Inverting thermal imagery of fumarole plumes to reveal gas fluxes

The mass and heat fluxes emitted by fumarole vents form a large portion of the total degassing budgets at volcanoes undergoing unrest, particularly at hydrothermal volcanoes. For example, the fumarole fluxes at la Soufrière represent around 78% of the total heat budget (Jessop et al., 2021). Pitot-tube instruments provide perhaps the most reliable data for fumarole fluxes but require the user to directly contact the hot, acidic, and toxic gases. Flux estimations from MultiGAS traverses still require the user to be in contact with fumarole gases but at a distance from the vent where the plume is much cooler. The errors associated with this method are substantial (c. 40%). Physically, fumarole plumes are similar to volcanic plumes produced by explosive activity in the absence of particles and hence similar models can be used (e.g. Woitischek et al., 2021). In this work, we propose using a wind-bent plume model (e.g. Woodhouse et al., 2013; Aubry et al., 2017) to match thermal images of the fumarole plume. Fumarole plumes typically are not opaque (see image) and so we develop a radiative model to account for this (cf. Cerminara et al., 2015). Our model requires the mass and heat flow rates to be specified at the vent. These parameters are retried by inverting our model with data extracted from thermal images acquired between 2017–2023 using a ground-based camera at the level of the vent. We find a slow but steady increase in vent flux over the period of study which is in keeping with data from MultiGAS and Pitot-tube measurements during the same period. The errors associated with our method are much lower than those of the MultiGAS traverse method and will allow for finer analyses of fumarole degassing data and its implications for understanding volcanic unrest. Aubry, T. J. et al. (2017). “Turbulent entrainment into volcanic plumes: new constraints from laboratory experiments on buoyant jets rising in a stratified crossflow”. Geophys. Res. Lett. 44.20, pp. 10198–10207. DOI: 10.1002/2017GL075069. Jessop, D. E. et al. (2021). “A multi-decadal view of the heat and mass budget of a volcano in unrest: La Soufrière de Guadeloupe (French West Indies)”. Bull. Volcanol. 3, p. 16. DOI: 10.1007/s00445-021-01439-2. Cerminara, M. et al. (2015). “Volcanic plume vent conditions retrieved from infrared images: a forward and inverse modeling approach”. J. Volcanol. Geoth. Res. 300, pp. 129–147. DOI: 10.1016/j.jvolgeores.2014.12.015. Woitischek, J. et al. (2021). “On the use of plume models to estimate the flux in volcanic gas plumes”. Nat. Commun. 12.1, p. 2719. DOI: 10.1038/s41467-021-22159-3. Woodhouse, M. J. et al. (2013). “Interaction between volcanic plumes and wind during the 2010 Eyjafjallajokülleruption, Iceland”. J. Geophys. Res.-Solid Earth 118.1, pp. 92–109. DOI: 10.1029/2012JB009592.