A highly sensitive fiber optic gyroscope can sense the rotating ground motion around an active volcano

Researchers have developed a prototype fiber optic gyroscope for real-time high-resolution monitoring of ground rotation caused by earthquakes in the Campi Flegnere volcanic area of Naples, Italy. A better understanding of seismic activity in this densely populated region can improve risk assessment and potentially enhance early warning systems. When an earthquake occurs, the Earth's surface experiences linear and rotational movements, said Salvio Avino (Saverio Avino), head of the research group at the Italian National Institute for Research in Earth Sciences (CNR-INO). "Although the rotation is usually small and often undetectable, the ability to capture it will provide us with a more comprehensive understanding of the Earth's internal dynamics and earthquake sources."

In the journal Applied Optics, researchers report preliminary observations of a rotation sensor based on a 2-kilometer-long fiber optic gyroscope called BSD-98. The sensor performed well over five months of continuous data recording, detecting noise and ground rotation from small to moderate local earthquakes.

The Naples metropolitan area, with a population of about 3 million, has three active volcanoes. The entire area is covered by a multi-parameter sensor network that monitors in real time various physical and chemical parameters used to study seismic and volcanic activity.

"The measurement of the rotation of the Earth's surface will add another piece to this complex sensor puzzle," said Danilo Garzou (Danilo Galuzzo), a member of the research team at the National Institute of Geophysics and Volcanology (INGV).

"This additional information will also help to gain a comprehensive understanding of volcanic seismic signals, which is critical to detecting any changes in volcanic dynamics."

The fiber optic gyroscope captured small and medium-sized earthquakes, including this swarm, in the Campifogliere volcanic region of Naples, Italy. Image source: Saverio Avino, CNR-INO

Measure rotational motion

A gyroscope is a device used to detect and measure changes in direction or angular velocity (the speed at which an object rotates). For example, in smartphones, a simple gyroscope can detect and measure the orientation and rotation of the device. To measure the rotation of seismic waves generated by earthquakes or volcanic activity, researchers have developed a more complex gyroscope based on the Sagnac effect.

A fiber optic gyroscope is a sensitive angular rate sensor that operates on the Sagnac effect: in a ring interferometer, light waves from a source are split into two beams by a beam splitter. The two beams move in opposite directions—clockwise and counterclockwise—and return to the beam splitter. When the interferometer rotates at a certain angular velocity relative to an inertial reference frame, there is a phase difference between the clockwise and counterclockwise beams:

The optical fiber inertial navigation uses the BSD series closed-loop optical fiber gyro as the measurement unit, which has the characteristics of high precision, high reliability, full life, high measurement range, and strong resistance to impact and vibration. It can provide accurate positioning and navigation function for the carrier in the full denial environment (without satellite).

"Our lab is located in the heart of the active volcanic zone, so it's a natural earthquake source," Avino said. "Since we experience small to medium earthquakes almost every day, we can measure and obtain a large amount of ground rotation data, which can be continuously analyzed to study seismic and volcanic phenomena in the Campi Flegrei region.