Acoustic pulses at the rate of 5 – 600 pulses/ min having BW of 12-25 degrees are transmitted vertically. Received echoes are converted to electrical signals by transducer. The electrical signals are sent to the stylus which produces corresponding marks on the recording paper.
Average speed of acoustic signals is 1505 ( approx 1500) m/s with conditions of 15 deg C & 3.4% salinity. Sound speed in sea water varies between 1445 and 1535 m/s depending on temperature, pressure and salinity. Standard value of 1500m/s gives soundings within 5% of actual depths even when extreme values of speed are encountered. If required depths can be corrected from sounding correction tables (NP 139)
Recording unit: It originates the sequence of events by transmitting an electrical signal to the pulse generator at the same instant as a stylus commences to travel across a sheet of sensitized paper.This is usually achieved by means of a small carefully regulated constant speed motor which actuates the stylus , a contactor being used to originate the signal to the pulse generator at the correct instant.
Pulse Generator: It contains a tuned circuit and a voltage multiplying device so that in response to the signal from he recording unit a heavy pulse of alternating current is discharged through the transmitter. A voltage doubling device can be made connecting the two capacitors in parallel when charging and in series when discharging. This arrangement, in conjunction with suitable inductances to give the required tuned frequency, produces the short high powered pulse necessary.
Transmitter: It may sometimes be a quartz oscillator (electrostrictive type working on the principle of peizo electric effect) but is usually of the magneto striction type.The latter depends on the fact that certain substances when subjected to magnetic fields tend to alter in size. Nickel is particularly susceptible to this effect and a bar of nickel placed in a solenoid tends to reduce in length when current is passed through the windings. This effect is independent of the direction of the field so that an a.c. would cause the bar to vibrate with a frequency twice that of current.
Receiver: After reflection from the sea bed the returning sound wave is received by a magneto striction type type receiving transducer. The nickel washers vibrate in tune with the incoming sound waves and, provided that it is slightly magnetized, an a.c. pulse will be induced in the windings. The current is very small and is transmitted to the amplifier.
Amplifier: It generally consists of more than one stage. Its function is to amplify the signal from the receiver to value which can be used in the recorder.
Recorder: It leads the amplified signal current to the moving stylus and on passing through the electrolytic paper to the earthed back plate a clearly visible mark is made.The position of the mark on the paper depends upon the distance moved by the stylus during the transit interval of the sound wave and this depends directly on the depth of water.
Range: Range can be increased by arranging that the stylus does not start to move until a fixed interval after the transmission ( phasing ) or by reducing the speed at which stylus sweeps across the paper.
Diffused reflection is better than specular reflection because of motion of ship. Higher frequency gives greater diffused reflection. And it is easier to concentrate the signals into narrow beams.
But higher frequencies have three disadvantages too:
More attenuation
Transducers have to be small which will not produce sufficient power
Thus the acceptable compromise is struck while deciding the frequency and optimizing the desired results.Moreover frequency should not be within audio frequency range otherwise audible noises ( engine, wake, water hitting ship’s hull etc) will also be picked up as echoes and appear as noise. ( audio range : 30 Hz – 20 KHz )
Pulse length: It lies 0.2 to 2 ms and varies with sets. For a given set shorter the range scale shorter the pulse length and vice versa. Stylus rotates at half the scale speed of acoustic signals i.e. at scale speed = 750 m/s.Echo cannot be received till transmission is over. And because stylus movement is synchronized with the leading edge of the pulse minimum measurable depth is given by ½ PL.thus for a PL of 2ms min. measurable depth will be 1/2 PL = ½*2/1000*1500 = 1.5 m
PRF: If PRF is 2 then in 1s 2 pulses are transmitted or each pulse is transmitted every ½ s.This means each pulse has ½ s at its disposal i.e. ¼ s to go and ¼ s to come back at the transducer. This means it will cover a distance of (¼+ ¼) * 1500 = 750 m which in turn corresponds to depth of 350 m. Thus each value of PRF corresponds to maximum critical depth that can be measured.
If actual depth is more than the critical depth, the second successive pulse will have left the transducer before the first pulse arrives as an echo. Correspondingly, stylus also would have completed its one rotation. Thus echo will be received on the second round of the stylus and is known as “second trace echo”. The correct depth of such echo is given by adding critical depth to the depth indicated on the recorder.
