Constraining water dynamics through unsaturated and saturated zones using fiber-optic seismic sensing data

Understanding the movement of water from the land surface through the vadose zone into groundwater is critical for studying the hydrologic cycle and predicting the Earth's critical zone response to extreme weather events. While recent research has shown that seismic velocity changes are largely linked to the groundwater variation, the role of vadose zone water and its interactions with groundwater has often been overlooked. Here, we use high-density seismic sensors by employing distributed acoustic sensing (DAS) with fiber-optic cables to estimate seismic surface wave phase velocity variations. This allows us to constrain annual water dynamics in both unsaturated and saturated zones. We incorporate rock physics modeling to analyze shear wave velocity variations, to identify key physical parameters - such as saturation, pressure, and fractures - that best describe the water movement in response to rainfall. Our findings reveal that changes of pore pressure and saturation in the unsaturated zone resulting from precipitation influence high-frequency velocity changes. In the saturated zone, dynamic pore pressure induced by groundwater levels and crack opening or closure are primarily drivers for low-frequency velocity changes. Additionally, we observe the hysteresis in velocity variations between drying and wetting cycles over different time periods, implying fluid redistribution in pores and crack opening or closure responding to drying and wetting processes. Our findings have important implications of using DAS with existing fiber-optic cables for understanding how the critical zone responds to future climate events, such as extreme weather conditions like rainstorms and drought.