4(:)) feet per second as the sound +elocity& .ecause theactual sound +elocity is ,oth +aria,le and some*hat faster than thisstandard( +aryin" *ith salinity( temperature( and pressure fi"& '#/'( somedifference almost al*ays e=ist ,et*een the actual depth and the indicateddepth& 0he actual depth is al*ays "reater than the indicated depth( thus

pro+idin" a small +aria,le safety factor&• 0here are other sources of error in the echo sounder depth as *ell& Sincethe transducer pulses propa"ate out*ard in the shape of a cone( the firstreflecti+e surface that the cone encounters *ill produce a reflectioninterpreted ,y the recorder as the depth immediately ,eneath the ship& -nactuality( ho*e+er the surface may lie off to one side( as sho*n in fi" '#/4&Such an echo returned from an ed"e of the sound cone is termed a sideecho&

• A third possi,le sources of error is the rollin" and to a lesser e=tent thepitchin" motion of a ship *hile under*ay& 0he position of the transducer onmost ship is fi=ed( causin" the transmission cone to ,e canted at an an"leto the +ertical as the ship proceeds& 0his effect is a factor that must ,e ta1eninto account re"ardless of the cone *idth& ;i" '#/?&



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%))

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% ) )

f t

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c t u a

l d e p

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< e p t h #

# ? f t

9o,,s( #$$4;i"& '#/4& Echo sounder depth error resultin" from a side echo



;i" '#/?& Echo sounder depth errors resultin" from ship motion 9o,,s( #$$7



auses of inaccuracies

• -n addition to the distortion caused ,y the ,eam *idth(there are t*o other ma2or sources of error ininterpretation of the echo sounder trace& 0he first is themultiple ,ottom return& 0he effect is most common in

relati+ely shallo* *ater *ith a hi"hly reflecti+e ,ottomsuch as sand or "ra+el( and is caused ,y re+er,erationof the sound pulses ,et*een the ,ottom and the *atersurface& A similar phenomenon sometimes occurs if the,ottom is co+ered to a depth of se+eral fathoms ,y a

material of poor reflecti+e 3ualities( such as soft mud&One return is formed ,y the *ea1 reflection from thesurface of the mud( *hile a second return results fromthe actual ,ottom reflection&



• A second possi,le source of error in echo sounder traceinterpretation is the deep scatterin" layer( a suspension of ,iolo"icalmatter such as plan1ton in a layer ,et*een the surface and the,ottom& 5hen a layer of this type is present( it usually rises to*ardthe surface at ni"ht and sin1s some*hat ,y day& Often the effect is+ery persistent in a "i+en area( and may lead to numerous reports ofshallo* *ater at locations *here the actual depth is +ery "reat & Afeature called the American Scout Seamount *ith depths of ') to $)fathoms *as reported east of @e*foundland so often ,et*een theyears #$4: to #$ 4 that it *as printed on se+eral contemporarycharts of the area& ontrolled sur+eys conducted in the #$ )s(ho*e+er( found no depths in the area less than %('?) fathoms& 0he

fictitious seamount reports are ,elie+ed to ha+e ,een caused solely,y the deep scatterin" layer 9o,,s( #$$76&



• A sound "enerated in the *ater *ill echo from the ,ottom and can,e recei+ed ,y a microphone& Since the appro=imate speed ofsound in *ater is 1no*n( the depth can ,e determined ,y measurin"the time inter+al ,et*een the "eneration of the sound and the returnof the echo( accordin" to the formula( depth speed = B timeinter+al ,et*een sound and echo&

• 0he speed of sound *a+es in *ater +aries *ith temperature( salinity(and pressureC ,ut an a+era"e +alue of 4(:)) feet per second issufficiently accurate for na+i"ational depth findin" particularly if thesame assumed speed *as used for the sur+eys *hich produced thedepth information on the charts used6& 0his ,ein" e3ui+alent to :))fathoms per second( an elapsed time of one second *ould indicatea depth of 4)) fathoms Maloney( #$7:6&

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Echo Sounders and Speed Measurements