OI² - A CLOSELY MONITORED VOLCANO
I - Introduction - The Observation Service
The Observation Service, OI², is a recognised component of the National Service for Volcanological Observations. The mission of OI² is the regular monitoring of ground movements brought about by the volcanic activity of Piton de la Fournaise volcano, Reunion Island, using InSAR data.
Movement of magma inside and beneath a volcano is generally manifested by inflations or subsidences of the ground surface. Various in situ geodetic methods (comprising a network of ground-based instruments which needs to be installed and maintained, such as GPS receivers and inclinometers) allow the evolution of the volcano's shape to be measured through time. These methods are routinely used in the majority of volcanic observatories.
The technique used by the Observatory Service OI², radar interferometry, has the advantage of not needing any ground-based installations since it is based on data acquired in space. Radar interferometry was developed at the start of the 1990s, following the launch of the first radar satellites (ERS-1 & -2). It consists of measuring the distance between the ground and the satellite using the return time of the radar pulse emitted by the satellite. A radar image of the distances between the Earth and the satellite can then be obtained by carrying out these measurements for a large number of points within a limited geographical area. Comparison of the acquired radar images at different periods in time allows changes in the Earth-satellite distance, due to ground movements, to be detected. When considering active volcanic terrain, these ground movements provide information on the physical nature of a volcano's magma feeder system (the underground pipe network) and on the way in which magma moves through this feeder zone. This information is valuable not only for ongoing volcano monitoring but also for the understanding of the fundamental processes which control its functioning and long-term evolution.
Thus, within the framework of the Observation Service OI², 429 radar images covering the Piton de la Fournaise have been obtained from the ENVISAT-ASAR satellite (operating at C-band), dating back to 2003. These images were supplied free to the "Observatoire de Physique du Globe" at Clermont-Ferrand by the European Space Agency. Based on these images, the ground displacements of many successive eruptions have been measured. This exceptional database of radar images means that Piton de la Fournaise is now a unique reference site worldwide in terms of the application of radar interferometry for monitoring volcanic activity. More recently, data from ALOS-PALSAR (operating at L-band), TerraSAR-X (operating at X-band), COSMO-SkyMed (operating at X-band) and RADARSAT-2 (operating at C-band) have been added to supplement the ASAR data. The combined contribution from these five sources broadens the sensitivity to movements and the acquisition geometry, which in turn means that the ground movements can be more precisely determined, as well as the driving processes behind these movements.