Guide to Marine Echo Sounders & Subsurface Depth Measurement -

Introduction

The echo sounder is a bridge instrument that measures the depth of water under the ship’s keel.

It is fitted on ships as required by SOLAS Chapter V. It is most important to monitor it during coastal navigation and whenever approaching shallow patches, to make sure the ship has enough water underneath to pass safely.

IMO Requirements for the Echo Sounder

Echo sounders must meet the performance standards in IMO Resolution MSC.74(69). The main points are:

The speed of sound in water is set at 1500 m/s.
Under normal propagation and seabed reflectivity conditions, the equipment must be able to measure between 2 m and 200 m under the transducer.
It must have at least two range scales: a shallow range covering 20 m and a deep range covering 200 m.
The display record must show at least 15 minutes of soundings.
The pulse repetition rate must not be less than 12 pulses per minute on the deep range and 36 pulses per minute on the shallow range.
Accuracy: ±0.5 m on the 20 m range scale, and ±5 m on the 200 m range scale or ±2.5% of the indicated depth.
An alarm must indicate when the depth falls below a preset value and when the power fails.

Refer to the resolution itself for the full detail.

What Is an Acoustic Wave?

An acoustic wave is a sound wave: a vibration that travels through a medium by alternately squeezing and stretching its particles. It needs a medium such as air or water to travel through. Radio waves do not travel well through sea water, but sound does, which is why ships use acoustic waves to measure depth.

Principle of the Echo Sounder

An acoustic signal is transmitted into the water by the transducer. It is reflected by the seabed and returns as an echo. The echoes are detected, amplified and processed to indicate the depth of the water. The depth is calculated with this formula:

D = v × t / 2

D is the depth
v is the velocity of the signal (1500 m/s)
t is the time measured from transmission to reception of the echo

Figure 1. The echo sounder times the pulse on its way down to the seabed and back up to the ship.

Why Divide by 2?

The time t covers the whole journey: down to the seabed and back up to the ship. The total distance is therefore twice the depth.

Distance = speed × time, so 2D = v × t, which gives D = v × t / 2

For example, if an echo returns 0.04 seconds after the pulse is sent, D = 1500 × 0.04 / 2 = 30 m.

Components of an Echo Sounder

Oscillator (pulse generator)
Transducer
Amplifier
Display unit

Figure 2. The oscillator makes electrical pulses, the hull transducer turns them into sound, and the echo is turned back into a signal, amplified and displayed.

Oscillator (Pulse Generator)

It is generally built into either the display unit or the transducer, or it can be a separate component. Its purpose is to produce electric pulses at the desired frequency. These pulses are passed to the transducer, which converts them into acoustic signals.

Transducer

The transducer is the key component. It converts the electrical signal into a sound signal when transmitting, and converts the returning sound signal (echo) back into an electrical signal when receiving. Two types are widely used.

Piezoelectric transducer: This works on the piezoelectric effect. Crystals such as quartz, or specially made ceramics, produce an electric current when a stress is applied to them. In the same way, they change shape when an electric current is applied, because the crystal expands and contracts. In the transducer the crystal sits between two steel plates. An alternating current disturbs the molecules of the crystal so that it expands and contracts, and this movement produces the sound energy. When an echo hits the crystal it disturbs the molecules again, charges line up positive on one side and negative on the other, and this induces an electric current.

Magnetostrictive transducer: A ferromagnetic component (nickel or iron) is wound with copper wire to form a solenoid. When current flows, a magnetic field forms around it and makes the component expand and contract, which produces the sound signal. When an echo hits the component, its magnetic properties change (the Villari effect), and by Faraday’s law this produces an electric current.

A piezoelectric transducer is usually fitted by piercing the hull, whereas a magnetostrictive transducer is installed inside the hull. With modern technology, a piezoelectric transducer can also be installed internally.

Amplifier

It increases the strength of the recorded echo before it reaches the display unit.

Display Unit

It shows the processed depth and the record of recent soundings.

Transmission Frequency

The choice of frequency depends on the size of the transducer, attenuation (loss of signal in the water) and the need for minimum noise. If the frequency were in the audible range, the receiver would also pick up the noise of the propeller, the ship’s engine and seawater flowing past the hull.

If the frequency is too high, the wavelength is short and the water absorbs more of the signal, and a small transducer may not be able to produce enough power. Considering all of this, echo sounders generally work between 50 kHz and 200 kHz, which is called the ultrasonic range.

