The shallow subsurface characterization in relation to geothermal resources: Harnessing Passive Seismic Signals for Subsurface Imaging

This research illustrates how seismic interferometry using body and surface waves can be applied to various passive noise sources such as trains and borehole drilling to image and characterize the shallow subsurface. The work has a particular focus on the characterisation of  geothermal resources. The initial scenario showcases seismic interferometry applications on a dense profile of 180 seismic sensors deployed along a railway for a 20-day period within a relatively simple geological setting. The recorded train-induced vibrations are used to image shallow subsurface structures (up to approximately 2 km deep). The outcome clearly reveals different interfaces and reflective surfaces that align with geotechnical data obtained from nearby boreholes. In the subsequent case, a configuration of 100 nodes was deployed along the railway for a two-week duration, situated in a geologically complex area characterized by fault structures believed to influence the flow of hot spring fluids. Here, seismic interferometry involving body and surface waves was conducted on the vibrations generated by passing trains to image shallow subsurface structures. The findings exhibit a strong correlation between variations in S-wave velocity, imaged interfaces, and lateral changes in near-surface lithology. The final case presents the outcomes of seismic interferometry performed on a dense network of 60 nodes deployed for a short period (e.g. three hours) near a geothermal drilling site in the Dublin Basin, Ireland. A comparison between the interferometry results and seismic sonic log data in the borehole, clearly highlights the potential of this technique, known as Seismic While Drilling (SWD), both for imaging deeper subsurface (beyond the depth of the borehole) structures and for monitoring the drilling site.