In a recently published study in the journal Applied Optics by the Optica Publishing Group, researchers from Italy constructed a three-axis fiber optic gyroscope that enhances the resolution and real-time monitoring of ground rotations caused by earthquakes. By setting up this gyroscope observatory, scientists can ensure better early warning systems and risk assessment.
Researchers built a prototype fiber optic gyroscope (pictured) for high-resolution, real-time monitoring of ground rotations caused by earthquakes in an active volcanic area. The fibers are precisely wound around an aluminum spool to form a gyroscope based on the Sagnac effect. Image Credit: Saverio Avino, CNR-INO
When seismic activity occurs, the Earth’s surface experiences both linear and rotational movements, although rotations are generally very small and not usually monitored, the ability to capture them would provide a more complete understanding of the Earth’s internal dynamics and seismic sources.
Saverio Avino, Research Team Leader, Consiglio Nazionale delle Ricerche Istituto Nazionale di Ottica
The researchers disclosed initial observational data from the rotating sensor, which is based on a two km-long fiber-optic gyroscope. Over the course of five months of continuous data collection, the sensor demonstrated good performance, detecting noise and ground rotations from minor to medium-sized local earthquakes.
The Naples metropolitan area has three active volcanoes and almost three million people. A grid of multiparametric sensors covers the whole region, enabling real-time monitoring of numerous physical and chemical characteristics used to track volcanic and seismic activity.
The measurement of ground rotations will add another tile to this complex mosaic of sensors, 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.
Danilo Galuzzo, Research Team Member, National Institute of Geophysics and Volcanology
Measuring Rotational Movement
Gyroscopes are instruments that track and measure angular velocity, or the speed at which an object rotates and changes in orientation. Simple gyroscopes, for instance, are used in smartphones to monitor and detect the orientation and rotation of the device. A more sophisticated gyroscope based on the Sagnac effect was created by the researchers to quantify rotation in seismic waves resulting from an earthquake or volcanic activity.
When light travels in opposite directions around a closed loop, it exhibits differing travel periods, a phenomenon known as the Sagnac effect. As a result, there are detectable interference patterns in the light based on the loop's rate of rotation. Measuring the light interference makes high-resolution detection of the angular velocity possible.
Avino said, “Our labs are located in the heart of an active volcanic area, thus creating a natural source of earthquakes, 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.”
Capturing Seismic Activity
The researchers used common laboratory equipment and parts to create a prototype fiber-optic rotational sensor. To test it, they sent light into a 2 Km long optical fiber cable, the kind used for optical telecommunication.
The fiber cable was accurately coiled around an aluminum spool with a diameter of 25 cm to produce a coil. This loop is formed where the input and output are joined, creating a continuous light route with no breaks.
For the duration of the research, the optical sensor was housed in a controlled laboratory setting in a structure perched on a caldera; a sizable depression created 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, it also had a very good duty cycle—the time percentage the instrument is measuring/acquiring data which allowed us to run the system continuously for around five months.
Marialuisa Capezzuto, Study First Author, Consiglio Nazionale delle Ricerche Istituto Nazionale di Ottica
Capezzuto also worked on the experimental apparatus.
Luigi Santamaria Amato from the Italian Space Agency said, “The prototype gyroscope can only 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.”
The researchers intend to set up a three-axis gyroscope when the single-axis system's resolution and stability have been enhanced. In the future, they hope to establish a permanent ground rotation observatory in the vicinity of Campi Flegrei.
Journal Reference:
Capezzuto, M., et al. (2024) A fiber-optic gyroscope for rotational seismic ground motion monitoring of the Campi Flegrei volcanic area. Applied Optics. doi.org/10.1364/ao.518354.