Spatiotemporal monitoring of soil moisture dynamics from Rayleigh-wave attenuation: A controlled field experiment

Seismic attenuation provides a highly sensitive constraint on fluid-driven processes in the shallow subsurface. These attenuation-derived spatiotemporal insights complement conventional seismic velocity monitoring and can be used for environmental monitoring and engineered subsurface infrastructure management. In this study, we implemented time-lapse Rayleigh-wave attenuation measurements during controlled shallow water injections to quantify the coupled evolution of seismic attenuation and pore-fluid infiltration. The monitoring experiment was conducted over a 14-day period at a localized test site where two vertical wells were hydraulically connected by a permeable pipeline. The frequency-dependent Rayleigh-wave attenuation coefficients are estimated from spectral-ratio slope fitting of multichannel active-source surface-wave records. These measurements are subsequently combined with phase velocities and S- and P-wave velocities to invert for depth-dependent energy dissipation factors  and  within a layered medium. The resulting attenuation variations are interpreted as proxies for changes in fluid saturation and hydrological properties in the shallow subsurface. The attenuation images clearly delineate the boundary between the pipeline and the surrounding medium and exhibit pronounced temporal variations driven by injection-induced fluid migration. Daily time-lapse variations of attenuation over the 14-day experiment reveal frequency-dependent responses to the intermittent injection schedule, with peak values near the period of maximum injection. These patterns reflect the migration and redistribution of pore fluids within the near-surface formation. The inverted Q images further identify localized low-Q zones around the pipeline and the two wells, indicating enhanced energy dissipation associated with fluid accumulation and increasing saturation. This study establishes a powerful framework for monitoring fluid migration and its physical impacts from time-lapse seismic attenuation. Our results highlight the importance of attenuation-based imaging for advancing high-resolution characterization of near-surface hydrological and engineered subsurface environments.