Monitoring Water Level of a Surficial Aquifer Using Distributed Acoustic Sensing and Ballistic Surface Waves

Groundwater resources play an increasingly crucial role in providing the water required to sustain the environment. However, our understanding of the state of surficial aquifers and their spatiotemporal dynamics remains poor. In this study, we demonstrate how Rayleigh wave velocity variation can be used as a direct indicator of changes in the water level of a surficial aquifer in a discontinuous permafrost environment. Distributed acoustic sensing data, collected on a trenched fiber-optic cable in Fairbanks, AK, was processed using the multichannel analysis of surface waves approach to obtain temporal velocity variations. A semi-permanent surface orbital vibrator was utilized to provide a repeatable source of energy for monitoring. To understand the observed velocity perturbations, we developed a rock physics model (RPM) representing the aquifer with the underlying permafrost and accounting for physical processes associated with water level change. Our analyses demonstrated a strong correlation between precipitation-driven head variation and seismic velocity changes at all recorded frequencies. The proposed model accurately predicted a recorded 3% velocity increase for each 0.5 m of head drop and indicated that the pore pressure effect accounted for approximately 75% of the observed phase velocity change. Surface wave inversion and sensitivity analysis suggested that the high velocity contrast in the permafrost table shifts the surface wave sensitivity toward the first 3 m of soil where hydrological forcing occurs. This case study demonstrates how surface wave analysis combined with an RPM can be used for quantitative interpretation of the acoustic response of surficial aquifers. Groundwater will potentially become the dominant source of fresh water as surface water accessibility deteriorates due to a warming climate. Therefore, sustainable aquifer management will become critical to changing climate adaptation. While direct measurement of water levels using wells is the primary tool in aquifer monitoring, it is inadequate for large-scale aquifer management due to its numerous limitations and intrinsic aquifer heterogeneity. We present a case study where seismic velocities recorded on a fiber-optic cable were successfully used to predict relative water table change in a shallow aquifer in Fairbanks, AK. The observed strong correlation between hydrologic processes and aquifer acoustic signatures was reconstructed using a model that accounted for physical processes associated with water level change. This study demonstrates that seismic velocities can be used to monitor shallow aquifer dynamics with adequate spatial and temporal resolution. Groundwater level variations can cause significant changes in seismic velocities Pore pressure effects are the predominant hydrological forcing responsible for surface wave velocity perturbations in surficial aquifers Relative velocity change can be accurately reproduced by a well-calibrated rock physics model