Echo Sounder

ECHOSOUNDER – is a device that measures depth of water by measuring the time intervals between pulses of acoustic sound propagation that travel to the bottom of the sea. The acoustic sound must be different from that of water. A typical echo sounder ranges from 0 – 100 meters. The sonic pulses are emitted from a transducer of the instrument.

The velocity of sound in water is taken as 1,500 m/s and the depth is calculated as:

Depth of water = ½ ( 1,500 x one complete sound travel ).

There are two types of echosounder: 1. chart recording echo sounder 2. electronic echo sounder

TECHNIQUE

Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in the water, which is approximately 1.5 kilometres per second. For precise applications of echosounding, such as Hydrography, the speed of sound must also be measured typically by deploying a Sound Velocity Probe into the water. Echo sounding is effectively a special purpose application of sonar used to locate the bottom.

Basic Principle:

Short pulses of sound vibrations are transmitted from the bottom of the ship to the seabed. These sound waves are reflected back by the seabed and the time taken from transmission to reception of the reflected sound waves is measured.  Since the speed of sound in water is 1500 m/sec, the depth of the sea bed is calculated which will be half the distance travelled by the sound waves.

COMPONENTS

Basically an echo sounder has following components:

Transducer – to generate the sound vibrations and also receive the reflected sound vibration.

Pulse generator – to produce electrical oscillations for the transmitting transducer.

Amplifier – to amplify the weak electrical oscillations that has been generated by the receiving transducer on reception of the reflected sound vibration. 

Recorder  - for measuring and indicating depth. 

CONTROLS:

An echo sounder will normally have the following controls:

Range Switch – to select the range between which the depth is be checked e.g.  0- 50 m, 1 – 100 m, 100 – 200 m  etc.  Always check the lowest range first before shifting to a higher range.

Unit selector switch – to select the unit feet, fathoms or meter as required.

Gain switch – to be adjusted such that the clearest echo line is recorded on the paper.

Paper speed control – to select the speed of the paper – usually two speeds available.

Zero Adjustment or Draught setting control – the echo sounder will normally display the depth below the keel.  This switch can be used to feed the ship’s draught such that the echo sounder will display the total sea depth.  This switch is also used to adjust the start of the transmission of the sound pulse to be in line with the zero of the scale in use.

Fix or event marker  - this button is used to draw a line on the paper as a mark to indicate certain time e.g. passing a navigational mark, when a position is plotted on the chart etc.

Transducer changeover switch – in case vessel has more than one switch e.g. forward and aft transducer.

Dimmer – to illuminate the display as required.           

More on the principle of Echo Sounding

Echo sounder equipment makes use of sound (or sonic) waves, which differ from radio waves in several ways. Sound waves vibrate in the direction of travel of the wave front Sound wave velocity is comparatively low (in comparison with radio waves). Sound waves can travel through a material medium such as a gas, a liquid or a solid, but not a vacuum.

Sound waves can be produced over a very wide range of frequencies, and these waves exert definite pressure variations, which under certain circumstances can be measured. The audible frequency or frequencies that human beings can detect varies, but an approximate range of detection (also called as the normal range) can be considered as lying between about 20 Hz to 20,000 Hz. Those above the normal range of the human ear are termed ultrasonic or supersonic frequencies.

There is a loss of energy when any waves are transmitted through a medium and the losses increase as the frequencies increase. Sound waves also suffer losses, however in water such losses are of the order of 1000 times less as compared to the loss in air. Supersonic vibrations are therefore much more suitable for transmission in water than in air. The losses do increase due to high frequency, however they do not become serious unless very high frequencies are used.

Water is an excellent sound transmitting medium as the velocity of sound in the water is known accurately and it does not vary more than about 3%, if temperature and salinity change.

The speed of sound increases as water temperature, salinity and water pressure increases, and all of the above vary with depth. The speed of sound varies from about 1432 metres/sec. in fresh water, to about 1535 metres/sec. in salt water of high salinity. For depth sounding equipment design purpose a sound speed value of 1500 metres/sec. are assumed.

For normal applications on merchant vessels, the indication of the depth value based on the average speed of sound (1500 m/sec.) causes a minor error when changing from Salt Water (SW) to Fresh Water (FW). Assuming there is a small correction adjustment for SW, the true depth in FW is found to be about 3% less than the indicated depth. As can be seen, this deviation is very small and thus insignificant for practical considerations.

When sound wave passes through an interface of two mediums, besides suffering loss of energy, they refract as well as reflect (critical refraction) at the boundary where the two media meet.

Echo sounding equipments may be divided into two main classes:

.1         Those that transmit and receive sound vibration through the shell plating of a ship, referred to as 1nternal installation" class.

.2         Those that are in direct contact with the sea generally referred to as "pierced-hull installation" class.

In the internal installation class, because of the shell plating, energy is wasted during transmission and reception. For a shell plating thickness of 9.5 mm, about 15 per cent of the energy gets through the plate and only 2 per cent gets through when the shell plating is 31.8 cm thick.

