Subsurface geothermal monitoring using fiber-optic technologies within an urban environment

In urban environments, shallow geothermal heating and cooling systems can play a crucial role in the transition towards renewable energy sources. One such site (USquare) is a transformed military barracks in Brussels, where over one hundred boreholes were drilled (~120 m) and equipped with heat exchangers as part of a low-enthalpy geothermal heating network for a multi-use urban development project. Fourteen of these were equipped with fiber optic cables that can provide continuous temporal monitoring of downhole conditions during operation. This includes Fiber Bragg Grating sensors (FBGs), providing point measurements of temperature, and Distributed Acoustic Sensing (DAS) towards recording strain along the length of the fiber. 

In this work, we present the experimental setup and initial results for subsurface monitoring at USquare. We compute the root mean square (RMS) amplitude of seismic noise across all channels, with promising results when compared with known hydro-geological logs. Consideration is also given to the impact of the urban environment on the stability of measurements due to variability in anthropogenic seismic sources. Finally, we show preliminary results applying seismic noise interferometry to downhole measurements. This includes computing auto-correlations from the individual channels and also cross-correlations with surface geophones. These findings highlight the potential of fiber optic sensing technologies for monitoring geothermal operations in urban environments, paving the way for more sustainable energy solutions.