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Volcano monitoring

Context

Natural risks (landslides, mudslides, floods, cyclones, eruptions) are very high on the Reunion island. They are linked with the combination of volcanic activity and exceptional rainfall, reaching sometimes world records for time intervals between 24 hours and 15 days, on porous and friable soils. Currently, two Natural Risk Prevention Plans have been completed for the Saint-Paul and Saint-Pierre municipalities and are under signature

Activities currently carried out by scientific teams

The Institut de Physique du Globe de Paris (IPGP) owns a Volcano observatory on the Reunion island, which aims at monitoring permanently the volcano activity using measurements of seismicity, geochemistry, magnetism or movements... The tasks performed by this Observatory on a routinely basis are completed by localized field trips regularly carried out by IPGP researchers and engineers.

In the field of ground motion study, the geodetic network for the supervision of Piton de la Fournaise has been set up in 1981, then progressively densified as technology, and more particularly GSP systems, changed. It currently includes several hundred points where measurements take place every year. Two motorised distancemeters assess every hour the position of 30 reflectors spread over the area. This device is completed by several stations performing local slope or ground extension measurements. A continuous thermal monitoring based on infrared cameras is under implementation. Several airborne photogrammetric surveys have been carried out in the past, allowing to tackle the issue of a remote sensing based monitoring of the volcano distortion.

Efforts are currently made in coordination with CNES to study the feasibility of sub-pixel correlation between space or ground images on the volcano, enabling the detection of small distortions.

Relevance of a reference remote sensing database

The most useful space data to monitor the volcano are:

- Archive or current SAR interferometric data (Envisat, Radarsat, TerraSAR-X), allowing to map the volcano distortion quite accurately;

- Optical imagery data which may give us information about changes in the volcano topography (Spot images for mid-resolution studies or Spot 5, Ikonos or Quickbird images to reach a better spatial resolution and a finer appraisal of altimetry);

- Low resolution thermal imagery data (100 meters or more), collected quite often, and high resolution ones (10 meters or less), to assess lava flows.

Space data must be combined with ground information:

- Distortion data collected through static or kinematic GPS,

- Airborne images or ground photos allowing to produce local DTMs in order to set and validate those developed from space data,

- Infrared radiometry field measurements carried out on lava flows to calibrate satellite thermal images.

For all types of space data, high resolution is made necessary because of:

- The size of the items to study: an eruptive crack or an erupting crater may have small dimensions (a few dozens to a few hundreds meters), even if the lava flows coming from these mouths stretch widely;

- The width of a lava flow, sometimes limited to a few meters;

- The thickness of active lava flows, rarely exceeding 2 meters in Reunion, which requires the use of a very accurate DTM to forecast the flow path;

- Accurate detection of ground distortions before, during and after an eruption.

A continuous monitoring of observations and a high information updating frequency (ideally a few hours) are also necessary for the volcano supervision. At last, recording all the data (space and ground) in a database meets at the same time a requirement to structure information (quite varied and changing) and to access it in an efficient way during crisis period.

All these arguments combine to bring about the development of the Kalideos Isle-Réunion database.

Contact point

This scientific coordination of this activity is ensured by Pierre BRIOLE from the Institut de Physique du Globe de Paris (IPGP), who coordinates GPS geodetic measurements carried out on Piton de la Fournaise and whose team also currently works on radar and optical high resolution imagery.