How an underwater sensor network is tracking Argentina’s lost submarine : Nature News & Comment

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Argentina’s ARA San Juan, a diesel-powered submarine, went missing this month off the east coast of South America.

On 15 November, Argentina’s Navy lost contact with the ARA San Juan, a small diesel-powered submarine that had been involved in exercises off the east coast of Patagonia.

About a week later, on 23 November, the Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) announced that its International Monitoring System — a network of sensors designed to detect nuclear explosions wherever they happen around the globe — had picked up a sound consistent with that of an explosion near the vessel’s last-known location. The submarine is carrying 44 crew members.

The CTBTO’s system has numerous scientific applications and this is not the first time that it has been put to use in the aftermath of a possible disaster. In 2000, for example, researchers searched its data for signs of the lost Russian submarine Kursk, and in 2014 they used it to try to determine the fate of Malaysian Airlines flight MH370. Nature spoke to CTBTO hydroacoustic engineer Mario Zampolli about the latest search.

CTBTO

Hydroacoustic engineer Mario Zampolli.

How does the international monitoring system work?

The system has 289 stations worldwide and, when complete, it will have 337. We use four different technologies: seismic, atmospheric infrasound and hydroacoustic, plus the radionuclide stations, which detect traces of radioactive isotopes from possible explosions. All the information goes to our international data centre in Vienna, and also to our analysts, to examine if there are any events that are of interest with regard to the detection of nuclear explosions. These systems record data 24/7. The signals are stored and are used for a variety of scientific applications and disaster mitigation.

How can you help in the search for the ARA San Juan?

Six of our underwater stations are equipped with hydrophones. Two stations picked up a signal: one in Ascension Island, slightly south of the equator in the Atlantic, and the other in the Crozet Islands in the Southern Indian Ocean, half-way between Africa and Antarctica. These two stations saw the same signal. Also, because each station has three sensors, based on the delay between the times when the signal reached each of the sensors, you can compute a bearing, and calculate the direction in which the signal was coming from. If you compute the geodesics starting at those points, the two lines cross in a location quite near to the point where the submarine last made contact.

Is this type of analysis done in real time?

For nuclear-explosion detection the CTBTO has a real-time processing pipeline. The automatic processing pipelines are optimized for detecting nuclear explosions. Whichever type of detector you build, you have to strike a balance between the probability of detecting something and the probability of false alarms. If the system is so sensitive that it detects everything, you will also have 100% probability of detecting false alarms. We would be completely swamped with events. To search for other signals, it becomes a manual job. We have to write ad-hoc pieces of software, compare signals and discuss them.

And what did your data show?

We found the location where the sound originated. It was estimated to have occurred 3 hours and 21 minutes after the last contact between the submarine and the base. We carried out a detailed analysis of the sound and are confident that this is not a natural event. It was an impulsive signal — short and sharp. The fact that it was detected with a good signal-to-noise ratio at Ascension and also at Crozet — 6,000 to 8,000 kilometres away from the source — means it must have been fairly loud. Some aspects of the signal are consistent with what has been seen in explosions before. But it is really very difficult to say that this was an explosion.

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