The Use of Surveillance Cameras for the Rapid Mapping of Lava Flows: An Application to Mount Etna Volcano

Mauro Coltelli, Peppe d'Aranno, Roberto de Bonis, Josè Guerrero Tello, Maria Marsella, Carla Nardinocchi, Emilio Pecora, Cristina Proietti, Silvia Scifoni, Marianna Scutti, Wissam Wahbeh
2017 Remote Sensing  
In order to improve the observation capability in one of the most active volcanic areas in the world, Mt. Etna, we developed a processing method to use the surveillance cameras for a quasi real-time mapping of syn-eruptive processes. Following an evaluation of the current performance of the Etna permanent ground NEtwork of Thermal and Visible Sensors (Etna_NETVIS), its possible implementation and optimization was investigated to determine the locations of additional observation sites to be
more » ... on sites to be rapidly set up during emergencies. A tool was then devised to process time series of ground-acquired images and extract a coherent multi-temporal dataset of georeferenced map. The processed datasets can be used to extract 2D features such as evolution maps of active lava flows. The tool was validated on ad-hoc test fields and then adopted to map the evolution of two recent lava flows. The achievable accuracy (about three times the original pixel size) and the short processing time makes the tool suitable for rapidly assessing lava flow evolutions, especially in the case of recurrent eruptions, such as those of the 2011-2015 Etna activity. The tool can be used both in standard monitoring activities and during emergency phases (eventually improving the present network with additional mobile stations) when it is mandatory to carry out a quasi-real-time mapping to support civil protection actions. The developed tool could be integrated in the control room of the Osservatorio Etneo, thus enabling the Etna_NETVIS for mapping purposes and not only for video surveillance. acquire dense 3D data over large areas, thus it is suitable to observe rapid evolving and widespread phenomena. Digital photogrammetry processing is based on the application of matching procedures between overlapping images using autocorrelation algorithms capable of working at subpixel level [5] . Direct georeferencing or, at least, the use of a pre-established network of GCPs (Ground Control Points) has reduced the need for ground survey operations [6] . The outputs of digital photogrammetry are multi-temporal Digital Orthophotos and Digital Elevation Models (DEMs), that allow implementing a quantitative comparative analysis. The systematic acquisition of airborne photogrammetric datasets can be adopted for long-term volcano monitoring and hazard assessment [1, 2] . During volcanic crisis, oblique digital images, frequently acquired by helicopter, can be processed, for example with the scientific tool Orthoview [7] that uses a straightforward photogrammetric approach to generate digital orthophotos. The processing, through photogrammetric software, of digital images taken from helicopter, light aircraft or ground can also be used to extract DEMs from points identified in multiple photos. The comparison of successive DEMs can be used to model the evolving morphology of a growing dome, as done for the 2004-2008 eruption of Mount St. Helens volcano, the 2009 eruption of Redoubt volcano, the active lava dome at Volcán de Colima, Mexico and the dome at the Soufrière Hills volcano on Montserrat 9]. Oblique stereo-pair time-lapse imagery acquired on the ground can also be processed by combining close-range photogrammetry and traditional stereo-matching software or by using a software based on structure-from-motion, to extract DEMs for analyzing active lava flows on Kilauea volcano, Hawaii and Mount Etna, Sicily [10] . In order to evaluate the temporal evolution of a lava flow field and estimate its effusion rate, thus enabling to perform analysis on the eruptive mechanism, a multi-temporal analysis is required. In the past, such analyses have been performed only on long-duration lava flows mainly using photogrammetric data, topographic maps and historical reports [11] [12] [13] [14] . An almost continuous survey can be obtained during emergency phases by processing images constantly acquired from a ground network of sensors, to implement a quasi-real-time monitoring for supporting civil protection actions. Previous works (discussed below) demonstrate that the routine and automatic application of photogrammetric techniques to process ground based images offers promising perspectives for operational applications, such as monitoring and early warning systems of natural disasters including volcanic eruptions, landslides, or analysis of glacier motion. Most of the previous applications were developed in support of portable video stations. More specifically, the approaches developed for landslides and glaciers monitoring are devoted to the extraction of single features displacement and velocity fields. A real-time mapping system was previously developed to extract ortho-images by rectifying digital landscape images acquired by a single ground-based digital camera. The rectification was carried out by matching between the common skyline edge that appears in the image and in a reference one generated from a DEM. The mapping system was tested during a survey of the Mayon Volcano, Philippines [15] . Oblique photogrammetric techniques have also been adopted to process visible and thermal images acquired on the ground with portable cameras. A minimum of four 3D targets, having known positions, on each visible image and a DEM of the investigated area are required to extract camera orientation. Thermal images are taken at the same location of the visible ones to allow the estimation of a suitable projective transformation for obtaining georeferenced maps used for further analysis of lava-flow development this method was tested during the 2004-2005 eruption of Mount Etna, Sicily [16] . A similar approach has been applied to map Etna's active flows on 27-29 September 2004 by moving a single pre-calibrated camera around the lava flow-fronts. A thermal image sequence was processed to analyze the flow of lava down a distal channel in order to assess flux and rheological properties [17] . A very-long-range terrestrial laser scanner was applied on active lavas at Mount Etna on 5-12 June 2009 [18] . To facilitate visualization and interpretation of the laser data, visible and thermal images were also acquired and processed as described in [16] . Visible imageries, acquired at the Mammoth Mountain (CA) fumarole area, were processed using the structure-from-motion
doi:10.3390/rs9030192 fatcat:q72oi337prab7a5tb6oxbfwubi