Exploring fracture networks beneath the Hengill Geothermal Field (Iceland) through probabilistic anisotropic P-wave tomography

Mapping of fracture networks is critical to the exploration and responsible exploitation of geothermal resources. Fractures provide the permeable pathways required for efficient heat extraction and knowledge of their subsurface distribution is necessary for optimal well placement and reservoir modelling. Additionally, fractures play a significant role in induced seismic hazards both by decreasing rock strength and by providing hydraulic connections between fluid injection/extraction sites and surrounding fault networks that may slip in response to pore pressure perturbations. However, constraining fracture distributions in 3D can be challenging. Geologic mapping provides limited information regarding how these systems evolve with depth and exploratory drilling is expensive and only provides point-wise constraints that may not reflect larger-scale trends. Seismic imaging utilising local earthquakes provides a cost-effective means to overcome these issues and map fractures at the reservoir scale. In this contribution, we constrain the anisotropic P-wave velocity structure of the Hengill Geothermal Field (Iceland) using arrival times from natural and induced seismicity. A Bayesian Monte Carlo sampling approach is used to construct likely velocity models and posterior parameter distributions from which we evaluate hypotheses for fracture properties. The imaged slow P-wave propagation directions constrain the average 3D fracture plane orientations while the degree of alignment and extent of fracturing is inferred from the strength of velocity anisotropy. Our models reveal significant spatial heterogeneity in these fracture properties throughout the Hengill geothermal system. We explore possible mechanisms behind this heterogeneity (e.g. deformation related to topographic loading, tectonic and magmatic stresses, and geothermal energy production) and its relationship to local seismicity patterns.