What is an EchoSounder?

The available sea depth, the distance from the mean water level to the seafloor, is often an essential parameter for consideration. For navigation of vessels (refer to the article on underwater keel clearance), fishing, oil explorations, rigging, research, and various purposes, estimating the depth of the ocean bottom is indispensable.

An echo sounder is a system that helps understand what is underneath and has been used by most seagoing ships for a long time. This system, which is one of the simplest applications of the Sonar (sound navigation and ranging) technique, uses the fundamental principle of acoustics to assess how much depth of water is available.

The theory behind this technique relies on the physics of underwater sound propagation and works by emanating sound/acoustic signals or pulses that rebound whenever they encounter an obstacle (like the seafloor) and travel back or echo again, giving us an idea about the time taken.

After that, from the first principles, with the help of the known velocity of sound waves and the recorded time, the depth of the water, that is, the linear distance from the mean sea level to the seabed level, can be approximated. While approximated is a more appropriate word for older versions of this technique, modern technologies for echo sounders are highly precise and reduce room for errors or inaccuracies.

Besides finding underwater clearance, echo sounders are also extensively used for purposes like finding shoals of fish, underwater explorations, etc.

Working of an Echosounder and Understanding Its Basic Components 

The main components of an echo sounder unit are 1) Transmitter, 2) Transducer, 3) Receiver, and 4) Display.

The transmitter generates short pulses of electrical AC signals from a voltage source directed towards a transducer, often through suitable power amplifiers that expand low-power signals to high-power.

The transducer, in this context, is a converter cum projector device that converts the electrical energy of the signals from the transmitter and emits them underwater.

These acoustic waves, emitted from the transducer unit, mainly located near the bottom of the ship hull, travel through the water, strike the seafloor, and ricochet back upwards. These reflected sound waves are then captured by the transducer, converted to electrical energy, amplified, and recorded by the receiver unit.

At this juncture, it is essential to note that the transducer unit serves two critical tasks:

i) An emitter and receiver unit that sends and receives sound wave signals to and from the vessel (or any other structure of interest)

ii) a converter unit that converts the input electrical energy to output acoustic or sound wave energy during transmission and converts the acoustic energy from the echo signals back to electrical signals during reception.

When the transducer transmits acoustic signals, it is often similar to a projector or speaker unit. While it receives the echo signals, it is analogous to a microphone or hydrophone unit.

During the transmission process, the input is essentially electric, and the output is acoustic, conversely, during reception, the input signals are acoustic or noise and output signals back along the reverse circuit are electrical waves again.

Some Time Base equipment records the time from beginning to end and is feedback-connected to the other significant components of the echo sounder system, helping estimate the distance. Moreover, in conjunction with the transmitter, they also control the pulse rate at which the signals or wave trains are generated.

Furthermore, as for the sound pulses both during emission and transmission to and from the seafloor, there can be a wide range of factors like losses, white noise, and various external disturbances. There are amplifiers along the circuitry that increase the amplitude of the electrical energy waves such that they can be well decoded.

The receiver unit reads the various parameters from the converted electrical pulse signals like amplitude, frequency, duration, etc. and computes the depth of the seabed using the simple relation:

Distance (d) = Velocity (v) X Time (t)/2

Here, the denominator, 2, takes care of the two-way transmission of the acoustic waves underwater (from ship to seabed and vice-versa), and t marks the total time taken. The calculated data is then displayed on the display unit for usage.

For all practical purposes, the speed of a normal sound wave in water is about 1500 m/s. However, this value may vary due to various factors ranging from weather to sea states, extreme salinity levels to extreme temperatures, and so on. Most modern vessels have means to take care of these errors and differences and accordingly account for water depth estimation.

In age-old methods, the calculation for estimating the depth was done manually based on the known velocity v and the recorded time interval between the emitted and recovered signal waves. Thereafter, devices like echo integrators were compounded to the sounding units to calculate the data and feed them to the display units for reference. Modern technologies include superfast and super-easy integrated digital systems that carry out the computations in no time and with full precision.

Echo-sounding can be of two kinds:

1) Single Beam and

2) multi-beam.

In simplest terms, single beams emit one particular beam of acoustic signals and cater for a smaller scope of area in determining the draft. Multi-beams, on the other hand, are more advanced systems that cover a wider range of area and use complicated wave mechanics like beamforming to have a better view of bathymetric distribution over larger swathes of seafloor area. We omit to discuss these in detail.

For all practical purposes, echo sounders emit acoustic signals in a conical manner, that is, divergent waves which spread over a certain area.

Though the values depend on the requirement, echo sounders must send short pulses (less than 10 milliseconds). Interestingly, the frequency of the wave signals also depends on water depth. Lower values within 20-25 kHz are used for deep waters, and shallower waters, higher order frequencies like 300-400 kHz or more are used.

You might also like to read-

• A Guide to Hydrographic Surveys
• Mastering Ship’s Navigation – Part 2
• What Is Deep Water Recovery?

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About Author

Subhodeep is a Graduate of Naval Architecture and Ocean Engineering. He has deep interest in marine structures and goal-based design aspects, and is dedicated to sharing and propagating technical knowledge of the industry.

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