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.
About Echo Sounder
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.
Understanding The Basic Components
The main components of an echo sounder unit are 1) Transmitter, 2) Transducer, 3) Receiver, and 4) Display.
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:
- An emitter and receiver unit that sends and receives sound wave signals to and from the vessel (or any other structure of interest)
- 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.
Types of Echosounding
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 that 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 for shallower waters, higher order frequencies like 300-400 kHz or more are used.
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Source: MarineInsight
