A lab-scale experiment to measure terminal velocity of volcanic ash

In this paper, a novel methodology to measure trajectory and terminal velocity of volcanic ash in laboratory is presented. The methodology consists of: i) planning a lab-scale experiment in order to reproduce the sedimentation processes of fine volcanic ash based on the principle of dynamic similarity; ii) realizing the experimental set-up using a glass tank filled with glycerine, a web-cam based vision system and a dedicated image post processing tool able to estimate the position and the terminal velocity of any particle falling in the tank; iii) performing a calibration procedure to accurately estimate the uncertainty on particle velocity; iv) comparing the experimental results with estimations obtained by some particle fallout models available in literature. Our results shows that there is a good agreement between experimental terminal velocities and those obtained applying a model which includes information on particle shape. The proposed methodology allows us to investigate how the particle shape affects the sedimentation processes. Since the latter is strategic to improve the accuracy on modeling ash fallout, this work will contribute to reduce risks to aviations during explosive eruptions

A Measurement Tool for Investigating Cooling Lava Properties

This paper presents the development and uncertainty characterization of a system for the direct measurement of heat transfer in cooling lava. The system continuously measures the parameters involved in the cooling process and, particularly, in the formation of the crust. The aim is to allow the future development of a physical model of the cooling process itself. In order to realize a system that will be effective in such a hostile environment, the principles on which the instruments for radiation thermometry are based have been thoroughly investigated. A virtual instrument has been developed, interfacing the measuring system and the user, processing the incoming data, and producing an estimate of the uncertainty of the measurement chain. The various sources of uncertainty have been taken into account to produce an accurate estimate of the uncertainty associated with the measured data. The results of experimental tests are presented

Studio della correlazione tra il tremore vulcanico e l’attività esplosiva dell’Etna nel Gennaio - Febbraio 1999 mediante il sistema VoTA (Volcanic Tremor Analyzer)

Il seguente lavoro presenta lo studio della correlazione tra il tremore vulcanico e l'attività esplosiva dell'Etna nel gennaio-febbraio 1999 effettuato mediante l’implementazione e lo sviluppo del sistema automatico per l’analisi e la visualizzazione del tremore vulcanico VoTA (Volcanic Tremor Analizer). In particolare, sono stati presi in considerazione cinque episodi eruttivi significativi dell’attività dell’Etna durante i quali il VoTA ha effettuato un’analisi on line del tremore vulcanico. Successivamente, tali dati sono stati confrontati con le immagini dell’attività eruttiva riprese dalla telecamera di sorveglianza dell’Etna, permettendo di ricavare delle correlazioni tra le diverse fasi dell’attività esplosiva ed i corrispondenti valori del tremore vulcanico

A quantitative approach for evaluating lava flow simulation reliability: the LavaSIM code applied to the 2001 Etna’s eruption

Many numerical codes have been developed to simulate the emplacement of lava flows for evaluating their possible evolutions and for defining, by a statistical approach, hazard maps useful for risk assessment and land planning. Although many examples of lava flow simulation can be found in literature, just a few of them attempted to quantify the correspondence between observed and simulated flows, nevertheless this is a crucial point especially if the codes are applied in real-time for risk managing. The aim of this work was to define a methodology to quantitatively evaluate the reliability of simulation codes. In particular, it applied the LavaSIM code (Hidaka et al., 2005) to simulate the main lava flow emplaced on the South flank of Mt. Etna (Italy) between 18 July and 9 August 2001 which represents an ideal test case for validating numerical codes (Coltelli et al., 2007). It is a single flow both for its geometry and its temporal evolution and, many data are available to be used as input of the simulations (lava composition, pre- and post-eruption topographies, final flow volume and thickness and temporal evolution of average volumetric flow rates) and for checking their results (2D temporal evolution). LavaSIM is the only full 3D model, thus able to account for the vertical variation of lava properties (temperature, viscosity, velocity and liquidus or solidus state). It is based on the 3D solution of the Navier-Stokes and the energy conservation equations and provides the most complete description of the lava cooling by considering radiation, conduction and convection. Its greatest peculiarity is to take into account crust formation by evaluating the enthalpy of every cell and by adopting an empiric threshold parameter (the solidification fraction of liquidity loss) to discriminate liquid and solid cells. Different values of input parameters (viscosity, solidification fraction of liquidity loss, eruptive enthalpy and lava emissivity) have been adopted for evaluating their influence on the simulated lava distribution and cooling. A simulation with constant lava discharge, averaged on the whole eruption, was also run for checking how the feeding affects the lava spreading and cooling. The results were first analyzed by comparing the planar expansions of real and simulated flows. A quantitative analysis was then carried out adopting two parameters for constraining both the lengthening and the planar expansion. For quantitatively verifying the correspondence between simulated and observed lengths, the Percent Length Ratio (PLR) was defined as the percentage ratio between simulated and observed lengths measured along the main flow direction. The second control parameter was the fitness function (e1) defined by Spataro et al. (2004) as the square root of the ratio between the intersection and the union of real and simulated areas. Since the e1 factor allows quantifying the simulated lateral spreading while PLR the flow lengthening, it is important to jointly analyze these two parameters. This work showed that by combining the fitness function of Spataro et al. (2004) with the Percent Length Ratio, here defined, it is possible to constrain both the lateral spreading (by e1) and the flow lengthening (by the PLR). The analysis here presented also demonstrated the capability of the LavaSIM simulation code to account for the vertical variation of the lava properties and to simulate the crust formation

Quantitative analysis of the 1981 and 2001 Etna flank eruptions: a contribution for future hazard evaluation and mitigation

