# echo sounder

The echo sounder is a device used in shipping for the electroacoustic measurement of water depths ( sounding ). The time that elapses between the emission of a sound pulse ( water- borne sound ) and the arrival of the sound waves reflected from the bottom of the water is measured . It is useful to differentiate between echo sounders for safe navigation, those for surveying purposes for creating sea charts and determining the need for dredging rings, in particular for waterways and research echo sounders. Active sonar devices have a very similar function . However, these are not used for vertical determination of the water depth, but for the predominantly horizontal underwater location, primarily for military purposes.

Sound propagation with the echo sounder (schematic)

## history

Record the echoes on a strip of paper

Shortly before the First World War, the echo sounder was developed in different countries simultaneously and largely independently of one another. In Germany, the physicist Alexander Behm achieved the breakthrough with his imperial patents No. 310690 from January 7th, 1916 and No. 367202 from June 1st, 1920. In 1920 he founded the Behm-Echolot-Gesellschaft in Kiel for the commercial exploitation of his invention .

## functionality

Transducer

An echo sounder requires a pulse-like sound source, the reflecting floor, a sound receiver, a device for measuring the time between the moment the pulse is sent and the reception of the echo signal, as well as a method of converting the transit time into the distance between the transmitter / receiver device and the floor.

Today, mainly short, sinusoidal, electronically generated signals are used, which are converted into an acoustic signal with a sound transducer - also known as a transducer. Cylinders made of piezoelectric lead zirconate titanate (PZT) are usually used as transducers, the height of which determines the resonance frequency and the diameter of which determines the directional resolution. Usually the acoustic received signal is also converted into an electrical signal with the same sound transducer. The received signals are usually displayed with a pen in vertical lines, modulated with the intensity of the received signal. This gives a profile line of the sea floor in the sequence of these lines. In the past, a wax carbon paper was used for this, in which the light wax layer was burned away depending on the intensity of the current. This echogram recorder has now been replaced by an electronic display that fulfills the same function.

It is not just the ocean floor that causes echoes, but also fish, aquatic plants and other objects. Air bubbles are particularly effective reflectors. The indication of fish is created by echoes of the air in the swim bladders of the fish. The rest of the fish body, on the other hand, has a similar acoustic impedance as the water and does not reflect the sound. These echoes provide valuable information for anglers and fishermen, for example about steep edges, holes and vegetation where fish particularly like to be. Correspondingly used echo sounders are called "fish finders".

Depending on whether it is a navigation sounder, a surveying sounder or an echo sounder for marine research, the design of the elements of an echo sounder differs considerably, so that they are discussed separately.

### Frequency, opening angle and pulse sequence

The frequency used depends on the task at hand. The sound attenuation in the water increases sharply with the frequency. Deep water echo sounders therefore use very low frequencies, usually 10 to 20 kHz. Even lower frequencies are chosen for sediment plumbing because of the strong attenuation of the sound in the ground. Nowadays navigation sounders mainly use frequencies above 50 kHz.

In the standard version, cylindrical transducers are used. The cylinder height is decisive for the frequency, while the cylinder diameter relative to the cylinder height (i.e. the wavelength of the sound) is decisive for the radiation angle. At 60 kHz and a diameter of the active transducer area of ​​5 cm, the angular range is approximately 30 °. The angle of radiation is the area in which the transmission power has dropped by 3 dB compared to the maximum sound level. This angular range is a function of the transducer diameter relative to the wavelength. The angular range is therefore approximately inversely proportional to the frequency. At high frequencies, a very high resolution is obtained even with small transducer areas, while at low frequencies for deep-sea sounders, very considerable transducer diameters would be required, which can no longer be technically realized with individual transducer elements. Groups of transducers are then used to achieve the required concentration.

The pulse repetition rate, that is how often an echo sounder can be carried out, depends primarily on the water depth, the depth range set for the graphic representation and the transmission power. Before a new impulse is sent, the ground echo must of course be awaited. If the transmission power is set too high, the signal reflected on the sea surface can be re-transmitted like the original transmission signal. As a result, very similar soil profiles can occur at exactly twice the depth, which are then received after the next transmission signal if the pulse sequence is too fast and are therefore received before the echo of this following signal and simulate a depth profile with a much smaller depth than the correct transmission signal (phantom echo). The transmission power must therefore be set depending on the water depth. In modern plumbing systems, this is done automatically by regulating the reception gain and adapting the transmission power to the set depth range. If the water depth range is set too low, phantom echoes can still occur. Such multiple echoes represent a significant problem for sediment echosounders and seismic sounders.