Paper Speed: Speed of paper over which stylus moves can be varied in addition to pre-programmed change of paper speed with range scale.
If bottom is uneven and a careful examination is required then the fast speed will give more separation of traces.
If echoes are faint e.g. at greater depth, slow paper speed will accentuate the echoes.
If running a line of soundings that require a change of scale , it is advisable to maintain a constant paper speed to assist subsequent analysis.
ERRORS: The position of transmission line or zero line should be carefully aligned with the transmission of pulse. This datum of the stylus can be adjusted as instructed in maker’s manual.
SOLAS CHAPTER V /Reg 12.
If the transmission line is set to the depth of the transducers, the recorded depth will be below the surface of the sea. If it is set to the zero of the scale, depths will be recorded below the transducers. Should the transducers be higher than the keel, by say 1m, then setting the transmission line to read -1 would give reorded depth below the keel.If transmission line is set to depth of transducers care is necessary when ship proceeds from salt to fresh water.
SPEED OF STYLUS: After adjusting T.L., the speed of the sounder should be checked and adjusted to correspond to a velocity of sound in water of 1500 m/s or such speed as the makers recommend. This is usually done by measuring with a stop watch the time for a given number of revolutions of the stylus as instructed in maker’s handbook.
FALSE ECHOES:
ROUND THE CLOCK ECHOES: False readings may be obtained from a correctly adjusted sounder when the returning echo is not received until after the stylus has completed one or more of its cycles and so repassed the transmission line and the next pulse has been transmitted.
If a sounder has its scale divided so that one complete cycle of the stylus corresponds to a depth of 300m, an indicated depth of 10m, could be sounding of 10. 310 or even 610m. Such false readings can sometimes be recognized if the trace appears weaker than normal for the depth recorded, or passes through the transmission line, or has a feathery appearance.
DOUBLE ECHOES: With many types of sounder, an echo may be received at about twice the actual depth. This mark on the trace is caused by the transmission pulse, after reflection from the seabed, being reflected from the surface and again from the seabed. It is always weaker than true echo, and will be the first to fade out if the sensitivity of the receiver is reduced.Its possible existence must always be borne in mind when a sounder is started in other than first phase setting.
MULTIPLE ECHOES: The transmission pulse in depths as great as several hundred meters may be reflected, not once but several times, between the seabed and the surface of the sea or the ship’s bottom before its energy is dissipated, causing a number of echoes to be recorded on the trace, These multiple echoes can be faded out by reducing the sensitivity of the set.. In the first phase setting, multiple echoes are too obvious to cause confusion, but should be guarded against in the second or subsequent phase setting. The sounder should always be switched on in the first phase and then phased deeper to find the first echo.
Echoes other than bottom echoes seldom have the reflective qualities to produce strong multiple echoes, and may sometimes be distinguished from the bottom echo by increasing the sensitivity of the set and comparing the multiple echoes.
In addition, they are caused by following:
Shoals of fish
Layers of water with deep scattering layer or set of layers believed to consist of plankton and fishes.It lies between 300 – 450 m below the surface during day and ascends to the surface by sunset and remaining there till sunrise. By day it is more pronounced when sky is clear than when overcast.
Submarine springs
Seaweed
Side echoes from an object not directly below vessel but whose slant depth is less than depth of seabed.
Turbulence caused by tidal streams or eddies with solid particles in suspension
Electrical faults or man made noises.
Sudden changes in temperatures and salinity can cause masking of bottom echoes
Weak Echoes: They are caused by:
Ship’s speed
Shape and condition of hull
Unsuitable siting of transducer
Sea and weather conditions
Aeration (caused by sternway, wake of vessel moving ahead, application of wheel, head trim)
SQUAT: consists of settlement and change of trim. It does not alter ship’s draft The effect depends on configuration of seabed, depth of water and vessel’s speed. The theoretical squat on a vessel drawing 10.0 m in a depth of 12.5 m will be 1.0 m at a speed of 15 knots.
In shallow water squat causes abnormal bow and stern waves to build up, which if observed should be taken as an indication that the ship is in shallow water with little clearance below the keel.. The amount of squat depends on many variables which differ, not only from ship to ship, but from place to place and can seldom be accurately predicted even in theory. So generous allowance should be made for this when ship is in shallow water.
