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.
The Practice of Echo Sounding
The echo
sounding principles is used as follows, to measure depth. A short sound pulse
is transmitted from the ship's bottom towards the seabed where it is reflected
back towards the hull as an echo.
The time
interval between transmission of the pulse and the receipt of the echo is
measured, and the depth is found from the expression:
Depth = velocity
X time divided by 2
The frequency of
the sound vibrations created in the water during pulsing is usually at the
upper end of, or above the audible range. A depth sounder instrument can be
obtained in three different forms based on the transmitted frequency:
.1 using 14khz frequency (Low end sounders)
.2 using 14 kHz to 30 kHz frequency (Normal
sounders)
.3 using 30 kHz frequency (High end
sounders)
The choosing of
frequency depends on the requirements of the user. The chosen frequency is
basically a compromise to avoid interference from audible ship motion noise,
and water losses through the seawater.
Generally low
frequencies are mostly affected by ship noise and high frequencies by water
losses, which are caused by absorption.
The sound pulses
are created by transducers, which convert electrical energy on transmission,
and reverse the process when the echo is received. The sound energy is always
transmitted as a beam in a particular direction (directional beam) and it would
be very wasteful to permit spherical radiation.
Transducers
are of two types:
Piezo-electric
transducer
Magneto-striction
transducer.
Piezo-electric transducer
This type makes use of the special
properties of crystals (e.g. crystals of barium-titanate and lead zirconate).
If an alternating voltage is applied to the opposite faces of a flat piece of
one of the above materials, the crystal will expand and contract, and hence
vibrate creating sound waves for as long as the vibrations continue. The
process is reversible, i.e. when varying pressure from a returning echo, is
applied to the opposite faces, an alternating voltage is generated across the
faces and the same can be further amplified and used to activate an indicator.
Magneto-striction transducer
In this type,
the use is made of the magneto-striction effect which is a phenomenon whereby
magnetization of ferromagnetic materials produce a small change in their
dimensions, and conversely the application of mechanical stresses such as weak
pressure vibrations, as from an echo to them, produce magnetic changes in them;
e.g. a nickel bar when placed in the direction of or strength of the magnetic
field. If the nickel bar is placed in a coil with an alternating current
flowing through it (a solenoid), the varying current and magnetic field will
cause the ends of the bar to vibrate and hence create a sound wave. This is
what happens when the transducer is transmitting.
Type with specs
50 kHz - 100 W: Maximum depth measurable - 700 metres Type with specs 20 kHz -
100 W: Maximum depth measurable - 400 metres
The
magneto-striction type would be fitted inside a cast elliptical housing or a
circular housing in such a way that the bottoms of the pistons are in contact
with the sea (i.e. pierced hull type - see details of this type later).
Another type,
which is fitted internally, consists of a ring of thick nickel discs enclosed
in a winding to which the AC is applied. This resulting sound pulse is directed
downwards through the steel bottom shell by a reflector. The advantages of
nickel ring types are that it is cheaper to construct and damping is greater.
In both designs
the sound pulses are directed down wards in a cone shaped beam to avoid loss of
sounding when the vessel is rolling. The process is reversible, as, when the
echo returns, it applies a varying pressure to the working faces of the
transducer, which causes the magnetic condition of the nickel to fluctuate at
the same frequency. This varying magnetic field strength induces a voltage in
the winding round each piston leg and this voltage is amplified before being
applied to the indicator.
As oscillators
must be in water dry forepeak tanks. Tanks may be flooded sufficiently to-keep
them submerged. Forepeak tanks are usually arranged so that when they are
pumped out, enough water is retained to keep the oscillators from becoming dry.
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 "internal 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.
Echo Sounding - Full cycle of operation
The full cycle
of operations for one sounding is as follows:
The recording
stylus starts each cycle as it moves pass the zero. It triggers an electronic
generator, which produces a known number of electrical oscillations, which are
applied to the transmitting transducer (Tr/Tx). The Tr/Tx creates the sound
pulse, which is injected into the sea, travels to the bottom, is reflected and
returns as an echo to the receiving transducer, where it is converted back into
an electrical pulse. This is amplified and applied to the stylus, which has
moved across the recording paper, to indicate the depth against a suitable
scale. The stylus moves across the paper at a constant speed which is decided
by the designer after he has decided the
- maximum depth to be displayed,
- width of the paper and the SW velocity to be used.
