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Transforming fiber optics on the seabed into a seismic network to detect earthquakes  
Transforming fiber optics on the seabed into a seismic network to detect earthquakes
This summer, a group of researchers from the Institut de Ciències del Mar (ICM-CSIC) in Barcelona, the Instituto de Óptica (IO-CSIC), the Universidad Alcalá de Henares (UAH) in Madrid and the Spanish network for the Interconnection of Computer Resources of Universities and Research Centres (RedIRIS) will convert the fibre optic cabling installed on the seabed in the Canary Islands into a seismic network for earthquake detection.

The researchers will use the underwater communication cables that connect the islands of Tenerife and Gran Canaria, located in an area of high seismic activity. The measurements will be carried out using DAS (Distributed Acoustic Detection) technology, which has great potential for monitoring seismic activity, since it allows converting a fibre optic cable into a very dense network of seismic sensors.

The DAS technology consists of a device that emits pulses of laser light through the optical fiber and measures the small signal fractions reflected in the microscopic imperfections inside the cable. These imperfections are converted into reference points that vary in position as a result of external factors such as ground vibrations and therefore change the properties of the backscattered light. Thus, a single cable connected to a single measuring device can become a network of thousands of sensors.

In order to have more spatial coverage, in this project two highly sensitive DAS devices developed by the IO and the UAH will be connected at the ends of the fibres on both islands.

The objective of the researchers, who have already started installing the devices, is to evaluate the detection level of this new technology and to improve the location of earthquakes in the region with respect to the existing terrestrial seismic network on the islands. The devices will be collected at the beginning of September, from which time the data obtained will be analysed.

"Despite the increase in the number of seismic stations in the Canary Islands in recent years, these are located on land, so the underwater areas are not well monitored", says Arantza Ugalde, from the Barcelona Center for Subsurface Imaging of the ICM-CSIC, who adds that "the availability of seismic data in this area will make it possible to characterize, with greater resolution, the seismically active structures between Tenerife and Gran Canaria".

Likewise, the experiment will make it possible to study other types of signals commonly recorded by marine seismic networks that may be caused by processes related to gases or deep ocean currents. Finally, the fibre optic cabling will be used to analyse non-seismic signals such as those emitted by some marine mammals, which will serve to study their behaviour.
"The distributed acoustic detection technology we have developed allows us to easily transform a fiber optic cable into a matrix of high sensitivity strain gauges," says Miguel González Herráez, from the UAH. According to the researcher, "this technology will revolutionize data collection in seismology, particularly in the underwater environment, where the installation of seismic sensors implies a great technical and economic challenge".
For his part, Hugo Martins, from the IO, assures that "thanks to this experiment more than 50 kilometres of fibre will be monitored in real time and close to the limits achievable by the technology in the laboratory, it being necessary only to connect a device to one of the ends of the fibre optic cable".




Finally, from RedIRIS, Esther Robles explains that "providing underwater fibre optics for this experiment to measure seismic activity has made us take another step forward in our vocation, which is to promote and facilitate the work of researchers with unique network solutions".
In the field of traditional seismology, seismic stations, installed either permanently or temporarily, act as individual sensors distributed around the globe. At present, one of the great challenges of seismology is the poor coverage of seismic stations located on the seabed due to their high cost.
Although it is still very incipient, the study of seismology by means of optical fiber has advanced a lot in the last 5 years and has been used successfully in several occasions. For example, in 2018, these DAS equipment installed in the Pasadena metropolitan area (California) detected, more than 9,000 kilometers from the epicenter, an earthquake that occurred in the Fiji Islands.
 
Investigación financiada por el Ministerio de Ciencia e Innovación y la Agencia Estatal de Investigación
Instituto de Óptica "Daza de Valdés"
(IO-CSIC)
C/ Serrano, 121
28006 Madrid (España)
Tel: 915 616 800
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