Heat flux-based strategies for the thermal monitoring of sub-fumarolic areas: Examples from Vulcano and La Soufrière de Guadeloupe

Although it is relatively easy to set-up, the monitoring of soil temperature in sub-fumarolic areas is quite rarely
used to monitor the evolution of hydrothermal systems. Indeed, measurements are highly sensitive to environmental
conditions, in particular daily and seasonal variations of atmospheric temperatures and rainfalls, which
can be only partially filtered by the established statistical analysis. In this paper, we develop two innovative processingmethods,
both based on the computation of the heat flux in the soil. The upward heat flux method (UHF),
designed for dry environments, consists in computing both the conductive and convective components of the
heat flux between two thermocouples placed vertically. In the cases of wet environments, the excess of total
heat method (ETH) allows the integration of rain gauges data in order to correct the heat balance fromthe superficial
cooling effect of the precipitations. The performances of both processing techniques are faced to established
methods (temperature gradient and coefficient of determination) on soil temperature time series from two test
volcanoes. At La Fossa di Vulcano (Italy), the UHF method undoubtedly detects three thermal crises between
2009 and 2012, enabling to quantify not only the intensity but also the precise timing of the heat flux increase
with respect to corresponding geochemical and seismic crises. At La Soufrière de Guadeloupe (French Lesser Antilles),
despite large rainfalls dramatically influencing the thermal behavior of the soil, a constant geothermal heat
flux is retrieved by the ETH method, confirming the absence of fumarolic crisis during the observation period
(February–August 2010). Being quantitative, robust, and usable in almost any context of sub-fumarolic zones,
our two heat flux-based methods increase the potential of soil temperature for the monitoring, but also the general
interpretation of fumarolic crises together with geochemical and seismological observations. A spreadsheet
allowing direct computation of UHF and ETH is provided as supplemental material.