报告内容简介
Water in the critical zone is vital but its current monitoring techniques are mostly for surface water content with few probing the vadose zone. To fill in this gap, timelapse seismology is a complimentary tool but suffers from large conventional sensor spacing. By transforming existing optic-fiber cables into thousands of seismic sensors, the emerging distributed acoustic sensing (DAS) technology provides an affordable and scalable solution for deploying large aperture and ultra-dense seismic arrays, thus feasible for the vadose zone monitoring. With two years of ambient noise recorded on the Ridgecrest DAS array, the time-lapse images of seismic changes (dv/v) reveal an unprecedented high-resolution spatiotemporal evolution of water saturation in vadose zone. A striking correlation between the dv/v amplitude and the sedimentary thickness is observed, while the frequency analysis of dv/v measurements suggests an uppermost 10 m hydrologic source as the cause for dv/v temporal variability. Our results indicate the great potential of DAS for long-term subsurface water monitoring.
