Evaluating the performance of distributed acoustic sensing for crosswell seismic imaging of abandoned mine geothermal systems

We explore the performance of distributed acoustic sensing (DAS) for crosswell seismic imaging at a shallow geothermal project in an abandoned mine. The UK Geoenergy Observatory (UKGEOS) research facility in Glasgow has repurposed an abandoned coal mine below the city with the goal to investigate the heat storage and heat recovery potential of flooded mines. Originally for distributed temperature sensing purposes, UKGEOS installed fiber-optic cables in boreholes less than 100 m deep passing through the mined coal seams now targeted for heat production. We first conduct a crosswell active-source seismic survey prior to heat pump installation to obtain a baseline velocity structure of the coal seams and surrounding lithologies. We acquire data simultaneously with DAS and a colocated hydrophone array to facilitate a comparison with conventional crosswell methods. We find that the noise levels are significantly higher for DAS, making first arrivals discernible only within a 30 m vertical offset between shot and channel depths, which limits the DAS monitoring depth range along the borehole. However, the DAS coda provides much greater spatial resolution that can capture refracted arrivals, allowing for accurate and precise identification of discrete layers. Future repeated surveys with DAS can thus shed important light on any changes within the seismic velocity structure between the boreholes, indicating potential changes in fracture zones and fluid pathways. Just as repurposing abandoned coal mines for geothermal energy has been shown to be a promising alterative geoenergy source, DAS could be an inexpensive and simpler alternative to drilling monitoring wells and deploying conventional seismic arrays.