Microbubbles catch a seismic wave

Trapped in Earth’s crust are vast quantities of methane, carbon dioxide, and other types of gas, much of it stored as microbubbles in the fluid-filled pores of rocks and ice. When that gas escapes to the surface, the consequences can be severe: Greenhouse gases released by melting permafrost, for example, threaten to accelerate global warming. A decade ago, researchers in the UK and Israel suggested that one might be able to identify subterranean stores of gas in the patterns of seismic waves. As microbubbles expand and contract under the oscillating pressure of a passing seismic wave, the thinking went, they should dissipate energy and attenuate the wave. A team led by Nicola Tisato of ETH Zürich and the University of Toronto has now reproduced that effect in the lab. The researchers submerged a porous, cylindrical block of Berea sandstone in a chamber filled with nitrogen- and CO₂-rich water. (The image shows Tisato working on the chamber.) When they then modulated the compressive force along the cylinder’s axis—to simulate a seismic wave traveling 1–2 km below ground—the cylinder periodically expanded and contracted, but with a slightly delayed phase. From the phase delay, Tisato and company could infer the rate of energy dissipation. That rate peaked at frequencies of about 5 Hz, just as the bubble-attenuation theory predicts. The researchers estimate that in a real-world scenario of, say, a tremor passing through a CO₂ reservoir in a carbon sequestration facility, bubble-induced attenuation would be strong enough to decrease the wave’s amplitude by nearly a third. (N. Tisato et al., Geophys. Res. Lett. 42, 3880, 2015. )