The research, published in the journal Science Advances, was carried out by a team of scientists from Pennsylvania State University. One of the co-authors is Polish geophysicist Rafał Czarny, PhD, currently affiliated with the Institute of Mine Seismology in Australia.

As the researcher told PAP, the whole process starts with lightning. A lightning discharge heats the surrounding air very strongly. The air expands rapidly, generating a shock wave that then transforms into an acoustic wave we perceive as thunder. After reaching the surface, part of the energy of the acoustic wave couples with the ground, exciting seismic waves, including Rayleigh surface waves. This creates very small vibrations called thunderquakes.

These vibrations proved interesting to scientists. Seismic waves are one of the basic tools for imaging the geological structure of the subsurface. Depending on the materials they encounter, they travel at different speeds. By recording these differences, it is possible to reconstruct the arrangement of underground layers and structures.

Typically, the sources of these waves are natural earthquakes or vibrations intentionally induced during geophysical surveys. However, the first solution is not universal because not all regions experience earthquakes. The second requires specialised equipment and field work, which increases costs and complicates research.

Scientists therefore decided to check whether storms could provide an additional, natural source of waves. They used a huge dataset collected on the campus of Pennsylvania State University. For more than two years, ground vibrations were recorded there using an approximately 4-kilometre fibre-optic cable running, for example, under sidewalks and streets.

‘Instead of classic seismometers and geophones, we used the Distributed Acoustic Sensing (DAS) device’, Czarny said. When connected to a fibre optic cable, the device sends laser pulses and analyses the light returning from the fibre. Ground vibrations induced by thunder minimally deform the optical fibre and change this signal, so they can be detected.

The scientist added that the great advantage of this method is that one long optical fibre can replace a very large number of separate seismic sensors. Instead of deploying multiple seismometers, scientists can take measurements at many points on the same cable.

‘In our case, a 4-kilometre fibre optic cable provided 2,137 measurement points spaced approximately every 2 meters. Thanks to this, we could precisely trace the speed at which seismic waves propagated in particular places, and on this basis reconstruct the structure of the subsurface’, Czarny added.

In total, scientists collected almost 180 terabytes of data. Detailed analyses allowed them to select 458 particularly distinct records of thunderquakes. They verified each of them using data from the American National Lightning Detection Network (NLDN), which provided information about the time and location of the discharge and its peak current.

They then compared many such events and analysed how quickly the thunder-induced seismic waves propagated along the optical fibre. On this basis, they reconstructed the speed distribution of transverse waves in the substrate to a depth of approximately 100 metres.

Importantly, imaging revealed four zones of reduced wave speed, interpreted as weakened, highly cracked or weathered parts of the karst substrate. Some of these structures had not previously been recognised.

According to Czarny, this stage was crucial for the entire method. If the obtained image had no connection with the actual geology of the area, it would be difficult to conclude that thunder actually allows scientists to see beneath the ground. Meanwhile, the results were consistent with independent data from boreholes and the results of MASW (Multichannel Analysis of Surface Waves) geophysical research, in which the structure of the shallow subsurface is determined based on the propagation of surface waves. Moreover, some of the detected weakening zones coincided with areas of surface deformation observed with InSAR satellite radar interferometry.

‘This confirmed that we were seeing real structures’, Czarny says.

The scientist also said that the method developed and verified by the team does not mean that geologists will now wait for a storm instead of conducting classic seismic research.

‘We already have good subsurface imaging methods; this new technique will likely be niche. It could prove useful primarily in areas where traditional seismic wave sources are difficult to deploy, or where natural earthquakes are very rare’, he said.

He gives the example of the eastern United States; it is far less seismically active than the West but experiences frequent storms. He also mentions Tasmania’s dense forests, where laying fibre-optic cable would be easier than bringing in heavy machinery or sending a crew into hard-to-reach terrain.

In the long run, the method could also find applications beyond Earth. If another celestial body experienced atmospheric phenomena capable of generating surface waves, recording them could provide information about the structure of deeper layers.

‘That is, of course, a prospect for the distant future. But our results show that we do not always need to directly induce ground vibrations to learn about subsurface structure. We can harness energy provided by the atmosphere’, Czarny concludes.

Rafał Czarny is a geophysicist and mining seismologist who specialises in analysing seismic activity in mines worldwide. His research interests also include seismic imaging, Distributed Acoustic Sensing (DAS), and the use of natural and anthropogenic seismic noise to study subsurface structures. His research on thunderquakes was one of several topics he pursued during his postdoctoral fellowship at Pennsylvania State University. He currently works at the Institute of Mine Seismology in Australia. (PAP)

PAP - Science in Poland, Katarzyna Czechowicz

kap/ bar/

tr. RL