UKC:
Reduced depth over submarine pipelines which may stand as high as 2m
Possible inaccuracies in offshore tidal predictions
Risk of negative tidal surges
Possible alterations in depths since last survey
Squat
Weather: rolling and pitching reduces UKC e.g. for a vessel of 50 m beam UKC is reduced by 0.5 m for a roll of 1 degree.
In certain areas where nature of bottom is unstable, depths may change by 1m or more, in a matter of months after a new survey.
Shifting banks or sand waves may themselves appreciably alter depths.
SETTING UP PROCEDURE: Prior to use, the back plate should be cleaned and the roll of recording paper fitted.
The equipment should be switched on; a range scale commencing at zero and covering the expected sounding should be selected e.g. 0 – 100 m.
The gain or sensitivity control should be adjusted until there is a clear trace from the sea bed. In the absence of a response, the maximum range scale having a zero should be selected and the gain set so that noise speckles are just marking the paper.
Adjust illumination if required.
Controls for paper speed (if provided) should be set at slow to permit trace integration of weak responses while making initial detection.
Alarms, if provided, may also be set.
Where a trace is already available, care should be taken in setting the gain control so as to avoid multiple traces which can result from signals bouncing between sea bed and the hull.
Choose the setting between UKC and depth of water.
Great care should be exercised when setting the gain control and using phased range scales.
ECHO SOUNDER GRAHICAL DISPLAY: should be sited on the bridge in position to facilitate easy access and viewing, and where the effect of any lighting for the equipment does not interfere with the keeping of an effective look out.
It should also facilitate easy servicing, repair and changing of paper rolls/stylus etc.
Because they are operated for long periods, adequate ventilation is essential. Thus sufficient space should be available around the display unit so as to avoid overheating and effect of fumes from some types of dry recorder papers.
SITING OF TRANSDUCERS:
To avoid putting them in the vicinity of underwater openings or projections from hull, such as plugs, anodes or other transducers, so that overall satisfactory performance is achieved.
The ideal position for transducer is one where there is ‘solid’ water from aeration beneath the transducer. And where the effect of surface, engine and propeller noise are at a minimum. Such positions are few on a ship and moreover position found to be satisfactory on one ship may not be suitable for another ship.
The principal source of aeration is the bow wave created by a moving ship. The wave rises some way up from the stem, curls over, and is the forced down beneath the ship, taking a quantity of air with it. The resultant bubble stream starts about a quarter length of the ship from the stem, and divides about three quarters of the length from the bow. The bubble stream varies in form and intensity according to speed, draught, shape of bow and hull, and the trim of the vessel and sea state. In particular in the case of bulbous bow, the only satisfactory site may be within the bulb, although the consequence of physical damage has to be recognized.
To avoid aeration, a position at the forepeak is desirable but it may be unsatisfactory in a ship with a light draught forward, especially in bad weather conditions. In addition the hull shape may make fitting difficult.
In a laden ship of normal design apposition within a quarter of the ship’s length from the stem will often be found to give satisfactory results. On small vessels damage may occur due to pounding and care should be taken when siting transducers. However it should also not be sited very close to propeller.
When separate transmitting and receiving transducers are fitted, they should be sufficiently separated to prevent interaction between them but it should also be kept as small as is possible to ensure accurate soundings in shallow waters.
Transducer should be installed in horizontal position.
If windowed transducer has to used, the window should be acoustically thin so that the range of the equipment is not adversely affected.
Siting near and particularly aft of ford propeller, bow thruster, water intake pipes, drain plugs and external speed measuring devices should be avoided.
To minimize effect of roll and pitch, centerline siting should be chosen.
Care should be taken to minimize interference between echo sounders and Doppler logs.
Information regarding the position of the transducer should be available onboard.
IMO standards: Should be capable of measuring any clearance under the transducer between 2 and 400 m.
Should provide min of 2 range scales one of which, the deep range, should cover the whole range of depth, and the other, the shallow range, one tenth thereof.
The scale of display should not be smaller than 2.5 mm per meter depth on the shallow range scale and 2.5 mm per meter depth on the deep range scale.
VOLUME REVERBERATION: is defined as scattering of sound waves from randomly positioned suspended particles, organism etc in water column. Intensity of sound reverberated from a unit volume of water is called volume reverberation. It is expressed in dB. In deep water it does no cause much problem, but in shallows reverberation can happen multiple times causing high background signal levels. On rocky bottoms it can cause multiple echoes.