While the signal length of a plumbing signal is usually on the order of 1 ms, the pulse repetition frequency for the reasons mentioned is at least 1.4 s for a depth range of 100 m.

### footprint

The opening angle results in a flat measuring area (footprint), the diameter of which increases proportionally to the depth. If the footprint is large, the echo starts abruptly from the point that is closest to the echo sounder within the footprint, i.e. only if the water depth is constant directly below the ship. The echo level does not decrease abruptly at the "edges" of the footprint, as the schematic image suggests, but gradually. Strong reflectors outside the footprint can also deliver echoes.

Opening angle and footprint
angle diameter
10 ° 0.17 × water depth
20 ° 0.35 × water depth
30 ° 0.53 × water depth
40 ° 0.72 × water depth
50 ° 0.93 × water depth
60 ° 1.15 × water depth
70 ° 1.40 × water depth
80 ° 1.68 × water depth
90 ° 2.00 × water depth
100 ° 2.38 × water depth
${\ displaystyle {\ text {diameter}} = 2 \ cdot \ tan (\ alpha / 2) \ cdot {\ text {depth}}}$

### Measuring range

The depth measuring range depends on the transmission power, frequency, as well as background noise and sensitivity. Frequencies between 15 and 50 kHz are used in echo sounders for deep water. Ground echoes from up to 100 m are already reached at 200 kHz.

### Two-dimensional solder strips

The conventional echo sounder delivers only one depth value per transmitted sound signal (ping), i.e. a sequence of depth profiles on a line. This is unsatisfactory for sea floor mapping because the measuring ship has to travel a tight network of lines in order to generate a sufficiently reliable flat depth map. There are two ways to get from this plumb line to a plumb line in the event of an overflow and thus reduce the number of necessary overflows. Both are so-called multibeam echo sounders . A whole series of plumb bobs can be arranged next to one another, each of which records parallel plumb lines with a low angular resolution (surface plumb bob), or the same can be achieved with an echo sounder system with high angular resolution with several plumb beams of different angular positions next to one another (fan plumb bob).

Transducer unit (folded) of a surface echo sounder; The two movable frame parts are folded outwards with transducers for measuring operation
Sounding ship "Laber",
oscillating unit extended

The area echo sounder is used to survey very shallow waters, especially rivers, canals and port areas. For this purpose, a number of transducers are used next to each other with a defined distance and a small opening angle. Neighboring transducers are operated simultaneously with different high transmission frequencies. The transmission frequency range, opening angle and transducer distance are determined by the measurement depth and the bottom condition of the measurement area and can usually be set or changed.

Fan echo sounder

When multibeam the overall width taking the solder strip in proportion to the depth. It is therefore unsuitable for very shallow water depths in comparison to the area echo sounder, which, conversely, can no longer be used at greater water depths.

In the case of the multi-beam echo sounder, a group of transducers in the longitudinal direction forms a directional beam of high focus (e.g. 1.5 °) but a large width of up to about 150 ° transversely to the direction of travel, i.e. a directional beam in the form of a fan. At the receiving end, a transducer group is used across the direction of travel. These converters are interconnected by delay time or phase rotation of the received signals in such a way that a large number of high-resolution individual lobes are formed simultaneously in the transverse direction. One can imagine this as a high-resolution beam being panned electronically. If this were to be carried out serially, from ping to ping (as is usual with radar), this would take too much time because the speed of sound is slow compared to the speed of light. The reception lobes are therefore formed simultaneously. Depending on the distribution of the rays across the direction of travel, a fan of 120 to 150 ° width is created. With modern systems, the compartment opening can be adapted to the water profile, the ship's speed and the water depth. Fan angles of up to 210 ° are used to measure port facilities. The fan captures strips of the sea floor along the ship's course. The greater the water depth, the wider the recorded strip.

If several overlapping strips are joined together, a digital terrain model can be calculated and finally a topographic map can be created. A prerequisite for surveying with a multi-beam echo sounder is a high-precision position sensor whose data can be used to correct the falsification of the depth data caused by the ship's movement. This is not without its problems:

1. The footprint becomes larger and larger as the angle becomes flatter and the echo signal input becomes more blurred because it no longer originates from the nearest point on the sea floor, but from the “edge” of the footprint;

2. angle errors lead to large errors in distance and thus to large errors in the calculated water depth; and

3. flatter rays are refracted in stratified water, which can lead to very large measurement errors if this is not taken into account precisely.

