A team of researchers of GEO3BCN-CSIC has started a new experiment aimed to use a buried fiber-optic cable as a seismic sensor in Barcelona. The experiment uses the DAS technology (Distributed Acoustic Sensing) which allows to measure the small deformations along the cable caused by vibrations by using laser pulses. The experiment is made possible by the collaboration of Reintel Company (Red Electrica group), who owns and provides the cable network, and the University of Michigan, owner of the DAS interrogator unit.
The experiment is aimed to explore and test the application of DAS technology in the urban seismology field. "The applications of the DAS technology are numerous and are still growing”, said Martin Schimmel, researcher at GEO3BCN-CSIC and coordinator of the experiment. "In our case, we are interested in assessing its potential use to monitor the variations of the subsurface seismic properties, study the shallow crust structure and also measure the city "pulse " by studying the ambient seismic noise data", explains Jordi Díaz, seismologist at GEO3BCN-CSIC. Moreover, the researchers expect to register some of the regional earthquakes that occur often in the Pyrenees and the Catalan coast area.
A 15 km-long underground fiber-optic cable that runs across the city underground from end to end and the “laser interrogation unit”, called "interrogator", are the two necessary elements to conduct the experiment. On 26th April, Zack Spica, researcher at the University of Michigan, and the members of the GEO3BCN-CSIC Seismic Laboratory (LabSis) installed and configured the interrogator unit, a DAS Febus Optics Ar1.
The interrogator unit, which was connected on one of the end of the cable, shoots laser pulses down the fiber-optic cable thousands of times a second (with a 5Hz frequency). The small deformations of the cable caused by, for example, ground vibrations, are determined by measuring the changes in the phase of the light beam reflected and dispersed. If the deformations move with the time, then a phase difference with the previous reflected pulse is calculated. "This allows us to use the cable as an array of thousands of seismic sensors by only emitting laser pulses and registering its reflection and dispersion on one of the ends of the cable", said Jordi Díaz.
The researchers have planned to acquire data continuously during a 2 to 4 weeks period in 1500 differents points distributed along the cable, which means a resolution sampling of 10 meters. Moreover, several seismometers placed along the fiber cable will be used to compare the registered data.
“All this information can help us to develop new structural and ground stability methods which can be relevant for future building and infrastructure construction work", said Martin Schimmel.
The researchers intend to use the registered and processed data to measure the changes in the subsurface properties caused by, for example, intense rainfall episodes. They also plan to compare the ground vibration levels caused by traffic along the cable, "which may provide new data that allow us to study the urban mobility".
The researchers highlighted that, although the DAS technology has already been tested in other cities, this experiment is a unique occasion to test this methodology in a densely populated urban area such as Barcelona.
"The population density, the constant traffic, its proximity to the sea and close seismic area give this project an outstanding potential to assess the capacity of DAS technology in urban environments", concludes Jordi Díaz.
The experiment is carried out within the SANIMS project, coordinated by Jordi Díaz and Martin Schimmel.
Reintel, the company owner of the fiber optic cable used in this experiment, is the subsidary company of Red Electrica Group, operates an optical fibre network comprising more than 50.000 km of cable deployed along the electricity transmission grid and the railway network. It is the major Spanish provider of the so called “dark fiber”, a robust, meshed and redundant telecom network devoted to data transmission.
Spica Z., Perton M., Martin E., Beroza G., Biondi B., Urban Seismic Site Characterization by Fiber-Optic Seismology, 2020, Journal of Geophysical Research (Solid Earth), doi:10.1029/2019JB018656.
Lellouch A., Yuan S., Spica Z., Biondi B., and Ellsworth W. L., Seismic velocity estimation using passive downhole distributed acoustic sensing records – examples from the San Andreas Fault Observatory at Depth, 2019, Journal of Geophysical Research (Solid Earth), doi:10.1029/2019JB017533