Short PL, narrow beams and absence of side lobes can minimize the volume reverberation and also reduce reverberation from seabed. This, in turn improves signal to noise ratio.
Average speed of acoustic signals is 1505 ( approx 1500) m/s with conditions of 15 deg C & 3.4% salinity. Sound speed in sea water varies between 1445 and 1535 m/s depending on temperature, pressure and salinity. Standard value of 1500m/s gives soundings within 5% of actual depths even when extreme values of speed are encountered. If required depths can be corrected from sounding correction tables (NP 139)
Recording unit: It originates the sequence of events by transmitting an electrical signal to the pulse generator at the same instant as a stylus commences to travel across a sheet of sensitized paper.This is usually achieved by means of a small carefully regulated constant speed motor which actuates the stylus , a contactor being used to originate the signal to the pulse generator at the correct instant.
Pulse Generator: It contains a tuned circuit and a voltage multiplying device so that in response to the signal from he recording unit a heavy pulse of alternating current is discharged through the transmitter. A voltage doubling device can be made connecting the two capacitors in parallel when charging and in series when discharging. This arrangement, in conjunction with suitable inductances to give the required tuned frequency, produces the short high powered pulse necessary.
Transmitter: It may sometimes be a quartz oscillator (electrostrictive type working on the principle of peizo electric effect) but is usually of the magneto striction type.The latter depends on the fact that certain substances when subjected to magnetic fields tend to alter in size. Nickel is particularly susceptible to this effect and a bar of nickel placed in a solenoid tends to reduce in length when current is passed through the windings. This effect is independent of the direction of the field so that an a.c. would cause the bar to vibrate with a frequency twice that of current.
Receiver: After reflection from the sea bed the returning sound wave is received by a magneto striction type type receiving transducer. The nickel washers vibrate in tune with the incoming sound waves and, provided that it is slightly magnetized, an a.c. pulse will be induced in the windings. The current is very small and is transmitted to the amplifier.
Amplifier: It generally consists of more than one stage. Its function is to amplify the signal from the receiver to value which can be used in the recorder.
Recorder: It leads the amplified signal current to the moving stylus and on passing through the electrolytic paper to the earthed back plate a clearly visible mark is made.The position of the mark on the paper depends upon the distance moved by the stylus during the transit interval of the sound wave and this depends directly on the depth of water.
Range: Range can be increased by arranging that the stylus does not start to move until a fixed interval after the transmission ( phasing ) or by reducing the speed at which stylus sweeps across the paper.
Diffused reflection is better than specular reflection because of motion of ship. Higher frequency gives greater diffused reflection. And it is easier to concentrate the signals into narrow beams.
But higher frequencies have three disadvantages too:
More attenuation
Transducers have to be small which will not produce sufficient power
Thus the acceptable compromise is struck while deciding the frequency and optimizing the desired results.Moreover frequency should not be within audio frequency range otherwise audible noises ( engine, wake, water hitting ship’s hull etc) will also be picked up as echoes and appear as noise. ( audio range : 30 Hz – 20 KHz )
Pulse length: It lies 0.2 to 2 ms and varies with sets. For a given set shorter the range scale shorter the pulse length and vice versa. Stylus rotates at half the scale speed of acoustic signals i.e. at scale speed = 750 m/s.Echo cannot be received till transmission is over. And because stylus movement is synchronized with the leading edge of the pulse minimum measurable depth is given by ½ PL.thus for a PL of 2ms min. measurable depth will be 1/2 PL = ½*2/1000*1500 = 1.5 m
PRF: If PRF is 2 then in 1s 2 pulses are transmitted or each pulse is transmitted every ½ s.This means each pulse has ½ s at its disposal i.e. ¼ s to go and ¼ s to come back at the transducer. This means it will cover a distance of (¼+ ¼) * 1500 = 750 m which in turn corresponds to depth of 350 m. Thus each value of PRF corresponds to maximum critical depth that can be measured.
If actual depth is more than the critical depth, the second successive pulse will have left the transducer before the first pulse arrives as an echo. Correspondingly, stylus also would have completed its one rotation. Thus echo will be received on the second round of the stylus and is known as “second trace echo”. The correct depth of such echo is given by adding critical depth to the depth indicated on the recorder.
Paper Speed: Speed of paper over which stylus moves can be varied in addition to pre-programmed change of paper speed with range scale.
