Seismic Evidence for Sustained Gas Migration in a Serpentinizing Aquifer

The production and transport of hydrogen during low-temperature peridotite alteration are essential for topics ranging from the origin of life to alternative energies, but are challenging to detect and monitor in situ . We present evidence from an ultra-dense geophone and borehole-hydrophone array at the Oman Drilling Program Multi-Borehole Observatory that gas fluxes from low-temperature alteration generated harmonic tremors that persisted for nine months of observation. The tremors consist of twelve harmonic modes with time-varying frequencies up to ~ 191 Hz, including a robust diurnal frequency variation of ~ 0.1%. The tremors are most likely generated by the resonance of a two-phase (gas and water) fluid in a meter-scale fracture in the aquifer. The fluid needs to contain at least 70% gas by volume (~ 10 ^− 4 –10 ^− 3 by mass) to generate the observed quality factors and frequencies of the tremor signals. Our results indicate that low-temperature peridotite alteration generates sustained free-gas transport through well-established flow paths within the fracture network, while suggesting that seismological techniques may be promising tools for characterizing these processes.