The Monitoring of volcanoes characterized by fumarolic activity can be usefully helped by thermal multitemporal surveys. Aimed at anticipating possible paroxysmic events, this kind of survey has been experimentally tested since the early '70s. In order to monitor the thermal activity of the fumaroles field over time (maximum value of temperature, total power, migration of the thermal baricentres, etc.) a very careful registration of the multitemporal thermal data is required. This paper presents a further approach to the problem based on the analysis of the symptoms accompanying the increasing or decreasing thermal transitory.
At the surface of a volcanic area showing fumarolic activity, the global thermal effect is given by the sum of the contribution of each single fumarole. Temperature near the fumarole duct decreases with the radial distance, following roughly a negative logarithmic law. Any variation of thermal power in the volcanic area modifies the radial profile of temperature around the fumaroles, till a new condition of stability has been reached. The ratio between the module of the radial temperature gradient |G(r)| and the temperature itself T(r), that is the normalized gradient |G(r)|/T(r), changes in such a way that by an increase of thermal power the gradient rises faster man the temperature and vice versa. The histogram of |G(r)|/T(r), called "characteristic function", related to surveys carried out at different dates, allows to understand if a given thermal distribution is in a phase of transitory compared to a previous one. In case of increasing thermal transitory the histogram of |G(r)|/T(r) "includes" the one referred to a previous date.
The mechanical equivalent model of a fumarole and its thermal environment is given by an elastic membrane fastened to a ring with a very large radius and forced at the center by a thin prop. If the prop is forced up, a transitory is induced and the radial slope of the elastic membrane changes faster than its height. If the prop reduces its force, coming a little back, radial slope varies slower than the height.
To assess the properties of the characteristic function, a series of thermograms have been collected on a physical model simulating the behaviour of a single fumarole. The model was formed by a conductive cylinder filled with sand and heated at the centre by a thin electrical resistor, A digital thermovision system was mounted so that it pointed vertically the centre of the model as close as to fill up its view field. The recording of thermograms has started with the start of heating of the resistor, and many thermograms have been collected during the warming up phase till reaching a steady state condition. Then the heating power has been reduced, in order to produce a decreasing thermal transitory.
Data have been processed to obtain the "normalized gradients" function and the corresponding histograms, the "characteristic function". Approaching the steady state configuration, the characteristic function tends to "include" the ones related to the increasing thermal transitory.
The method was applied to two thermal surveys on the crater of Vulcano island, collected by night by an airborne line scanner respectively in 1993 and 1994. The characteristic function has shown that in the year 1994 the volcano had slightly reduced its fumarolic activity in comparison with the previous year 1993.
The comparison of the characteristic functions of multitemporal thermal surveys allows to show the stability or, on the contrary, a trend towards an increase or a decrease of the volcanic activity, with a very good sensitivity and using just a comparison among corresponding areas, without any accurate registration among the set of multitemporal thermal images.
