Researches have managed to create a submarine seismic network by utilizing communications lines established in the North Sea and connected to the shore.
Monitoring seismic activity undersea is a challenging task. Normally, it would require the installation of a wide sensor network across a vast area, a project that would not be practical or financially-efficient.
To address this difficulty, a scientific team, led by the California Institute of Technology (Caltech) deployed an intelligent method that involves the utilization of existing communications cables in the oceans to create virtual seismic sensors. In particular, researchers established an array of 4,000 sensors in a 40-kilometer fiber optic cable that links the shore with a wind farm in the North Sea.
A study on the North Sea project was published in Nature Communications in December 2019. "Fiber optic communications cables are growing more and more common on the seafloor. Rather than place a whole new device, we can tap into some of this fiber and start observing seismicity immediately," Ethan Williams, lead author of the study and a graduate student from Caltech, stated. "With the flip of a switch, we have an array of 4,000 sensors that would've cost millions to place," Williams added.
Scientists used a photonic sensing technology capable of converting typical communications fiber into an array of distributed acoustic sensors known as DAS. Those sensors were initially used for energy exploitation purposes but they have proved to be useful for seismology endeavors. The technology is based on manipulating minuscule defects in the communication cables which reflect small amounts of light and act as "waypoints". During a seismic incident, those points slightly alter their position and the travel time of the reflected light subtly changes, allowing scientists to detect the propagation of a seismic wave.
The sensitivity of the network is sufficient to detect seismic noise that is not associated with earthquakes known as microseisms. Therefore, scientists gathered data and found evidence that this noise is associated with ocean waves and could be proof of the existence of Scholte waves, surface waves that propagate at an interface between a fluid and an elastic solid medium, on the seafloor.