Listening to the ocean Drawing, description

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Listening to the ocean Children's Science Lab

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You have probably paid attention: in relation to the seas, oceans, the word "mystery" is used as often as in relation to space. This is no coincidence. Ocean exploration is very, very difficult. And although knowledge about this element is constantly accumulating, there is still a lot of incomprehensible today.

What are the difficulties? Indeed, from the board of a research vessel, instruments can be lowered to any depth and the composition of sea water, salinity, speed and temperature of the currents can be determined. Deep-sea television cameras help to monitor the life of the inhabitants of the sea. There are also bathyscaphes in which you can descend to great depths.

All this is so. But the sea is changeable. And if the so-called stationary currents, day after day, year after year, following in the same direction and at the same depth, are really relatively easy to study, then what about water disturbances that arise and disappear within a few hours? How to investigate the ring underwater vortices, generating, according to scientists, cyclones or anticyclones that change the weather around the globe? After all, there is simply no time to "feel" them, probing with depth instruments. Even keeping track of the movement of schools of fish in order to give clear commands to fishing vessels is not easy and expensive. To do this, it is necessary to maintain almost a whole air fleet, and its effectiveness is not so great, since a jamb can be detected from the air only at a relatively shallow depth. Therefore, for a long time now, scientists have been looking for a method that would make it possible to obtain a detailed and complete picture of the phenomena occurring in the sea, and not just fragmentary data obtained at points where research vessels lowered their measuring instruments.

Of course, it would be most tempting to illuminate the water column with some kind of radiation, just as an X-ray machine shines through the concrete panels of houses, showing all their defects on photographic film. But in water, X-rays fade before they have traveled a dozen meters. Radio waves decay just as quickly. So the radar under water would be blind. The light rays also dissipate quickly. The sound remains...

Experts have long known that sound travels considerable distances in water. But is it suitable for use in an underwater locator?

listening to the ocean
This is how the underwater "speaker" works

To answer this question, scientists from the Institute of General Physics of the USSR Academy of Sciences set up the following experiment: a sound emitter was fixed on the underwater part of a research vessel - a massive metal cylinder with two membrane covers and an electromagnet inside. An audio frequency voltage generator was connected to the electromagnet windings, and the ship went out to sea.

listening to the ocean
Here is another one of the mysteries of the ocean: the farther the ship goes from the coast, the greater the amplitude of the sound that the sonar receives.

Time passed. The ship went farther and farther, and the hydrophone installed near the shore confidently received its signal. Even 400 kilometers of distance almost did not weaken the sound thread connecting the ship with the shore - the hydrophone still clearly received the sound of the emitter.

It turned out that near the coast it is possible to receive the sound echo of processes occurring in the sea and thousands of kilometers from the hydrophone. They tried to do this, but after listening to the hydrophone signals, which in another experiment were recorded by a tape recorder for several days in a row, scientists discovered something that could not be deciphered: a chaotic mixture of all possible sounds, from infra-low to ultra-high, appeared on the magnetic tape. No computer would help to understand such a sound mess.

It became clear that listening to the sea is futile. You need to probe it, just probe it with your own sound, just like a locator does. However, the direct principle on which the locator works was not suitable for physicists. You probably know that the locator sends radio signals into the sky and picks up their reflection. It could be assumed that a school of fish in water is also capable of reflecting a sound signal that has fallen on it - its density differs from the density of water. But an annular vortex or flow will most likely not reflect the sound or reflect it very weakly. Water, after all, is water, and it makes no difference to sound whether it is still or moving. Therefore, they decided to separate the sound emitter and the hydrophone at a distance of tens of kilometers. The calculation was that the perturbations of the water or the same shoal of fish that appeared between them, at least a little, would prevent the sound from propagating in the water, distort its amplitude or phase. And in order to prevent extraneous signals from entering the hydrophone amplifier, they decided to build in a filter very precisely tuned to the frequency of the sound emitter.

Then it was necessary to think about the complete scheme of sound sounding of the sea. And here physicists first of all remembered the Doppler effect.

You have probably experienced this effect more than once. Remember: when the train approaches the station, its beep is higher than when it passed by. This is because at first the speeds of sound and trains add up, the sound flies faster, and its frequency for a stationary observer becomes higher. Then the speed of the train is already subtracted from the speed of sound. Its frequency is decreasing.

For a broadband audio receiver like our ear, it doesn't matter. But if it is tuned only to the frequency of the horn, as the hydrophone is to the frequency of the transmitter, then neither higher nor lower frequencies will be heard. Therefore, they decided to install the sound emitter at the bottom of the sea, motionless, and not on a ship, which, by its movement, could change the frequency.

listening to the ocean
This figure clearly shows how the phases of the signals differ due to the fact that the cable with hydrophones is not laid exactly along the radius.

One hydrophone for accurate analysis was, as the scientists reasoned, not enough. To cover as much space as possible, sound receivers need at least a few dozen. Then it will be possible not only to register a school of fish or an annular vortex, but also to monitor their movements. That is, it will be possible to create a certain spatial picture of disturbances in the sea and find out what caused these disturbances.

You can tell for a long time how the equipment for the experiment was prepared - special pre-amplifiers were built into the hydrophones, capable of both hearing weak signals and not "deafening" from too strong ones, how they looked for ways to protect them from water pressure and corrosion, how they chose the most interesting from the point of view view of science, a section of the sea ... There were many difficulties in the preparation. They waited for scientists during the experiment.

After the sound emitter and fifty hydrophones on a common cable were immersed to the bottom of the sea and all the devices were turned on, instead of the expected signal, the researchers saw fifty signals with different phases on the oscilloscope screen - all hydrophones did not work together, but out of order.

The reason turned out to be simple: in order for all hydrophones to work, as they say, in unison, the distance from each of them to the sound emitter must be the same. Then all signals will come to them in one phase. But after all, a cable cannot be laid perfectly evenly to a depth of a hundred meters, with an accuracy of microns. How it falls to the bottom is a matter of chance.

And yet, hydrophones managed to be made to work in one team. Physicists have aligned the phases with very high accuracy by developing special electronic phase-shifting devices. And now the stationary track - that's how the experts called their underwater sound locator - is already providing information. Now theorists are analyzing it, looking for patterns that will make it possible to determine exactly what this or that signal distortion means, what phenomenon in the sea it corresponds to.

In the future, scientists are thinking of installing such routes on all seas and oceans. And, apparently, the time is not far off when they will have much fewer secrets.

Author: A.Fin

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