Sunday, December 5

How do you convert a fiber optic cable into a seismic station? The key is that the network is faulty

There is a submarine cable between La Palma and Tenerife that has gone from being used for fiber optics to becoming a seismic network with thousands of aligned sensors, capable of detecting a greater number of earthquakes, even those in principle imperceptible, and of deciphering the next chapters of the volcanic eruption in Cumbre Vieja, which does not seem to show signs of weakness a month and a half after robbing the palm trees of sleep.

A group of researchers from the Institute of Marine Sciences (ICM-CSIC) of Barcelona has coordinated the work to install a DAS interrogator (from the English Distributed Acoustic Sensing) in the fiber optic cabling already used by the Roque de los Muchachos Observatory, of the Instituto de Astrofísica de Canarias, to connect to the Spanish Academic and Research Network (RedIRIS).

This instrument is not new. It has been used in civil protection for some time, especially in the surveillance of buildings and infrastructures. A clear example: in 2020 a pilot test of the project Smanslope, an initiative that uses fiber optic DAS technology to identify rockfalls that occur around the railway line on the Manresa-Tarrasa line, some 60 kilometers, and thus respond to geological phenomena, such as slope movements and landslides .

This can be a bit confusing, because thinking of a simple cable as a huge seismic or surveillance network is not normal. But the truth is that the very operation of fiber optics, as well as the channel through which it is transmitted, favors the deployment of this type of alternative applications. In addition, it starts from a premise: the more defective the material used, the worse for telecommunications and the better for other uses.

Fiber optics, roughly, is a means of communication between two points. From the transmitter, to put a starting point, a very strong flash of light is emitted that must travel a distance, which can be centimeters or kilometers, to reach the receiver in the form of a message. Countless pulses of light that travel at millions of meters per second pass through the fiber optic cabling, encoding a signal and transmitting it. Light is refracted within the wire, so it never escapes. And so, the information that was issued at the beginning reaches the end with practically no alterations.

But even though they are built with a resistant material, such as copper, fiber optic cables have some imperfections inside that are the key to agreeing their transfer to a more technical work, how can seismology be. The best way to detect these faults is to analyze the reflection carried by the light pulses. These, each time they are launched, leave a small trace behind, tiny, which will be greater as long as the element with which the connection path has been created registers a multitude of deficiencies.

In this way, by presenting a wider and larger fan of traces, the quality of the optical fiber worsens, it loses intensity and energy. And the reflections of the flashes of light are taken to the field of science for analysis. The mechanism is very simple. When two pulses of light are sent very closely, with an output difference of thousandths of a second, the normal thing is that the reflections of both are identical. However, if this is not the case, if there were a microscopic change, this could indicate that a seismic wave has caused a slight variation in length along the fiber. An earthquake has almost certainly occurred.

Luca D’Auria, director of the Volcanic Surveillance Area of ​​the Volcanological Institute of the Canary Islands (INVOLCAN), is working with the DAS system to study the seismic activity of the La Palma volcano to the millimeter. With this new tool, which was installed a few days ago, very small earthquakes that might otherwise be escaping the eyes of scientists can be detected, as well as their location and depth. So far, these two variables of seismicity on the island are experienced with a certain “uncertainty”, says D’Auria. Thanks to the DAS system, the precision will increase.

“Because La Palma is an island, we see that the seismic network is connected above the surface. However, very deep earthquakes are occurring right now [a más de 30 kilómetros]Therefore, the stations, by their nature, start with uncertainty in the location of these earthquakes. Having something like hundreds or thousands of virtual sensors available, it will be possible to improve on this ”, explains the expert.

Since the Cumbre Vieja volcano erupted, the seismic activity on La Palma has been set at two points: most earthquakes occur about 10-15 kilometers from the surface and many others at about 30-35 kilometers. The latter suggest that the volcano is feeding on a magma reservoir located far from the ground, from where the material moves from the furthest pocket to the closest one to end up coming out with force through the mouths of the volcanic fissure.

The logical thing is to think that if the earthquakes continue at that depth, the volcano will continue to feed and eject material. That is why D’Auria highlights the importance of the DAS system, which will determine if there is a relevant change in the location of the earthquakes and, therefore, in the development of the eruption. “As in El Hierro, seismicity is an important indicator. It not only serves as a precursor [un enjambre sísmico alertó de la posibilidad de que estallara un volcán en La Palma], but also completion ”, highlights the volcanologist.

The fiber optic cable must be well anchored to the surface, otherwise it may not correctly capture seismic waves. According to an experiment that INVOLCAN coordinated last summer, the DAS system can capture even the seismic ear, a vibration of the ground that has its origin in the sea waves. D’Auria says that can be annoying because it masks other signs. But it also has its advantage, as it can be used to understand the structure of the subsoil.

In that experiment in which INVOLCAN debuted with the tool, D’Auria and his colleagues relied on a fiber optic cable that connects Tenerife with Gran Canaria to analyze the area, where seismicity is “very interesting,” says the expert. . “Not only from a scientific point of view, but also from a civil protection point of view. It is an area that must be studied in detail because from time to time there are earthquakes up to magnitude 5, like the one in 1989 ”. This year An earthquake measuring 5.3 on the Richter scale caused glass breakage and furniture displacement in Tenerife. It brought panic to the island, but there were no fatalities or building collapses.

The INVOLCAN seismologists appreciated the usefulness of the DAS system, from which D’Auria believes that even more information can be obtained than the current one, and they contacted the Photonics Engineering Group of the University of Alcalá de Henares (UAH) and the CSIC Optics Institute (IO-CSIC) to borrow it during this eruption. They already plan to buy one that would land at the end of the year.

The cable, for its part, does not stop broadcasting data day and night, functioning as “a dense seismic network with significant capacities” from which scientists extract the information in the form of files. “When we see an interesting event, like a deep volcanic tremor, we download the file,” says D’Auria. At that moment, behind a screen, the analysis of one of the many signs that mark volcanic activity on La Palma begins.