Monitoring groundwater flow using fibre optic sensing

Locating and monitoring groundwater flow is key to understand the health of aquifers, and identify possible pathways for groundwater flooding events that can affect citizens and infrastructures. Geophysical methods such as Electrical Resistivity Tomography have been widely used to image the location of water-saturated areas in the shallow underground (<<1 km), but are mostly limited to local, temporary deployments. Seismology has been successfully used to sense groundwater-related tremor, and seismic stations are well-suited to long-term deployments, but dense, local seismic networks are difficult and costly to deploy.

In this work, we are going to test the potential of Distributed Acoustic Sensing (DAS) on telecom fibres for monitoring groundwater flow. Telecom fibre cables are readily available in many areas where groundwater monitoring is key, such as densely populated cities and major roads, and DAS offers the possibility to achieve great spatial resolution (down to 1 m) with little deployment cost (provided access to the fibres).

Firstly we test the capabilities of DAS compared to traditional seismometers by converting seismic records to virtual DAS. We use records from a dense deployment over an underground, water-filled cave in County Roscommon, Ireland recorded in 2020 by DIAS. By rotating and differentiating the horizontal seismic velocities, we simulate the geometry, spatial averaging and strain rate from a virtual DAS cable that follows the seismometer profiles. Our results show that horizontal, axial strain can successfully sense the weak groundwater-related tremor picked up by the seismometers and identify its frequency content. Secondly, we use the Amplitude Source Location Method to track the source location of the tremor on both the seismometer and the simulated DAS data to assess DAS performance in locating subterranean groundwater flow.

Finally, we will present the results from a DAS survey on telecommunication fibres in Ireland to map groundwater flow we will perform in February 2025. The acquisition will target both well-known water bodies (rivers) along the fibre path for benchmarking as well as areas of past groundwater flooding along major roads in County Galway, Ireland. This experiment will be one of the earliest uses of DAS to locate and monitor groundwater flow, setting the stage for the use of optical fibre networks as a tool for high-resolution, long term aquifer and flood monitoring in urbanised areas.