Characterising vp/vs ratios beneath geothermal systems of the Hengill volcano in Iceland using a global-local approach

The Hengill volcanic system in Iceland is of exceptional geological interest and energetic potential. Hengill sits on the mid-Atlantic ridge, on a triple junction, and close to the Icelandic hotspot. It also hosts strong geothermal activity, as apparent at the surface through pools of boiling water scattered across the flanks of the mountain (see figure attached). This geothermal activity has been exploited for electricity production and heating. 

 

While Hengill is of great geological and energetic interest, the geological processes occurring beneath the surface remain only partially understood. Recently, the site has been increasingly instrumented, in particular with large deployments of seismic nodes and with distributed fiber-optic sensing. These give us an unprecedented opportunity to understand processes at work beneath this exceptional volcanic system and shed light on new geothermal energy reservoirs. 

 

Due to their sensitivity to fluids, v_p/v_s ratios are a parameter of choice to characterise geothermal systems. However, for practical reasons, these ratios also prove difficult to estimate. In this study, we use two overlapping and complementary techniques to infer v_p/v_s ratios beneath the Hengill volcanic system. First, we use a 'local' technique: the method of double-differences to estimate v_p/v_s ratios within clusters of earthquakes. These estimates have great accuracy, but they are limited to the locations of the clusters, with a resolution the size of the clusters. Second, we use a 'global' technique: a multi-parameter implementation of Eikonal tomography to map the 3D distribution of v_p/v_s. This technique offers a global view at the scale of the volcanic system but suffers from resolution artefacts and uncertainty inherent to seismic tomography. These 'local' and 'global' approaches overlap, producing results that can be used to validate each other, and are complementary, allowing us to better characterise the Hengill geothermal system.