If bottom is uneven and a careful examination is required then the fast speed will give more separation of traces.
If echoes are faint e.g. at greater depth, slow paper speed will accentuate the echoes.
If running a line of soundings that require a change of scale , it is advisable to maintain a constant paper speed to assist subsequent analysis.
ERRORS: The position of transmission line or zero line should be carefully aligned with the transmission of pulse. This datum of the stylus can be adjusted as instructed in maker’s manual.
SOLAS CHAPTER V /Reg 12.
If the transmission line is set to the depth of the transducers, the recorded depth will be below the surface of the sea. If it is set to the zero of the scale, depths will be recorded below the transducers. Should the transducers be higher than the keel, by say 1m, then setting the transmission line to read -1 would give reorded depth below the keel.If transmission line is set to depth of transducers care is necessary when ship proceeds from salt to fresh water.
SPEED OF STYLUS: After adjusting T.L., the speed of the sounder should be checked and adjusted to correspond to a velocity of sound in water of 1500 m/s or such speed as the makers recommend. This is usually done by measuring with a stop watch the time for a given number of revolutions of the stylus as instructed in maker’s handbook.
FALSE ECHOES:
ROUND THE CLOCK ECHOES: False readings may be obtained from a correctly adjusted sounder when the returning echo is not received until after the stylus has completed one or more of its cycles and so repassed the transmission line and the next pulse has been transmitted.
If a sounder has its scale divided so that one complete cycle of the stylus corresponds to a depth of 300m, an indicated depth of 10m, could be sounding of 10. 310 or even 610m. Such false readings can sometimes be recognized if the trace appears weaker than normal for the depth recorded, or passes through the transmission line, or has a feathery appearance.
DOUBLE ECHOES: With many types of sounder, an echo may be received at about twice the actual depth. This mark on the trace is caused by the transmission pulse, after reflection from the seabed, being reflected from the surface and again from the seabed. It is always weaker than true echo, and will be the first to fade out if the sensitivity of the receiver is reduced.Its possible existence must always be borne in mind when a sounder is started in other than first phase setting.
MULTIPLE ECHOES: The transmission pulse in depths as great as several hundred meters may be reflected, not once but several times, between the seabed and the surface of the sea or the ship’s bottom before its energy is dissipated, causing a number of echoes to be recorded on the trace, These multiple echoes can be faded out by reducing the sensitivity of the set.. In the first phase setting, multiple echoes are too obvious to cause confusion, but should be guarded against in the second or subsequent phase setting. The sounder should always be switched on in the first phase and then phased deeper to find the first echo.
Echoes other than bottom echoes seldom have the reflective qualities to produce strong multiple echoes, and may sometimes be distinguished from the bottom echo by increasing the sensitivity of the set and comparing the multiple echoes.
In addition, they are caused by following:
Shoals of fish
Layers of water with deep scattering layer or set of layers believed to consist of plankton and fishes.It lies between 300 – 450 m below the surface during day and ascends to the surface by sunset and remaining there till sunrise. By day it is more pronounced when sky is clear than when overcast.
Submarine springs
Seaweed
Side echoes from an object not directly below vessel but whose slant depth is less than depth of seabed.
Turbulence caused by tidal streams or eddies with solid particles in suspension
Electrical faults or man made noises.
Sudden changes in temperatures and salinity can cause masking of bottom echoes
Weak Echoes: They are caused by:
Ship’s speed
Shape and condition of hull
Unsuitable siting of transducer
Sea and weather conditions
Aeration (caused by sternway, wake of vessel moving ahead, application of wheel, head trim)
SQUAT: consists of settlement and change of trim. It does not alter ship’s draft The effect depends on configuration of seabed, depth of water and vessel’s speed. The theoretical squat on a vessel drawing 10.0 m in a depth of 12.5 m will be 1.0 m at a speed of 15 knots.
In shallow water squat causes abnormal bow and stern waves to build up, which if observed should be taken as an indication that the ship is in shallow water with little clearance below the keel.. The amount of squat depends on many variables which differ, not only from ship to ship, but from place to place and can seldom be accurately predicted even in theory. So generous allowance should be made for this when ship is in shallow water.
UKC:
Reduced depth over submarine pipelines which may stand as high as 2m
Possible inaccuracies in offshore tidal predictions
Risk of negative tidal surges
Possible alterations in depths since last survey
Squat
Weather: rolling and pitching reduces UKC e.g. for a vessel of 50 m beam UKC is reduced by 0.5 m for a roll of 1 degree.
