Underwater acoustic communication: diving into a field with many possible applications

With the expansion of human activities at sea, underwater acoustics finds a wealth of applications in the most various fields, that are just waiting to be developed. IMT Atlantique has a research team with a recognised expertise in this field.

Communicating under the sea

How do you communicate in "the world of silence"? How do you exchange messages, images and even videos in the depths of the sea, and, if possible, over long distances? A small team at IMT Atlantique in Brest has a recognised expertise in this area, including abroad.

TASM team
Christophe Laot, Sébastien Houcke and François-Xavier Socheleau

Communicating under the sea is no simple matter. "With radio waves, the range is limited to a few dozen metres. Only acoustic waves can cover long distances underwater - several dozens of kilometres, or even more under certain conditions. But there are lots of constraints and limitations," explains Christophe Laot, an associate professor in the Mathematical and Electrical Engineering department and a specialist in underwater acoustic communication. The aquatic environment poses a number of problems: signal reflection or "multipath", interferences, wave attenuation depending on frequency and distance, etc. As a result, transmitting a file, even a small one, can take several seconds. And to cover long distances, you have to use low frequencies, which means lower transmission speeds.

Acoustic underwater transmission

Nevertheless, serious progresses have been made in recent years. Nowadays, it is possible to send images and exchange messages that are akin to text messages. In Italy, the CMRE (1), a NATO research group, has standardised a system that enables interoperability between submarines and surface ships - albeit with a data rate limited to a few hundred bits per second.

The IMT Atlantique team is involved in a variety of projects, most often with an industry partner, as part of theses or research contracts. Researchers also have a dedicated platform, TASM for Underwater Acoustic Transmission, that received the Carnot Télécom and Société Numérique label, which brings together a range of equipment (transducers for transmission, power amplifiers, hydrophones, etc.). All of which can be used to carry out its own tests.

Several studies have been carried out with DGA-TN (2) and SERCEL to demonstrate the feasibility of transmitting images and text. One of these projects is a kind of underwater telephone that uses acoustic waves to transfer digitised voice information. With a range of around one kilometre, it could be of interest to divers, but this type of device is expensive. Another obstacle are the very bulky antennas.

A video transmission system is also being studied with Thalès. "Video transmission, on the other hand, remains a challenge," observes Christophe Laot. "A few tests have been carried out, but we don't yet have a robust system." Here again, the range will be limited: only a few hundred metres. To boost throughput, the team plans to use "multi-antenna" systems - with several dozen sensors. But we are talking about gigantic dimensions... "We're making progress, but there are physical limits," admits the expert. "Multiplying the throughput by 10 would already be enormous." Now, the objective would be to achieve a "proof of concept".

Another area of work with Naval Group is the security of underwater communications. The aim is both to make the signal difficult to detect by an eavesdropper and inoperable when intercepted.

A final example: the team has been working with Exail Technologies and IFREMER on devices to improve underwater tracking - a challenge for many players, given the absence of an underwater GPS. This could, for example, enable the movement of tectonic plates to be tracked with a high degree of accuracy (of the order of a few centimetres per year).

Spot schools of fish, monitor wind turbine cables, predict a tsunami...

With the growth of human activity beneath the surface of the sea, the need for communication is multiplying. Underwater acoustics therefore finds a wealth of applications, some of them unexpected, in many fields. The defence sector is, of course, one of the primary parties involved. The oil industry is also using these technologies, for example to position and locate oil fields using tomography of the seabed. Offshore wind farms are also interested in underwater communications, particularly for monitoring cables and anchors.

There are also numerous applications for oceanography and ocean monitoring. Thanks to underwater acoustics, underwater observatories can communicate information on the impact of climate change on the ocean, and even predict a tsunami, thanks to sensors placed on the seabed that transmit information via a wireless link. The IMT Atlantique team has even worked with SHOM (3) to monitor the movement of pebbles in areas with strong currents, to study the potential impact on tidal turbines. In the future, deep-sea exploration will make extensive use of drones and underwater telecommunications.

Underwater acoustics is also helping fishermen. In particular, it enables them to spot schools of fish. Marport company in Lorient even uses an acoustic link to ensure optimal positioning of fishing gear...
"Generally speaking, working underwater is very complicated," observes Christophe Laot. "So communication is very useful in many areas - even if it's not always essential. Underwater acoustic communication is an emerging field...". The subject is also an opportunity, during the institution's open days, to carry out demonstrations that are quite astonishing for lay people.

  • (1) Centre for Maritime Research and Experimentation.
  • (2) Direction Générale de l’Armement – Techniques Navales
  • (3) French Navy Hydrographic and Oceanographic Service
Published on 21.03.2024

by Pierre-Hervé VAILLANT

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