Lava flows produced during Etna flank eruptions represent severe hazards for the nearby inhabited areas, which can be protected by adopting prompt mitigation actions, such as the building of diversion barriers. Lava diversion measures were attempted recently during the 1983, 1991-93, 2001 and 2002 Etna eruptions, although with different degrees of success. In addition to the complexity of barrier construction (due to the adverse physical conditions), the time available to successfully slow the advance of a lava flow depends on the lava effusion rate, which is not easily measurable. One method to estimate the average lava effusion rate over a specified period of time is based on a volumetric approach; i.e. the measurement of the volume changes of the lava flow over that period. Here, this has been compared to an approach based on thermal image processing, as applied to estimate the average effusion rates of lava flows during the 1981 and 2001 Etna eruptions. The final volumes were measured by the comparison of pre-eruption and post-eruption photogrammetric digital elevation models and orthophotographs. Lava volume growth during these eruptions was estimated by locating the flow-front positions from analyses of scientific papers and newspapers reports, as well as from helicopter photographs. The analyses of these two eruptions contribute to the understanding of the different eruptive mechanisms, highlighting the role of the peak effusion rate, which represents a critical parameter for planning of mitigation actions and for hazard evaluation

Monitoring and forecasting Etna volcanic plumes

In this paper we describe the results of a project ongoing at the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The objective is to develop and implement a system for monitoring and forecasting volcanic plumes of Etna. Monitoring is based at present by multispectral infrared measurements from the Spin Enhanced Visible and Infrared Imager on board the Meteosat Second Generation geosynchronous satellite, visual and thermal cameras, and three radar disdrometers able to detect ash dispersal and fallout. Forecasting is performed by using automatic procedures for: i) downloading weather forecast data from meteorological mesoscale models; ii) running models of tephra dispersal, iii) plotting hazard maps of volcanic ash dispersal and deposition for certain scenarios and, iv) publishing the results on a web-site dedicated to the Italian Civil Protection. Simulations are based on eruptive scenarios obtained by analysing field data collected after the end of recent Etna eruptions. Forecasting is, hence, supported by plume observations carried out by the monitoring system. The system was tested on some explosive events occurred during 2006 and 2007 successfully. The potentiality use of monitoring and forecasting Etna volcanic plumes, in a way to prevent threats to aviation from volcanic ash, is finally discussed

On-line image analysis of the Stromboli volcanic activity recorded by the surveillance camera helps the forecasting of the major eruptive events

The typical activity of Stromboli consists of intermittent mild explosions lasting a few seconds, which take place at different vents and at variable intervals, the most common time interval being 10-20 minutes. However, the routine activity can be interrupted by more violent, paroxysmal explosions, that eject m-sized scoriaceous bombs and lava blocks to a distance of several hundreds of meters from the craters, endangering the numerous tourists that watch the spectacular activity from the volcano's summit located about two hundreds meters from the active vents. On average, 1-2 paroxysmal explosions occurred per year over the past century, but this statistic may be underestimated in absence of continuous monitoring. For this reason from summer 1996 a remote surveillance camera works on Stromboli recording continuously the volcanic activity. It is located on Pizzo Sopra la Fossa, 100 metres above the crater terrace where are the active vents. Using image analysis we seeks to identify any change of the explosive activity trend that could precede a particular eruptive event, like paroxysmal explosions, fire fountains, lava flows. From the day of the camera installation up to present 12 paroxysmal events and lava flows occurred. The analysis include the counting of the explosions occurred at the different craters and the parameterization in classes of intensity for each explosion on the base of tephra dispersion and kinetics energy. The plot of dissipated energy by each crater versus time shows a cyclic behavior with max and min of explosive activity ranging from a few days to a month. Often the craters show opposite trends so when the activity decreases in a crater, increases in the other. Before every paroxysmal explosions recorded, the crater that produced the event decreased and then stopped its activity from a few days to weeks before. The other crater tried to compensate increasing its activity and when it declined the paroxysmal explosion occurred suddenly at the former site. From September 2001 an on-line image analyzer called VAMOS (Volcanic Activity MOnitoring System) operates detection and classification of explosive events in quasi real-time. The system has automatically recorded and analyzed the change in the energetic trend that preceded the 20 October 2001 paroxysmal explosion that killed a woman and the strong explosive activity that preceded the onset of 28 December 2002 lava flow eruption

Monitoring Active Volcanos Using Aerial Images and the Orthoview Tool

In volcanic areas, where it can be difficult to perform direct surveys, digital photogrammetry techniques are rarely adopted for routine volcano monitoring. Nevertheless, they have remarkable potentialities for observing active volcanic features (e.g., fissures, lava flows) and the connected deformation processes. The ability to obtain accurate quantitative data of definite accuracy in short time spans makes digital photogrammetry a suitable method for controlling the evolution of rapidly changing large-area volcanic phenomena. The systematic acquisition of airborne photogrammetric datasets can be adopted for implementing a more effective procedure aimed at long-term volcano monitoring and hazard assessment. In addition, during the volcanic crisis, the frequent acquisition of oblique digital images from helicopter allows for quasi-real-time monitoring to support mitigation actions by civil protection. These images are commonly used to update existing maps through a photo-interpretation approach that provide data of unknown accuracy. This work presents a scientific tool (Orthoview) that implements a straightforward photogrammetric approach to generate digital orthophotos from single-view oblique images provided that at least four Ground Control Points (GCP) and current Digital Elevation Models (DEM) are available. The influence of the view geometry, of sparse and not-signalized GCP and DEM inaccuracies is analyzed for evaluating the performance of the developed tool in comparison with other remote sensing techniques. Results obtained with datasets from Etna and Stromboli volcanoes demonstrate that 2D features measured on the produced orthophotos can reach sub-meter-level accuracy

Radiosondaggi atmosferici nell’area etnea

RADIOSONDAGGI ATMOSFERICI NELL’AREA ETNE