Hydrothermal fluid flow disruptions evidenced by subsurface changes in heat transfer modality: The La Fossa cone of Vulcano (Italy) case study

Abstract

International audienceDetecting volcanic unrest is of primary importance for eruption forecasting, especially on volcanoes characterized by highly dangerous , and often seemingly unpredictable, phreatic or phreatomagmatic eruptions. We present a simple and innovative analysis of shallow vertical temperature profiles to depths of 70 cm. These data were recorded at La Fossa cone of Vulcano (Aeolian Islands, Italy), during an episode of increased hydrothermal and seismic activities that occurred between September and December 2009. This work involves the use of the coefficient of determination (R 2) on vertical temperature profiles in order to identify changes in conductive versus convec-tive heat transfer modality. The increase in convective heat transfer can be related to the disruption of the hydrothermal system due to its pressurization and/or variation of ground permeability between the hydrothermal system and the surface. While raw temperature data do not evidence any significant variation during the period investigated and the classic temperature gradient is highly influenced by seasonal variations, the fluctuation of R 2 displayed striking spikes that coincided with the seismic swarm inside the volcanic edifice. Such a low-cost device associated with easy real-time data processing could constitute a very promising, yet deceptively simple, technique to monitor hydrothermal systems, in order to assess the hazard posed by high-energy eruptions for populations living close to active volcanoes

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

“…Temperature measurements in a steady state allow describing the vertical structure of sub fumarolic areas in three successive zones (Aubert, 1999;Gaudin et al, 2013;Ricci et al, 2015). In the deepest zone (convective zone, see Fig.…”

“…. For weaker fluxes, atmosphere dra matically cools the most superficial layers of the ground and steam con denses mostly in the soil in the so called sub fumarolic zones (Aubert, 1999), and a significant fraction of the heat flux is transported to the surface by conduction (Sekioka and Yuhara, 1974;Gaudin et al, 2013Gaudin et al, , 2015Ricci et al, 2015).…”

“…Being at the same time cheap and reliable, ground temperature re mains the most widely used technique for thermal monitoring (Peltier et al, 2012;Ricci et al, 2015). Measurements must be performed in the shallowest tens of centimeters of sub fumarolic zones in order to avoid the "buffer effect" of the steam, occurring when steam at saturat ing pressure coexists with liquid water, thus forcing the temperature to remain close to water boiling temperature (Ricci et al, 2015). At shallow depths, ground temperature is highly sensitive to atmospheric condi tions, in particular daily and seasonal temperature variations and rain falls (Peltier et al, 2012;Gaudin et al, 2015;Ricci et al, 2015).…”

“…The first one consists in averaging the temperature over 24 h and computing the vertical temperature gradi ent. More recently, Ricci et al (2015) proposed to compute the coeffi cient of determination (R 2 ) between the depth and the measured temperature, allowing distinguishing convective and conductive re gimes in the ground. This easy to use method is quite efficient to filter the conductive effects of seasonal and diurnal temperature variations from those due to changes in the geothermal heat flux, but i) it does not allow a quantification of the geothermal flux and ii) it does not take into account the disruption produced by the largest rainfalls that may occur on tropical volcanoes, thus severely limiting the range of use of these techniques at these latitudes.…”

The imprint of thermally induced devolatilization phenomena on radon signal: implications for the geochemical survey in volcanic areas

“…There are many physicochemical phenomena that develop between radon escaping from a volcanic rock in the ground and the radon measurement at ground surface. For example, thermal gradients at subvolcanic conditions (Scarlato et al 2013) and in geothermal areas (Finizola et al 2010;Ricci et al 2015) Thermally induced devolatilization phenomena 559 volcanoes around the world are characterized by continuous injections of magma that stall at very shallow levels or feed complex dyke networks, even at a few metres from the ground surface. The conductive heat flow may produce thermal gradients in the host rocks from 1100 • C at the contact to 200 • C at a distance of thousands of metres, incorporating several cubic kilometres of the substrate (Bonaccorso et al 2010;Mollo et al 2012;Heap et al 2013).…”