In certain areas where nature of bottom is unstable, depths may change by 1m or more, in a matter of months after a new survey.
Shifting banks or sand waves may themselves appreciably alter depths.
SETTING UP PROCEDURE: Prior to use, the back plate should be cleaned and the roll of recording paper fitted.
The equipment should be switched on; a range scale commencing at zero and covering the expected sounding should be selected e.g. 0 – 100 m.
The gain or sensitivity control should be adjusted until there is a clear trace from the sea bed. In the absence of a response, the maximum range scale having a zero should be selected and the gain set so that noise speckles are just marking the paper.
Adjust illumination if required.
Controls for paper speed (if provided) should be set at slow to permit trace integration of weak responses while making initial detection.
Alarms, if provided, may also be set.
Where a trace is already available, care should be taken in setting the gain control so as to avoid multiple traces which can result from signals bouncing between sea bed and the hull.
Choose the setting between UKC and depth of water.
Great care should be exercised when setting the gain control and using phased range scales.
ECHO SOUNDER GRAHICAL DISPLAY: should be sited on the bridge in position to facilitate easy access and viewing, and where the effect of any lighting for the equipment does not interfere with the keeping of an effective look out.
It should also facilitate easy servicing, repair and changing of paper rolls/stylus etc.
Because they are operated for long periods, adequate ventilation is essential. Thus sufficient space should be available around the display unit so as to avoid overheating and effect of fumes from some types of dry recorder papers.
SITING OF TRANSDUCERS:
To avoid putting them in the vicinity of underwater openings or projections from hull, such as plugs, anodes or other transducers, so that overall satisfactory performance is achieved.
The ideal position for transducer is one where there is ‘solid’ water from aeration beneath the transducer. And where the effect of surface, engine and propeller noise are at a minimum. Such positions are few on a ship and moreover position found to be satisfactory on one ship may not be suitable for another ship.
The principal source of aeration is the bow wave created by a moving ship. The wave rises some way up from the stem, curls over, and is the forced down beneath the ship, taking a quantity of air with it. The resultant bubble stream starts about a quarter length of the ship from the stem, and divides about three quarters of the length from the bow. The bubble stream varies in form and intensity according to speed, draught, shape of bow and hull, and the trim of the vessel and sea state. In particular in the case of bulbous bow, the only satisfactory site may be within the bulb, although the consequence of physical damage has to be recognized.
To avoid aeration, a position at the forepeak is desirable but it may be unsatisfactory in a ship with a light draught forward, especially in bad weather conditions. In addition the hull shape may make fitting difficult.
In a laden ship of normal design apposition within a quarter of the ship’s length from the stem will often be found to give satisfactory results. On small vessels damage may occur due to pounding and care should be taken when siting transducers. However it should also not be sited very close to propeller.
When separate transmitting and receiving transducers are fitted, they should be sufficiently separated to prevent interaction between them but it should also be kept as small as is possible to ensure accurate soundings in shallow waters.
Transducer should be installed in horizontal position.
If windowed transducer has to used, the window should be acoustically thin so that the range of the equipment is not adversely affected.
Siting near and particularly aft of ford propeller, bow thruster, water intake pipes, drain plugs and external speed measuring devices should be avoided.
To minimize effect of roll and pitch, centerline siting should be chosen.
Care should be taken to minimize interference between echo sounders and Doppler logs.
Information regarding the position of the transducer should be available onboard.
IMO standards: Should be capable of measuring any clearance under the transducer between 2 and 400 m.
Should provide min of 2 range scales one of which, the deep range, should cover the whole range of depth, and the other, the shallow range, one tenth thereof.
The scale of display should not be smaller than 2.5 mm per meter depth on the shallow range scale and 2.5 mm per meter depth on the deep range scale.
VOLUME REVERBERATION: is defined as scattering of sound waves from randomly positioned suspended particles, organism etc in water column. Intensity of sound reverberated from a unit volume of water is called volume reverberation. It is expressed in dB. In deep water it does no cause much problem, but in shallows reverberation can happen multiple times causing high background signal levels. On rocky bottoms it can cause multiple echoes.
Short PL, narrow beams and absence of side lobes can minimize the volume reverberation and also reduce reverberation from seabed. This, in turn improves signal to noise ratio.