### Consideration of the speed of sound in water

Echosounders measure the time from the transmission of the transmission pulse to the reception of the echo, but usually indicate the water depth, i.e. a distance. The conversion takes place with the help of the speed of sound, which is not constant but depends on the temperature, the salt content and the pressure. The assumption of a constant speed of sound, usually 1480 m / s for echo sounders, leads to depth errors of up to 5%, which is justifiable for many purposes of the sounders. The speed of sound can often be adjusted. Then a sound speed of 1450 m / s is more appropriate in fresh water, which reduces the depth error to less than 2%.

For precise measurements, these parameters must be determined and taken into account when converting the transit time into a water depth. There are various empirical formulas for this. Simple approximation formula:

${\ displaystyle c = 1448 {,} 6 + 4 {,} 618 \, T-0 {,} 0523 \, T ^ {2} +1 {,} 25 \ cdot (S-35) +0 {,} 017 \, D}$

It means:

c = velocity of propagation (m / s)
T = water temperature (° C)
S = salinity (‰)
D = water depth (m)

This empirical formula describes the relationship for 0 ° C <T <40 ° C and 0 ‰ <S <40 ‰.

## Types of echo sounders

There are very different echo sounders, from handheld echo sounders to deep-sea fan-shaped sounders, the installation of which requires structural consideration even in large research vessels.

• The most common echo sounders are the navigation sounders, which are found on almost all watercraft from larger recreational sailing and motor boats and are often linked to the function of a fish finder.
• Surveying plumb bobs have to meet significantly higher requirements for the accuracy of the water depth determination and the positioning of the depth measurement.
• Research plumb bobs cannot easily be distinguished from survey plumb bobs, but they have to fulfill a broader range of tasks.

### Navigation plumbs and fish finders

Navigation plumbs are primarily used to prevent the carrier vehicle from hitting the ground and to support safe navigation. They usually have a plumb line pen or, more modern, a mostly colored display. To protect against ground contact, it is not necessary that the plumb bob has enough transmission power for depth ranges of a few 100 m. The requirements for determining the depth accuracy also decrease with increasing water depth under the keel. It is therefore not necessary to correct the speed of sound for the conversion from echo time to water depth. Furthermore, a navigation plummet does not require a high angular resolution. Within the footprint, the next point is always measured, which with a large measurement angle range can only be directly below the ship or at a smaller distance. If the angular resolution is low, vertical stabilization is not required to compensate for the heel .

In addition to warning of immediate ground contact, navigation plumbs are important navigation aids. The planned courses are drawn in pencil by the ship's command on nautical charts, which also contain depth lines and individual depth measurements related to the nautical chart zero . A comparison of these depths with the soldering depths provides information on correct navigation. In addition, in areas with strong water depth fluctuations , it is often necessary to travel through courses at too high tides , which according to the chart (water depth at the lowest expected water level) are not sufficient. The echo sounder gives information about the current water level together with the nautical chart. But even for this purpose, a high level of accuracy is not required at great water depths, because, because of the expected depth fluctuations, a longer prognosis until the shallow depth is reached is no longer accurate.

Air bubbles are very good acoustic reflectors. That is why fish deliver easily recognizable echoes. The echoes are practically only triggered by the swim bladder and not by the whole fish, the density and compressibility of which is too similar to that of the surrounding water. So you cannot see the shape in the echo sounder and even with great experience you can only estimate its size to a very limited extent from the strength of the echo. Nevertheless, the echo sounder provides the fisherman or angler with important information about the number of fish under the boat. For this reason echo sounders are often used as " fish finders ". While one does not strive for a high resolution for the navigation plumbing function, it is more of an advantage for the fish finding function. That is why echo sounders are now being equipped with a further, significantly higher transmission frequency, especially in the recreational boat sector. Because the angular resolution is proportional to the transducer diameter in relation to the wavelength, it is significantly higher at the second, higher frequency (usually 200 kHz) than at the frequency of 50 to 80 kHz otherwise used.

In addition to the fish finder function of a navigation plummet, larger commercial fishing vessels usually use other special plumbs to locate fish and schools of fish, with the help of which schools of fish can be tracked and with the help of which the net depth can also be set in pelagic deep-sea fishing . The functioning of these sounders is often similar to that of a fan-type echo sounder or a high-frequency sonar device .

