The source is free and arrives on schedule most summer afternoons. Penn State geophysicist Tieyuan Zhu's team reports Friday in Science Advances that buried telecommunications fiber can turn ordinary thunderstorms into images of the shallow ground beneath a city, down to about 100 meters. [1][3]

The method is distributed acoustic sensing, and it is elegantly indirect. The researchers pulsed a laser down roughly 2.5 miles of pre-existing cable buried about a meter beneath about four kilometers of State College, Pennsylvania. When thunder's acoustic pressure waves couple into the ground, they induce tiny strains that stretch the fiber minutely, shifting the phase of backscattered light, a detectable stutter sampled hundreds of times per second at intervals of a few meters along the glass. [1][2]

Two and a half years of recordings yielded more than 450 thunderquakes, which the team converted into tomographic images of the campus subsurface. Penn State sits on karst, limestone riddled with caverns, and the images revealed previously undetected weak zones, corroborated against borehole logs and satellite-measured surface deformation, precisely where sinkholes might one day open. [1]

Lead author Nolan Roth calls it the first successful seismic imaging using thunderquakes; the technique suits places earthquakes rarely visit, and one storm's signal covers tens of square kilometers at zero marginal cost. [2] Zhu muses about running the same trick on Titan, which has megastorms and no fiber. [1] Cities have both. They simply never asked their phone lines to listen.

-- KENJI NAKAMURA, Tokyo