Fibre optic distributed sensing of geothermal reservoirs: first results from the University of Leeds Geothermal Campus boreholes

Fibre optic distributed sensing is an emerging technology that allows long-term high-resolution subsurface monitoring. We report on experimental fibre optic surveys using distributed acoustic sensing (DAS) in boreholes in the University of Leeds campus. These boreholes were drilled during 2024 as part of plans to decarbonise campus heating and cooling operations. Drilled to depths up to 250 m, they investigate the Elland Flags and Rough Rock sandstones and their suitability for geothermal reservoirs and include fibre optic instrumentation for DAS monitoring. DAS data, recording in autumn 2024, show that seismic arrivals are visible through the 250 m depth extent of the borehole, and seismic velocities can be derived to ~160 m depth. Furthermore, these appear consistent (to within ±100 m/s) with velocities observed in wireline sonic logs. With additional insight from ambient noise tomography, DAS monitoring could potentially detect local changes in seismic velocity associated with changes in elasticity of the reservoir’s pore water on heating. These data provide a valuable baseline ahead of planned thermal testing and subsequent system operation.