Figure 3. The working frequency is a compromise between noise, transducer size and loss in the water.

Pulse Length

The duration of the pulse in an echo sounder is generally 0.2 to 2 milliseconds. The pulse is short for the shallow range and long for the deep range. The pulse length decides the minimum depth that can be measured:

D_min = v × l / 2

D_min is the minimum depth
v is the speed of sound
l is the pulse length (in seconds)

For example, if the pulse length is 1 millisecond:

D_min = 1500 × (1 / 1000) / 2 = 1500 / 2000 = 0.75 m

So the minimum depth measured is 0.75 m, and any depth less than this will not be recorded.

Figure 4. A pulse that is still being sent when its echo returns hides the echo, which sets the minimum measurable depth.

Why the Minimum Depth Depends on Pulse Length

An echo from depth D returns after a time t = 2D / v. If this is shorter than the pulse length (t < l), the echo comes back while the pulse is still being transmitted. The two overlap and cannot be separated. The smallest usable depth is where the echo returns just as the pulse ends:

2D_min / v = l, so D_min = v × l / 2

Pulse Repetition Frequency (PRF)

The number of pulses transmitted per second is the pulse repetition frequency. For the shallow range it should not be less than 36 pulses per minute, and for the deep range not less than 12 pulses per minute. The maximum depth that can be measured is decided by the formula:

R = v × t / 2

where t is the time between two successive pulses, in seconds.

Shallow range: minimum 0.6 pulses per second, so t = 1 / 0.6 = 1.67 s and R = 1500 × 1.67 / 2 ≈ 1250 m.
Deep range: minimum 0.2 pulses per second, so t = 1 / 0.2 = 5 s and R = 1500 × 5 / 2 = 3750 m.

Figure 5. The slower the pulses, the deeper an echo can return before the next one is sent.

Why the Maximum Depth Depends on the Pulse Interval

An echo from depth R takes 2R / v to return. It must arrive before the next pulse is sent, otherwise it is confused with the echo of the next pulse. So 2R / v must not be more than t, and the limiting depth is R = v × t / 2.

Errors of the Echo Sounder

Velocity of Propagation in Water

The speed of sound varies with the pressure, temperature and salinity of the water. The echo sounder uses a preset value of 1500 m/s, which causes an error in other waters.

Figure 6. The depth is calculated with a fixed sound speed, but the real speed varies from place to place.

Aeration

Vibration causes air bubbles, which restrict the echoes. The echoes can suffer a tremendous loss before reaching the receiver, so no echo appears. It happens particularly when the ship goes astern.

Figure 7. Bubbles under the hull weaken the pulse and the echo so much that the display may show nothing.

Multiple Echoes

The echo may be reflected a number of times between the keel and the seabed, giving multiple depths. In that case, the first depth is the correct depth.

Figure 8. Repeated reflections give false readings at two, three and more times the real depth.

Pythagoras Error

This is caused when two transducers are used, one for transmission and the other for reception. The error can be found with the formula:

e = d − √(d² − x² / 4)

d is the recorded depth
x is the spacing between the transducers

This error is negligible when one transducer is used for both transmitting and receiving.

Figure 9. With separate transmitter (T) and receiver (R) the slanted path makes the recorded depth d larger than the true depth h.

Proof of the Pythagoras Error Formula

Let h be the true depth. By symmetry, the sound reaches the seabed halfway between the two transducers, so each leg of its path is the long side of a right triangle with sides h and x / 2.

Step 1: The echo sounder assumes a path straight down and up, so the depth it records is the length of one slanted leg:

d = √(h² + x² / 4)

Step 2: Square both sides and solve for the true depth:

d² = h² + x² / 4, so h = √(d² − x² / 4)

Step 3: The error is the recorded depth minus the true depth:

e = d − h = d − √(d² − x² / 4)

If one transducer does both jobs, x = 0 and the error disappears.

Thermal and Density Layers

The density of water varies with temperature and salinity, which tends to form different layers. Echoes can return from the surfaces of these layers, so a faint line may appear between zero and the actual depth.

Figure 10. A layer boundary returns a weak echo of its own, which is not the seabed.

Conclusion

The echo sounder is a simple but vital instrument. By timing a sound pulse to the seabed and back, it tells the officer of the watch how much water is under the keel. Knowing its principle, its limits and its errors helps you judge when to trust the reading, especially in coastal waters and near shallow patches.

Guide to Marine Echo Sounders & Subsurface Depth Measurement