The advantages of having an internal installation are:

.1         Equipment may be fitted without dry-docking the ship.

.2         Projectors or oscillators may be serviced or changed while the ship is afloat.

Sound wave energy is wasted if it is required to pass through a plate. The plate will prevent sound waves to pass through, if the thickness of the plate is close to a quarter wavelength of the sound wave; but if thickness of the plate is about a half wavelength then the steel plate becomes transparent to the sound wave.

For a pierced-hull installation, the shell plating of the ship is first pierced and the gap filled in by a thinner plate. If a steel plate is to be fitted, then the physical dimensions of the steel plate needs to be small and the plating will have to vary in thickness from ship to ship because of different frequencies used.

Thus for pierced hull installation the problem of using a frequency suitable for reasonable penetration no longer applies and higher frequencies can be used.

With a very low frequency, the size of the oscillator becomes inconveniently large; secondly, there is lack of selectivity from water and other noises within the audible range and finally, less directivity.

The higher frequencies gives more improved selectivity from noise and better directivity is possible, but there is less penetration.

Interpretation of SoundingFalse Bottom Echoes. Second Trace Echoes Echoes, which are received at a properly

adjusted sounder, until after the stylus has completed one or more passes across the paper and the next pulse have been, transmitted cause false readings. Example of one revolution represents 1600 metres, and an indicated depth of 50 metres could be sounding of 50 or 1650 or even 3250 metres. The correct depth can be ascertained if the transmission circuit can be switched off with the stylus still moving. After switching off, on the switch and then count the number of times the stylus crosses the paper before the echo re-appears.

Reflection echoes a) Double Echoes Echoes received after reflection from the seabed,

but which the hull or the sea surface back to the bottom and then reflects thence to the transducer. They produce a second weaker echo at approximately double the correct depth. It will fade out if sensitivity is reduced (may be received up to several hundreds metres).

b) Multiple Echoes

Echoes received after being reflected several times between the seabed and the surface or the ship's bottom before the energy is lost. It causes equally spaced echoes on the trace. Reduce sensitivity to fade out. Switch on to first phase and then phase deeper to locate first echo.

c) Variable Echoes These are varying reflecting surfaces on the seabed. In general

hard sand, coral, chalk and rock are good reflectors and thick mud is a poor reflector. Stepped formation of rock result in side Echoes from an object not immediately below the vessel but whose slant depth is less than the depth of water.

d) Electrical faults, or man made noises. Other False Echoes These do not normally obscure the bottom echo and may be

caused by .1         Shoals of fish .2         Layers of water of differing sounding velocities

(salinity etc.) .3         The deep scattering layer, which is a layer or set

of layers, in the ocean, believed to consist of plankton and which attenuate, scatter and reflect sound pulses. They lie between about 300-450 metres below the surface by day, and near the surface between sunset and sunrise (by day, it is more pronounced when the sky is clear, than when overcast).

.4         Kelp or weed. .5         Turbulence from the interaction of tidal streams

or eddies with solid particles in suspension.

SPEED ERROR

The speed of the recorder motor must be proportional to the velocity of sound in seawater and the velocity is known to vary. The recorder motor running at an incorrect speed causes the speed error. If the motor speed is too fast, it will record a greater depth and if it is too slow than a lesser depth.

Other errors include Pythagoras error, error due to maladjustment, ECHO SOUNDER CONTROLS

Mains Dimmer Range/Phasing/ scale Gain Other controls Speed control Zero adjustment/Draught setting Change over transducer Minimum depth alarm PULSE length Number of pulses per sec. Checks on echo sounders Twice yearly with hand lead, if reading is too high, then

motor is going too fast.

ERRORS

Velocity Error - Increase in temperature and salinity of water increases velocity of sound in water thus giving rise to an error in the depth displayed. 

Aeration – Presence of air bubbles below the transducer gives rise to false echoes. Air bubbles are normally caused when a vessel goes astern, turbulence when rudder is put hard over or due to pitching when vessel is in light condition.

Multiple echoes – This is caused in shallow waters with a rocky bottom due to some of the sound pulses reflecting up and down between the ship’s keel and the sea bottom before being recorded on the display. The first echo is the correct reading. 

False echoes – In deep waters, by the time the sound pulse returns from the bottom, the stylus may have already finished  more than one revolution and thus the echo which will be recorded will be a false one and the depth indicated will be much lower than the actual depth.

Pythagoras Error – If the vessel has one transducer for transmitting and one transducer for receiving, separated by some distance, the distance travelled by the pulse will be greater than the depth of the sea bed in shallow waters.

Comply with the maintenance instructions given in the manual.  Normally it is just a monthly cleaning of carbon / dirt deposits from the inside of the recorder.

Keep a stock of at least 1 spare stylus and 3 months stock of recording paper.

Compare the soundings obtained with the soundings given in the chart.

Maintain a log to enter the soundings obtained.

Some echo sounders have an alarm to alert the navigator when the sounding goes below the set sounding.