Monitoring shallow geothermal aquifers with active-source distributed acoustic sensing: A prognostic feasibility study on the University of Leeds Geothermal Campus

The United Kingdom's shallow aquifers offer significant but underdeveloped geothermal energy resource. Seismic methods can help address this by improving the understanding of aquifer heterogeneity and potentially allowing long-term monitoring of thermal plumes. Here, we consider the derivation of seismic velocity and attenuation models from borehole distributed acoustic sensing (DAS) data, recorded on the University of Leeds Geothermal Campus. We record active-source seismic shots in a 250 m-long vertical cable that samples two fractured sandstone aquifers (Elland Flags and Rough Rock). Compressional wave velocities (vP) are obtained to 160 m depth and are typically 3100 ± 50 m s−1 through the Elland Flags. To predict the value of vP for thermal monitoring, we simulate groundwater warming in a Hashin–Shtrikman petrophysical framework, which suggests that ±50 m s−1 precision is sufficient to detect heating >26°C (>14°C above ambient temperature). Although this is at the upper range of likely temperature change in a shallow geothermal system under typical usage, UK environmental legislation restricts heating to <25°C; therefore, our approach could be used to support the monitoring of operational compliance. Refining both the DAS acquisition and measurement of seismic quantities (vP and quality factor) should improve precision and thus facilitate monitoring of more subtle heating expressions.