Tarmo Soomere, president of the Academy of Sciences and a marine scientist, said that submarines and other undersea activity can be detected using underwater currents as well. "A submarine only generates underwater currents when it is moving in an environment, where the density changes vertically. For example, there is denser water deeper down and less dense in shallower waters," Soomere told ERR.

"The problem with detecting submarines in Norway is that underwater currents only occur in a few isolated places and at certain depths. Fortunately, in the Baltic Sea, most of the water masses are variable density from the surface to the bottom. If the density of the water changes vertically, any moving object will generate internal waves just as an object moving on the surface of the sea generates surface waves," explained the marine scientist.

This is why, according to Soomere, the chances of detecting submarines in the Baltic Sea are orders of magnitude better than, for example, on the Norwegian coast. Nonetheless, submarines moving at slower speeds are quite difficult to spot, even in this area, he said. "It is very difficult to detect submarines that are moving very, very slowly. Then the disturbances they cause are also very small. Another sad fact is that there are very few specialists left in the whole Baltic Sea basin. There is also no help to be sought from neighboring countries," he continued.

Soomere said that the classic method of detecting submarines is sonar. Active sonar emits directional sounds and underwater microphones – hydrophones – pick up reflections from various objects. The analysis compares the times between sound generation and reception. In this way, it is possible to determine the distance and direction of different underwater objects.

Passive sonar is mainly a hydrophone that, so to say, listens to the surrounding marine environment. "Sound travels very well through water, so submarine activity and other underwater activity can be separated from ambient noise. Hydrophones should be installed at regular intervals along the cable or pipe.Technically it is not very complicated but, of course, the practical implementation can take a lot of time and effort," Soomere explained.

Fiber optic hydrophones can be a solution

Professor Aleksander Klauson, who studies underwater acoustics at Tallinn University of Technology (Taltech), confirmed that acoustic waves are virtually the only ones capable of traveling through water. "Therefore, everything about the underwater environment – communication, detection, positioning – is acoustics-based, for which sensors or hydrophones are employed," Klauson told ERR.

"Considering the exceptionally low visibility conditions and the limited distance light can travel through water, this strategy is essentially the only viable alternative in the Baltic Sea. Also, radio waves also have limited transmission in water: at very low frequencies they can travel minimal distance, and the incredibly long wavelengths of these waves present their own special challenges too," he said.

"At Tallinn University of Technology, for example, we already use individual hydrophones with autonomous sound recorders to monitor underwater sound in the Baltic Sea. So that we can track how the ambient sound has changed in a specific location," Klauson explained.

However, one hydrophone cannot accurately determine sound direction. Klauson suggested connecting several hydrophones together and fixing them at a certain distance from each other in grids of different shapes. This then can show where the sound pressure has risen and where it comes from. "Behind warships, there are long cables and interconnected hydrophones at the ends. With such a long antenna, very small changes in sound pressure in the water can still be detected," the professor gave an example.

Unfortunately, the disadvantage of a single hydrophone or an cluster of hydrophones is that it can only be in control of a certain part of the sea. As mentioned above, Klauson believes that hydrophones could be installed in certain sections at intervals to cover, for example, the entire Estlink or the Balticconnector.

"For example, if the length of the pipe is 80 kilometers, I am not sure that 80 kilometers can be checked from one point. There should probably be several sections. In that case, the entire length of the pipe could be checked in case of an abnormal increase in sound pressure level," the professor said.

Fiber optic hydrophones, which could be installed along undersea cables, are already in use today. This type of sensor uses the underwater signal to induce changes in the intensity of light propagating through the optical fibre. Compared to conventional piezoelectric hydrophones, fiber optic hydrophones have much better long-range signal transmission, very low frequency response and wide bandwidth, Klauson explained.

"I haven't used such devices myself, but I think it would be a very good solution. I really hope that Estonia and Finland could do something like this together. This would undoubtedly be a big investment, but in reality, all kinds of things that work underwater are very expensive to install, inspect and maintain," Klauson said.

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