The pulse length
to be used for transmission is governed by a number of factors. The minimum
theoretical depth that can be measured is equal to half a pulse length. Since
sound travels at approx. 1500 metres per second, a pulse length of 1 millisec
(ms) will mean that the theoretical minimum depth, which could be measured,
would be 1.5 metres. In practice it would be about this value.
The difference
between the theoretical and the practical values is because the transducer
being a resonant device does not stop oscillating immediately the electrical
pulse ceases. It shows a tendency to "ring" when energised and this
is usual for the time taken for 10 to 12 cycles. If depths less than 1.5 m are
to be measured then a shorter pulse length is required. One sounder has a pulse
length of 0.3 m, which gives a theoretical minimum sounding of 0.225 m and a
practical minimum of 0.45 m.
If a very deep measurement is to be made
then more energy is required. This could possible be achieved by increasing the
amplitude of the pulse, but this is usually limited by the output of the active
element in the transmitter and therefore it is injected directly into the
water. Sounders, which have to cover very shallow and very deep sounding on
different ranges, will usually be designed so that the pulse length can be
changed as the range is changed.
PULSE LENGTH
Shallow 0.3 milli sec Up to 200 or 400 metres
l
to 5 milli sec 2000 metres or more
The commonest form of echo sounder has a
display, which records the depth on electro-sensitive paper. It may take the
form of rotating arm moving anti-clockwise across the paper, which is marked by
the stylus at the end of the arm when a DC pulse is applied on receipt of the
echo. Another type has a moving belt to which the stylus is attached and which
is made to move across the paper from top to bottom at a constant rate, which
is decided by the depth scale displayed. The paper is marked in the same way,
and the indicated depth is measured from the top of the paper by a suitable
vertical scale at the side.
Another type of
display more suitable for shallow depths consists of a disc or arm carrying a
neon lamp at its extreme edge, which is spun round at constant speed. A scale
is fitted round the edge of the area covered by the spinning neon which is made
to flash at zero on transmission and again on receipt of the echo at the point
in its revolution appropriate for the depth measured. The overall recording
accuracy claimed for one echo sounder is close to +/- 2% of the actual depth.
Recording Paper may be of two kinds,
moist and dry. The moist paper is impregnated with a solution of potassium
iodide and starch. When a direct current is passed through it from the stylus
to the metal plate at the back, it releases iodine and causes a brown stain to
appear. The stylus is tipped with iridium. This action only takes place when
the paper is damp - it becomes an insulator when dry.
This type of
paper should be kept in its airtight tins before use. If an echo trace on damp
paper is to be kept for reporting or other purposes, a line should be drawn
down each side of the paper while it is still damp to indicate the limits of
the scale. The bottom trace and transmission line should be drawn in pencil,
the paper dried, preferably in a dim light, and then the paper should be rolled
up to prevent fading. An indelible pencil should preferably be used or a
ballpoint pen for all writing.
The Dry paper is
a carbon impregnated paper base, metallised on one side and covered on the
other with a very thin film of fight coloured semi-conducting chemical. The
metallised side makes contact with the metal plate at the back, and the stylus
moves over the chemically treated side. When the echo returns, a pulse of
current is applied to the stylus which destroys the chemical film and exposes
carbon beneath to show a black record in contrast to the gray paper, carbon
dust and possibly fumes will be released and these may be a health hazard. The
recorder must be sited so that adequate ventilation its possible. The dust,
which is deposited on parts of the recorder must be removed at require
intervals using a soft rag or brush.
TRANSDUCER SITING
Satisfactory
operation of an echo sounder depends on the transmission and reception of the
largest possible signal for a given amount of power. The siting of the
transducer is important in this respect to reduce attenuation on transmission
and reception as far as possible. The ideal position is one in which there is
"solid" water free from aeration beneath the transducer, and where
the effects of surface, engine and propeller noise are at a minimum. There are
few positions which are suitable in every respect and a position found to be
satisfactory in one design of ship will not necessary give equally good results
in another.
The principle source of aeration is the
bow waves created by the ship. This wave rises some way up the stem, curls
over, and then is forced down beneath the ship, taking a quantity of air with
it.
