**Automatic Mapping of the Lava Flows at Piton de la Fournaise Volcano, by Combining Thermal Data in Near and Visible Infrared**

Z. Servadio1,2, N. Villeneuve1 and P. Bachèlery3,4 *1Laboratoire Géosciences Réunion, Université de la Réunion, Institut de Physique du Globe de Paris, CNRS, UMR 7154, Géologie des Systèmes Volcaniques, Saint Denis 2Institut de Recherche pour le Développement, US 140, BP172, 97492 Sainte-Clotilde Cedex 3Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, CNRS, UMR 6524, Observatoire de Physique du Globe de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand 4IRD, R 163, LMV, F-63038 Clermont-Ferrand France* 

#### **1. Introduction**

200 Remote Sensing – Applications

Zhang, D. (1993). Preliminary Study on Visual Interpretation Marks of Building Damages

ISSN 1000-3274.

Caused by Earthquakes on Aero Photograph, *Earthquake*, Vol.13, No.1, pp. 26-30,

Knowing the eruptive history of a volcano is an essential key to the understanding of its functioning, and therefore of the evolution of the character of dangerousness of its eruptions. For an essentially effusive basaltic volcano such as the Piton de la Fournaise, the spatial and temporal distribution of lava flows allows to deduct numerous parameters of its activity, on a magmatic and a structural point of view. Satellite imaging brings more advantages than the methods used in aerial pictures studies, especially by supplying bigger temporal and spectral series. The revisiting of satellites over a region can allow the generation of dynamic mappings of the implementation of the lava flow, and also bring information on the phenomenology of the eruptions: Surface, volume, flow, spatial distribution…

Furthermore, satellite images have the advantage of supplying data that grant a global visualization of the study area, and information on not easily accessible areas. The interpretation of these satellite data enables obtaining information on the surfaces and volumes of the lava field flows, but also on its nature and behavior. In a tropical environment such as La Reunion, where the climatological context presents a strong cloudiness, a satellite revisit is statistically necessary.

The optical satellite images have already been successfully used to realize mappings of lava flows. For example, in Nevado Subancaya in Peru (Legelay-Padovanie et al., 1997) or in Etna in Italy (Honda et al.,2002), the combination of spectral and morphological properties helped to elaborate surface lava flows mappings and also allowed to individualize the main

Automatic Mapping of the Lava Flows at

defined by Bachèlery (1981).

(Bachèlery, 1981).

Piton de la Fournaise Volcano, by Combining Thermal Data in Near and Visible Infrared 203

Fig. 1. Major morphological figures of the Piton de la Fournaise, the grey area is the rift zone

intrusions situated along the rift zones (Bachèlery, 1981). The intrusions are interconnected at the same level as the central cone. Eruptions are of three kinds: summit zone eruptions, proximal eruptions, and distal eruptions (Peltier et al. 2009). It is clear that in the recent history of the Piton de La Fournaise, summit zone eruptions are the most frequent (Villeneuve et Bachèlery, 2006; Peltier et al., 2009). Most of these eruptions take place according to leveled cracks that progressively migrate by furthering from the central cone

Degassing at the event during eruptions can generate lava fountains which cause pyroclastic deposits and cones of several meters high. Two morphologies of lava flow are observed at

The occidental part of the Enclos Fouqué is largely recovered by a vast field of lava that Lénat et al. 2001 name the CLEF (Champ de Lave de l'Enclos Fouqué). This field of lava, essentially formed by a pahoehoe lava type flow, may have been constituted from slow

At the Piton de la Fournaise, the flow of lava emitted by eruptive cracks represent long (several meters to several kilometers), thin, (about ten meters), shallow (one meter in the slopes to 5 meters in flat zones (Letourneur et al., 2008)) lava flows, which shows the poor viscosity of the emitted magma. The juxtaposition of several individual flows during a same phase will contribute to the constitution of fields of lava, and particularly for long time eruptions (more than one month). In this case, in regards to the initial lava flow, the new income contributes essentially to the thickening and widening of the lava field flow. In this

The spectral properties of the lava flows differ according to the type of the surface (mainly 'a'a type at Piton de la Fournaise), but also according to the age, either because of a chemical

Piton de la Fournaise: 'a'a type lava flows and pahoehoe type lava flows.

emissions from the volcano's summit zone, between 1750 and 1794.

case, the lava field will be considerated here as a same unit.

structures. Lu et al. (2004) propose an association of OPTICAL and RADAR imaging in order to define more accurate outlines for the lava flows. In the later example, the downstream part of the lava flow presenting vegetation, the discrimination was realized by the infrared. The upstream part being a heavily snow covered zone, RADAR images properties become a precious source of information. Thanks to the use of LIDAR (Airborne Light Intensity Detection and Ranging) on volcanoes, Digital Ground Models of very highresolution can be generated and various retro-reflecting properties of the lava's different textures can be studied (Favalli et al., 2009) .The comparison of the different DGM produced at different dates allow Favalli et al. (2009) to obtain mappings of the thickness and outlines of the lava flow.

The use of thermal satellite imaging to characterize the relative chronology of the implementation of lava flows was the aim of Kahle et al. (1998)'s work. Abrams et al. (1991), used it in association with optical satellite imaging to realize a chronological mapping of the lava flows in Hawaii. Other authors have realized mappings of lava flow's temperature (Hirn et al., 2005) and implementation maps by using a thermal camera (Harris et al., 2007; James et al., 2007; Lombardo et al., 2009).

At the Piton de la Fournaise, the first research that used photogrammetry to realize mappings of the outlines and thickness of lava flows took place on lava flows dated back to 1972 and 1976 (Lénat, 1987). Bonneville et al. (1989) mapped the main geological units by using SPOT1 images. After which, Despinoy (2000) realized a mapping of the lava flows above Les Grandes Pentes using a CASI hyperspectral sensor. Villeneuve (2000) realized outlines, volume calculations and a chronological follow up of the implementation thanks to stereophotogrammetry and the use of DGPS (Digital Global Positioning System). Lénat et al. (2001) associated RADAR and SPOT images to map the field of lava of the Enclos Fouqué's caldera. Recently, De Michele et Briole (2007) used a technic of correlation of images to extract lava flows which implemented between two series of aerial pictures. The study of the incoherencies in the interferograms helped realize a dynamic follow up and a mapping of the lava flows (Tinard, 2007 ; Froger et al., 2007 ; Froger et al., in press).

The aim of this article is to propose an original approach by combining thermal images with ones acquired in the visible and near infrared in order to extract independent outlines for each lava flow. The extraction is therefore independent of the operator's subjectivity. Only one automatic extraction is possible when associating thermal and optical images for the implementation of a lava flow. An automatic extraction of the outlines of lava flow is realized and then compared to a mapping of references realized by photo-interpretation. We can therefore estimate the precision of the automatic extraction.
