Seasonal Seismic Velocity Variations Measured Using Seismic Noise Autocorrelations to Monitor the Dynamic of Aquifers in Greece

Abstract

Monitoring groundwater levels in aquifers is crucial for water resources management on a global scale. In Greece, water stress is particularly high due to the expansion of agricultural land, urbanization and tourism, leading to aquifer over‐exploitation. In this study, we investigate the possibility of monitoring groundwater reservoirs from seismic velocity changes (δv/v) measured using noise autocorrelations. The results obtained at short periods (0.5–3 s) indicate that in several regions, seismic wave velocity varies according to an annual cycle mainly controlled by aquifer loading and discharge. We also predict seismic velocity variations from a simple model using precipitation records. In addition, this model provides a proxy for the characteristic time of aquifer discharge rate and allows us to distinguish shallow alluvial aquifers (∼100 m) from deep karst systems (∼1 km). Finally, we study the effect of water pumping from groundwater reservoirs. To that end, we combine Global Positioning System (GPS), hydraulic heads and velocity variations measurement. The results indicate that confined and unconfined aquifers respond differently. While for unconfined aquifers, pumping water implies an uplift of the surface, we show that for confined aquifers a subsidence occurs, due to the consolidation of the sediments at depth. Being an in‐situ and volumetric measurement, the δv/v is primarily sensitive to the amount of water stored in the entire aquifer system, whereas GPS measurements provide the deformation at the surface induced by both deep sediment compaction and water level variations. These observations show that GPS, hydraulic head, and δv/v are complementary tools to monitor aquifers.