The resultant
bubble stream normally starts about a quarter length of the ship from the
stern, and divides about three quarters of the length from the bow. The bubble
stream varies in form and intensity according to the speed, draught, shape of
bow and hull, the trim of the ship as well as the sea state. In ships with a
bulbous bow the wave appears to dip water just abaft the stem, so that the flow
of bubbles is over almost the whole length of the vessel and the only
satisfactory forward site may be within the bulb. In oil tankers the after
position is invariably chosen, usually under the fore part of the engine room.
Classification Society Lloyd Register does not permit oscillators to be fitted
underneath cargo space on vessels classed for carrying petroleum in bulk.
A position in
the forepeak may appear to be the best, but in bad weather and light ship it
would be unlikely to give good results and may also be difficult to fit there.
In laden ship of normal design a position about a quarter of the length from
the stem will often be found to give satisfactory results. Ships often making
long passages in ballast e.g. tankers, often find an after position about three
quarters of the length from the stem gives better results. If two are fitted,
one is fitted at one quarter and one at three quarter length abaft the stem.
Care must be
taken to make sure a receiving transducer is a sufficient distance from the
propeller, and tests should be carried out to ensure this. They need to be
sufficiently separated to prevent interaction between them, but the separation
should be as small as possible to ensure accurate sounding in shallow water.
Positions either side of the keel is often satisfactory.
Other factors,
which should be borne in mind, are: fit in a horizontal position, sometimes
slightly projecting but faired off to avoid aeration. Avoid sites near bow
thruster units, water intake pipes and underwater log units.
Internal access
to the transducer should be possible for maintenance. Any junction box should
be in a dry space and if possible the transducer should be in a dry place.
NOISE
All transmission
systems are subjected to interfering signals of some kind.
CROSS NOISE
It is caused by
vibration of the energy, which is transmitted out by a ship and goes directly
to the echo sounder receiver. The recorder shows a broad line on zero reading
and this can mask echoes totally.
THERMAL NOISE
It is generated in electronic devices by
random movement of electrons in components and this is amplified in the
receiver in any radio system. In sonar system, using sonic waves below 50 kHz,
noise level can usually be ignored, as it is very small compared with the sea
noise.
SEA NOISE
They are of two
main kinds, the first are interfering wave action, and may be thought of as
background noise. Sources are fish, other ships, and noise from one's own ship
particularly in bad weather and close to land. For most purpose, the amplitude
of disturbances at any instances is unpredictable and taken, as a whole may be
considered random. For this reason, the designer must make sure that the signal
is always recognisable above the noise level.
The second is
noise produced by the interaction of the sea and the sonar system. This is
generally called "reverberation noise" and when transmitted into
water, all the small reflectors in the water such as bubbles, marine life, and
mud and sand particles immediately affect it.
These multiple reflector produces a
return signal (echo), which is theoretically continuous since they exist at all
depths. However the intensity of the transmitted pulse is reduced as it moves
away from the transducer and the intensity of the return signal also reduces in
accordance with the same law. The result is that after the end of transmission,
the reverberation signal decreases with time according to an inverse square
law. Its effect can be considerably reduced by the use of time variable gain or
"initial suppression". This circuit is set to reduce the gain of the
receiver to a very low level immediately following transmission, but then
allows the gain of reverberation noise after the same has fallen below that of
background noise.
Interpretation of Sounding
False 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).
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.
NAVIGATION AT OPERATIONAL LEVEL
Objective of the module:
Learn operation of the echo
sounder
Carry out operational checks and
adjust the equipment to proper performance
Apply information obtained to
ascertain the ship’s position
Carry out maintenance of the echo
sounder as laid out in the equipment manual.
Learn procedure for changing stylus and recording
paper.
PRINCIPLES
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:
Ø Transducer – to generate the sound
vibrations and also receive the reflected sound vibration.
Ø Pulse generator – to produce
electrical oscillations for the transmitting transducer.
Ø Amplifier – to amplify the weak electrical
oscillations that has been generated by the receiving transducer on reception
of the reflected sound vibration.
Ø 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.
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.
MISCELLANEOUS
§ 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.
Figure in the
module
Figure 5901: 2
types of Echo Sounder recorders
Figure 5902:
Block diagram and principle of Echo Sounder
 |
Reference:
Radar and Electronic
Navigation by G. J. Sonnenberg
Marine Electronic
Navigation by S. F. Appleyard
Bridge Equipment and
Watchkeeping notes by Edrich Fernandes
Various Equipment Manuals
