Space-time monitoring of groundwater fluctuations with passive seismic interferometry.

Historic levels of drought, globally, call for sustainable freshwater management. Under pressing demand is a refined understanding of the structures and dynamics of groundwater systems. Here we present an unconventional, cost-effective approach to aquifer monitoring using seismograph arrays. Employing advanced seismic interferometry techniques, we calculate the space-time evolution of relative changes in seismic velocity, as a measure of hydrological properties. During 2000-2020 in basins near Los Angeles, seismic velocity variations match groundwater tables measured in wells and surface deformations inferred from satellite sensing, but the seismological approach adds temporal and depth resolutions for deep structures and processes. Maps of long-term seismic velocity changes reveal distinct patterns (decline or recovery) of groundwater storage in basins that are adjacent but adjudicated to water districts conducting different pumping practices. This pilot application bridges the gap between seismology and hydrology, and shows the promise of leveraging seismometers worldwide to provide 4D characterizations of groundwater and other near-surface systems.

Observing the subglacial hydrology network and its dynamics with a dense seismic array.

Subglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming.