**2. Context**

The Piton de la Fournaise (Reunion Island, Indian Ocean) is a basaltic volcano whose functioning is connected to the activity of a hot spot (Courtillot et al. 1986). The eruptions take place inside the Enclos Fouqué's caldera and have contributed to the creation of a volcanic cone whose summit is occupied by two craters: the Bory on the east side and the Dolomieu on the west side (figure 1).

During the past three decades and besides the 1986 and 1998 exceptions, the lava flows have been taking place inside de Enclos Fouqué's caldera (figure 1 and 4). They are mainly fed by

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

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;

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 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

The Piton de la Fournaise (Reunion Island, Indian Ocean) is a basaltic volcano whose functioning is connected to the activity of a hot spot (Courtillot et al. 1986). The eruptions take place inside the Enclos Fouqué's caldera and have contributed to the creation of a volcanic cone whose summit is occupied by two craters: the Bory on the east side and the

During the past three decades and besides the 1986 and 1998 exceptions, the lava flows have been taking place inside de Enclos Fouqué's caldera (figure 1 and 4). They are mainly fed by

the lava flows (Tinard, 2007 ; Froger et al., 2007 ; Froger et al., in press).

can therefore estimate the precision of the automatic extraction.

of the lava flow.

**2. Context** 

Dolomieu on the west side (figure 1).

James et al., 2007; Lombardo et al., 2009).

Fig. 1. Major morphological figures of the Piton de la Fournaise, the grey area is the rift zone defined by Bachèlery (1981).

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 (Bachèlery, 1981).

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

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

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

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

Automatic Mapping of the Lava Flows at

**3.1 Optical and thermal data** 

Pentes area as for the Piton de la Fournaise.

(VNIR) to 90m (TIR).

**3. Method** 

Piton de la Fournaise Volcano, by Combining Thermal Data in Near and Visible Infrared 205

The originality of this research states in the use of thermal data as an analyze mask. In spite of its low spatial resolution (90m), thermal data brings essential information in our automatic mapping method. It allows determining with certitude the zone where the newly implemented lava flow is localized. The automatic extraction of the outline can be realized in this analyze mask. Also, its utilization enables treating the lava flows separately from one another because for one thermal image, only one lava flow is associated in this methodology. This is particularly adapted in the case of the constitution of a lava field flow.

The automatic extraction method has been realized by the combination of thermal and optical data. SPOT and ASTER data have been used. SPOT data have **a** wavelength from the visible to mid infrared, and a spatial resolution from 2,5 to 20 meters. ASTER data have a spectrum from visible to thermal infrared and a spatial resolution that varies from 15m

The ASTER TIR thermal data have to be acquired at the end of the eruption or very little time afterwards, in order for the thermal anomaly to be clearly visible on the entire zone. The maximum post eruption delay of acquisition is variable and depends on the thickness of the lava flow and therefore on its speed of cooling. In most of the cases, it is less than a month. ASTER VNIR and SPOT data can be acquired long after the lava flow's implementation. The principal is not having a new lava flow implementing on the same zone. The recent lava flows present low reflectances between the visible and mid infrared wavelength. The basalt spectrum, in the visible and short wavelength of the infrared (0.4-2.4 µm), is dominated by the presence of iron, which, at different levels of crescent oxidation, increases the reflectance (Despinoy, 2000). In the same way, the presence of lichen that grows on the lava flow increases the reflectance. In near and medium infrared, the presence of chlorophyll in the vegetation induces a strong signal (Kahle et al. 1995), permitting to discriminate precise outlines in zones with vegetation cover, especially near the Grandes

The KALIDEOS project from the CNES (Centre National d'Etudes Spatiales, and GEO Grid (AIST/METI), and the NASA, grant free satellite data in the case of a research program. The data used in this article are from SPOT data from the KALIDEOS program, and from the GEO Grid program for the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data. The climatological conditions must be optimal because the presence of clouds masks, deforms or reduces the thermal anomaly, which in a tropical zone and especially in the volcano's zone is frequent. Only seven ASTER TIR data (the ASTER satellite was released in orbit in December 1999), acquired at the end of the eruption don't present these types of issues, and have therefore been used for the treatments (Table 1 and figure 3).

Optical data acquired after the eruption have been associated to the former images.

interpretation, and aerial pictures from IGN. This base is considerate as a reference.

The precision of the outlines extracted by automatic method leans entirely on their comparison with a base of outlines realized from very high-resolution satellite data' photo

**3.2 Error calculation and outlines precision of photo-interpretation** 

transformation of the rock on the surface, or because of the implantation (always very fast in Reunion Island) of a vegetation cover (lichen, moss, shrub…). Lava flows which implementations were separated by several years can therefore be distinguished by their spectral properties. For the summit zone of the volcano, all the more in the Dolomieu crater, where the rocks are superimposed with only a few years or a few months of interval, the spectral properties of the diverse lava flows can then be very similar.

The oldest known eruption at the Piton de la Fournaise dates back to 1644. About 200 events have been counted since that date thanks to archives, 95% of them took place in the caldera (Lacroix, 1936; Stieltjes et al., 1989; Peltier et al. 2009). Never the less, this database is incomplete, particularly in the case of short time and low scale eruptions, before 1980. The mean average magma emission at Piton de la Fournaise estimated over a century, is 0,01 km3.an-1 (Lénat et Bachèlery, 1987), or 0.3 m3.s-1. The debit estimations show a temporal estimation. For example, Stieltjes et al. (1989), calculate a mean debit of 0.3 m3.s-1 over 54 years (1931-1985), but obtain 0.78 m3.s-1 for a period of 25 years (1960-1985). These variations are partly due to the existence of long periods of inactivity. For example, no eruption took place during 1992 and 1998; witch is to say 6 years of inactivity. Also, another inactivity as long was observed between 1966 and 1972 (Villeneuve, 2000). Peltier et al. (2009) illustrate these debits variation and show a more important activity since 1998. Between January 1990 and January 2010, 61 eruptions have been registered with a total volume of emitted lava estimated at 473 Mm3 (figure 2), and 33 eruptions between 1998 and 2010, with a total volume of emitted lava of 313 Mm 3 (Peltier et al. 2009, OVPF 2009; 2010). From these observations, we have calculated a mean debit estimated between 0.45 m³.s-1 and de 0.82 m3.s-1, from 1980 to 2010. These estimations are superior to those obtained by Stieltjes et al. (1989), on former periods.

Fig. 2. Estimation of the cumulative volume of lava emitted from 1980 to 2010 by the Piton de la Fournaise.
