A recent paper published on nature.com describes how researchers can remotely use fibre optic cables to observe geological activity on volcanoes. How can fibre optic cables be used to monitor vibrations, what did the researchers achieve, and how will this change geological monitoring of the future?

How can fibre optic cables be used to monitor vibrations?

Fibre optic cables have been one of the most important inventions in the field of telecommunications due to their extremely large bandwidth, immunity to electrical noise, and ease of installation. Interestingly, there is a myth that fibre optic cables are better than copper as photons travel at the speed of light, whereas electrical current doesn’t. However, the reality is that signals travel slower in fibre optic cables compared to copper due to the nature of photons travelling through solid materials (the speed of an electrical pulse down a cable is 2.3 × 108 m/s while photon pulses are 2.0 × 108 m/s down an optic fibre cable). The reason for the increased bandwidth for using fibre-optic cables comes from their ability to switch significantly faster, the use of independent light spectrums that do not interfere, and their significantly higher signal to noise ratio.

However, it turns out that fibre optic cables can be used for more than just data transmission, with researchers often using buried fibre optic cables to monitor vibrations in the ground. Simply put, small vibrations in a fibre optic cable change the internal structure of the cable, and these minute changes affect how photons travel down the cable.

The use of very sensitive receivers can detect these minute changes, and reflections from pulses can be used to determine the nature of the vibration. This method of detecting vibrations, called Distributed Acoustic Sensing, has been used by researchers in experiments for years thanks to the help of telecom companies who have buried large amounts of fibre optic cables that remain unused. During their initial installation, it was quickly discovered that the expensive cost of labour to bury cables made it economical to install far more cables than needed for future growth.

Researchers use fibre optic cables to monitor volcanic activity

Taking advantage of vibration detection in fibre optic cables, researchers recently used fibre optic cables to observe geological activity near Mount Etna in Italy. The goal behind the research was to overcome some of the many challenges when using seismic detectors around active volcanoes, including a limited number of sensor locations, uncertainty in the underlying ground, the dangerous conditions, and the detection of subtle changes in the ground.

Under normal circumstances, scientists would turn to unused buried fibre optic cables to perform research, but as Mount Etna does not have any telecommunication infrastructure, the researchers had to bury their own cable. The 1.3km cable was buried between 15cm and 25cm into the ground, located around 2km from 5 active craters. The use of an interrogator (intelligent distributed acoustic sensing) allowed for the detection of changes in the cable.

Overall, the researchers were able to distinguish key events at the volcano, including small eruptions, shifts in the Earth’s crust, localised tectonic activity, and atmospheric phenomena, including hail and thunderstorms. To validate the data, seismometers and geophones were used to monitor Mount Etna and confirm changes in the optical signals resulting from the geological activity.

How can optic fibres change the field of geology?

While the ability of fibre optic cables to monitor geological activity is useful in its own right, the ability to do such monitoring remotely in particularly harsh environments is highly advantageous for potential early warning systems. The ability for the entire length of an optical cable to be a detector helps improve sensitivity to geological activity and allows for more precise mapping of underground faults.

Such sensors may find themselves used in dormant and active volcano sites that pose a significant threat to nearby populations who may rely on the fertile grounds, location, or accessibility. The ability to pick up extremely small vibrations also gives the potential for the optic cables to detect activity well in advance of earthquakes which is currently impossible to do (due to the fact that earthquakes happen without warning).

What the researchers are doing with fibre optic cables is nothing new, but using them for geological purposes could provide key insights into the Earth’s geology while providing humanity with an early warning system for devastating natural disasters.