Fiber Optic Gyroscope Monitors Active Volcanoes
FLORENCE, Italy, May 29, 2024 — A research collaboration between the Consiglio Nazionale delle Ricerche Istituto Nazionale di Ottica (CNR-INO), the National Institute of Geophysics and Volcanology (INGV), and the Italian Space Agency (ASI) has built a prototype fiber optic gyroscope for high resolution, real-time monitoring of ground rotations caused by earthquakes in the active volcanic area of Campi Flegrei in Naples, Italy. The device is expected to provide greater insights into seismic activity in the region, which could provide opportunities for improved risk assessment and potentially for early warning systems.
In the study, the researchers reported preliminary observational data gathered from the rotational sensor over the course of five months. The sensor, based on a 2 km-long fiber optic gyroscope, detected noise and ground rotations from small to medium local earthquakes.
“Our labs are located in the heart of an active volcanic area, thus creating a natural source of earthquakes,” said research team leader Saverio Avino from CNR-INO. “Because we experience small/medium earthquakes almost every day, we can measure and acquire a large number of data on ground rotations, which can be successively analyzed to study seismic and volcanic phenomena of the Campi Flegrei region.”
According to Avino, seismic activity brings out linear and rotational movements that can be detected in the Earth's surface. Rotations, he said, are generally small and not typically monitored, but the ability to capture them would grant a more complete understanding of the earth's internal dynamics and seismic sources.
The researchers built a fiber optic gyroscope for monitoring linear and rotational activity in the Earth’s crust caused by seismic activity. Based on the Sagnac effect, the gyroscope is formed from fibers wound around an aluminum spool. Courtesy of Saverio Avino, CNR-INO.
Naples, a city of around 3 million people, has three active volcanoes. The area is covered by a grid of multiparametric sensors that provide real-time monitoring of various physical and chemical parameters used to study seismic and volcanic activity.
“The measurement of ground rotations will add another tile to this complex mosaic of sensors,” said research team member Danilo Galuzzo from INGV. “This additional information will also aid in the comprehensive understanding of volcanic earthquake signals, which are crucial for detecting any changes in the dynamics of volcanoes.”
Gyroscopes are devices used to detect and measure changes in orientation or angular velocity –– the rate at which an object rotates. For example, in smartphones simple gyroscopes detect and measure the device's orientation and rotation. To measure rotation in seismic waves from an earthquake or volcanic activity, the researchers developed a more complex gyroscope based on the Sagnac effect.
The Sagnac effect occurs when light traveling in opposite directions around a closed loop exhibits different travel times. This leads to measurable interference patterns in the light that depend on the rotation rate of the loop. By measuring the light interference, the angular velocity can be detected with high resolution.
The researchers assembled a prototype fiber-optic rotational sensor using standard laboratory instrumentation and components. To test it, they injected light into a 2 km-long optical fiber cable, similar to the ones used for optical telecommunication. The fiber cable formed a loop where the input and output are connected, creating a continuous light path with no breaks, and was precisely wound around an aluminum spool with a diameter of 25 cm to form a coil.
A seismic map of the Campi Flegrei volcanic area. Blue triangles mark the monitoring network’s seismic stations, while the orange circle marks the researchers’ gyroscope, and the red circles represent recorded seismic events. Courtesy of Courtesy of Saverio Avino, CNR-INO.
During the experiments, the optical sensor is kept in a controlled laboratory environment in a building that sits on top of a volcano caldera –– a large depression formed when a volcano erupts and collapses.
“This first version of the system showed a resolution comparable to other state-of-the art fiber-optic gyroscopes,” said the paper’s first author Marialuisa Capezzuto, who is from CNR-INO and worked on the experimental apparatus.
According to Luigi Santamaria of ASI, the prototype is only able to measure one of the three directional components of the rotation movement. "However, combining three of the same gyroscopes, each oriented to capture a different axis of rotation, could be used to capture all three components,” he said.
Once the researchers have improved the resolution and stability of the single-axis system, they plan to set up a three-axis gyroscope. Eventually, they want to create a permanent ground rotation observatory in the Campi Flegrei area.
The research was published in Applied Optics (www.doi.org/10.1364/AO.518354).
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