Tracking groundwater level in the Perth Basin using WA-Array and passive seismic interferometry

Groundwater level variations alter pore pressure within the rock formation and induce measurable changes in seismic velocity.  Using continuous passive seismic data from Phase 1 of the WA-Array, Western Australia, we estimated time-lapse velocity changes in the Perth Basin and used it for tracking groundwater level throughout 2023. By analysing early coda waves from ambient noise cross-correlations, we revealed shear-wave velocity changes (dv/v) up to 0.4% with an annual cycle. The velocity increases during summer and decreases in winter, corresponding to Perth’s dry and wet seasons. These temporal velocity variations show a clear inverse correlation with the groundwater level measurement from borehole, as higher groundwater levels increase pore pressure and reduces seismic velocities. Spatial mapping of the velocity changes using coda-wave sensitivity kernels shows in general consistent results, however, the resolution is limited by the large station spacing (~40 km). Complementary to traditional hydraulic-head observations, our results demonstrate the importance of having long-term seismic networks (arrays) in metropolitan area for cost-effective long-term groundwater level monitoring.