### Survey echo sounders

Completely different requirements are placed on echo sounders for depth measurement for data acquisition and correction of sea charts as well as for determining needs and billing in dredging than on navigation sounders. The IHO has determined the necessary accuracy of depths and positions

Depth [m] Depth error [m] Position error [m]
5 1 20th
20th 1.2 21st
100 2.5 25th
500 11.5 45
Accuracy of depth and position according to IHO

These errors contain the sum of all measurement errors such as navigation accuracy, heeling of the ship, correctness of the speed of sound, determination of the echo time and current water level. In ports and other special locations, much higher levels of accuracy are required. In addition to measures that do not affect the echo sounder, such as very precise navigation and taking into account the current water level, the depth of the echo sounder must be calculated using the currently measured speed of sound and a well-bundled transducer that is vertically stabilized is required for greater water depths. The close bundling and vertical stabilization serves less for the exact localization of the measuring point than for avoiding depth errors on inclined seabeds or heeling measuring ship.

In addition, it is more efficient to use multibeam echo sounders, because otherwise too close-meshed plumb lines are necessary for a depth measurement. The so-called surface plumb bobs are particularly suitable in very shallow waters because they can meet the accuracy requirements relatively easily. Fan echo sounders are unavoidable in this case at greater water depths. However, the accuracy of the depth measurement decreases with increasing distance of the respective measuring beams from the vertical, because the footprint of the measuring beams increases, which increases the effect of inclined seabeds and errors in the heel correction. In addition, the measuring beams are refracted when the speed of sound is dependent on the depth, which can lead to considerable measuring errors even with a complex correction. It requires evaluation by a good specialist who can assess the fan angles up to which the required accuracy can still be achieved. But despite these restrictions, the use of a fan-shaped plumb bob makes sense. Vertical stabilization is easier with fan-shaped perpendiculars than with single-beam perpendiculars, because it can be done electronically by assigning the beams to the angles. In addition, small-scale depth deviations that could otherwise be overlooked between the course meshes can be recorded at least qualitatively so that they can be measured precisely by a supplementary overflow.

### Echo sounders for research

Special echo sounders are often used on research vessels that go beyond the needs of general shipping and the measurement of water depths. Above all, these are the often very complex plumb bobs, especially for deep-sea use. Fan-shaped perpendiculars require a high angular resolution, i.e. large transducer apertures . Because of the frequency-dependent sound absorption, however, a low transmission frequency (10 to 20 kHz) is required at great depths of water. Therefore, very large groups of vibrators are necessary. In the past, so-called shelf plumb bobs with a low frequency (10 to 20 kHz) and high angular resolution were used, especially at the inclined shelf edges, which had to be stabilized against the heel. They have largely been replaced by fan-shaped perpendiculars, where stabilization can be carried out electronically. In addition, the three-dimensional representation of the fan-shaped perpendiculars offers great advantages, especially on the shelf slopes.

Even lower frequencies are required for sediment plummets, where echoes from the sea floor are also evaluated. They must also have a high angular resolution, because the soil surface almost always provides the strongest echo, but the weaker echoes from the sediment layer can be obscured by later echoes from the footprint. For the same reason, a high distance resolution is required, and therefore a very short transmission signal. Both are in contradiction to the low frequencies.

For this reason there is a special form of sediment echo sounder , the parametric echo sounder. Here, the low frequency is not generated in Lot itself, but two high frequencies are emitted at the same time. At a very high power density, the sound propagation is no longer linear . During the transmission, a signal is then formed from the difference between the two primary frequencies in front of the transducer. This gives a high angular and distance resolution without the transducer having very large dimensions. This is why this method is worthwhile in spite of the low level of efficiency in generating the secondary frequency. Since the secondary frequency is only formed in the water, the parametric echo sounder requires a greater water depth.

The hardness of the sediments on the sea surface can be estimated with a special evaluation of the received signal with any echo sounder, which is why the echo sounders are often equipped with the ability to estimate echo strengths on research trips. This can reduce the need to take soil samples.

## Echolocation

The echolocation is a closely related process which is also the location of objects, such as ships or fishing can determine. Echolocation is used for searching submarines and for searching for mines (active sonar method). High-resolution devices are used to search for wrecks and to search for people and corpses. Toothed whales and bats use echolocation for orientation.

## Related procedures

A comparable method is occasionally used in aviation . The Sonic Altimeter is used to measure the altitude of aircraft over the ground.

The autofocus system of some cameras works with an ultrasonic distance measuring device.

Changes in the ground are made visible with the geoscanner .

Humans can also develop a certain feeling for the size of rooms if they have some reverberation, which is particularly used in the dark or by blind people.

## Bathymetric map

General Bathymetric Chart of the Oceans (GEBCO) is a bathymetric , worldwide data set that is based on data collected from ship echo sounders. The maps created from this show the exact shape of the sea bed with its ridges and valleys. Google and OpenSeaMap use such maps.