Shallow subsurface velocity changes and hydrological responses revealed by distributed acoustic sensing

Shallow subsurface velocity variations provide key constraints for understanding the migration of soil water and associate hydrological processes. In this study, we use a distributed acoustic sensing (DAS) unit to record high-frequency ambient noise and apply ballistic wave seismic monitoring (Mi et al., 2025) to retrieve velocity variations within the upper 10 m of the shallow subsurface in Hefei, Anhui Province, China, over a two-month period. During non-precipitation periods, low-frequency (< 22 Hz) phase velocities exhibit a negative correlation with temperature, whereas high-frequency (>22 Hz) phase velocities show a positive correlation. Following precipitation events, phase velocities decrease significantly across all frequencies. Based on a reference 1-D shear-wave velocity model, we further invert the time-dependent phase velocity perturbations to obtain depth-dependent shear-wave velocity variations (Haney and Tsai, 2017). Integration with borehole observations reveals contrasting responses between shallow (<5 m) and deeper (>5 m) layers to evaporation, infiltration, and loading: diurnal temperature variations regulate soil moisture and thereby control velocity changes during dry periods, while rainfall-induced infiltration becomes the dominant factor during precipitation. Our results demonstrate the effectiveness of DAS-based time-lapse velocity monitoring for characterizing shallow soil water cycling and highlight its potential for high spatiotemporal resolution hydro-geophysical monitoring of the near surface.

Raeferences

Haney, M.M. & Tsai, V.C., 2017. Perturbational and nonperturbational inversion of Rayleigh-wave velocities, Geophysics, 82, F15–F28.

Mi, B., Xia, J. & Li, J., 2025. On the measurement of relative phase velocity changes for ballistic wave seismic monitoring, Geophysical Journal International, 234, 1-9.