Tracking surficial aquifer state using DAS and ballistic Rayleigh waves

Long-term monitoring of groundwater level and other geological properties is a key requirement for safe and sustainable aquifer development. Temporal perturbations in seismic velocity observed in measured data have shown to have a strong correlation with water head but their quantitative interpretation has remained uncertain. We present the results from a field experiment where surface wave monitoring has been used to monitor near surface hydrogeophysical state using distributed acoustic sensing (DAS). The observed variations in Rayleigh wave velocity have been linked to fluctuating groundwater level and underlying pore pressure and partial saturation effects, both incorporated into a 2D rock physics model. The modeling results support the observed low sensitivity to underlying permafrost properties due to high velocity contrast and shallow boundary between the aquifer and the permafrost. A narrow band of frequencies are sensitive to near-surface hydrologic state, also accurately predicted by the model. Even though the estimated velocity changes are lower than experimentally measured, we believe that with better model calibration, perturbations in velocity can be accurately inverted to recover groundwater level variations.