The rumble of thunderstorms can reveal the structure of the ground down to about 100 meters deep, thanks to simple buried optical fibers.
Thunder does more than just make the air vibrate. Some of its energy reaches the ground and creates weak seismic waves. Researchers led by Pennsylvania State University have shown that these vibrations can be used to examine what lies beneath the surface, without causing an explosion or waiting for an earthquake.
Wave paths during the generation of a thunderquake by thunder.
Direct acoustic waves from the lightning strike (T1 and T2) couple with the ground and the environment, creating body waves and surface waves. The sum of all these waves makes up the wavefield of the observed thunderquake.
The team used about 4 km of a telecommunications cable already buried beneath the University Park campus in Pennsylvania. A laser sent through the fiber detected tiny deformations in the cable. It acted like a long series of sensors, with measurements taken every few meters and hundreds of times per second.
When a clap of thunder occurs, its sound waves reach the ground at different locations. They generate vibrations there that the researchers call “thunderquakes,” literally earthquakes caused by thunder. Their propagation depends on the properties of the layers they pass through, making it possible to infer the structure of the subsurface.
The researchers analyzed 2.5 years of continuous recordings. They selected 458 high-quality events, verified using lightning data. By combining numerous signals, they were able to track seismic waves propagating near the surface and reconstruct an image of the ground beneath the campus.
This image reaches a depth of about 100 meters. It reveals several weak zones that had not previously been detected. Some correspond to deformations observed at the surface by satellite radar. Measurements from boreholes and engineering studies were also used to verify the results.
The method could be useful in places where natural earthquakes are rare. Conventional techniques sometimes require specially deployed seismic equipment or artificial sources. Here, the storm provides the energy and an existing telecommunications cable performs the measurements, limiting fieldwork.
In practice, this technique could help study risks such as subsidence or landslides. It could also help locate groundwater, monitor mining areas, or examine regions that are difficult to access. Its effectiveness nevertheless depends on the presence of thunderstorms and usable fibers.
The researchers now want to test the method on a larger scale and under different weather conditions.