Observatoire volcanologique du Piton de la Fournaise
14 RN3 Le vingt-septi譥- Km 27
97418 La Plaine des Cafres (R鵮ion)
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Aline Peltier - Director of the Piton de la Fournaise Volcano Observatory at Martinique
Val鲩e Ferrazzini - Seismology
Christophe Brunet - Engineer
Allan Derrien - Volcanology
Philippe Kowalski - Electronics
Philippe Catherine - Electronics
Jacques Lebreton - Field Assistant, Mechanics
Nicole Richter - Volcanology
Gian Luciano Garavaglia
Andrea Di Muro
Piton de la Fournaise Volcanological Observatory is one of the three french volcanological observatories with Soufri貥 de Guadeloupe and Montagne Pel饠(Martinique) observatories. The headquarter of french volcanological observatories (Director: Jean Louis Chemin饩 is at the Institut de Physique du Globe de Paris (IPGP).
Close co-operation exists with the Department of Earth Science of the Universit頤e la R鵮ion, in particular through Patrick Bach謥ry : volcanology - petrology - geochemistry -deformation.
The Piton de la Fournaise Volcanological Observatory ensures the good working of the seismic stations of Karthala Volcano Observatory (Gran Comoro, FIR of Comoro).
Permanent monitoring networks consist now in about 80 geophysical and geochemical stations, distributed over the volcano. Equipments are grouped into geophysical stations, that are composed of :
- various sensors (seismometers, electronic tiltmeters, magnetometers, extensometers, automatic EDM, continuous GPS, 222-Rn detectors and temperature probes).
- a protection frame, that supports antenna and a solar power supply, and that contains 12 V batteries, a field computer and various electronic devices (VCO, radio-transmitters, ...).
Monitoring networks can be devised in 5 main disciplines :
1. Seismicity, with 19-station array distributed over and around the volcano. 15 stations are either 1 vertical-component (14) or 2-component (1) Schlumberger short period seismometers. 2 stations are 3-component high dynamics (120 dB) Lennartz seismometers.
The radio telemetered analogical signals of the Schlumberger stations are received at the Observatory where they are digitised. The three component Lennartz stations supply digitised data that are directly transmitted and stored on a magnetic tape. A real time analogical control is supplied at the Observatory for the 8 most significant stations by a strip-chart recorder and a recording sequence on a magnetic tape is triggered when seismic events are detected by at least three stations.
Moreover, 2 vertical long-period Geotech seismometers allow to identify long-period teleseisms. Finally, a three component high resolution broad band seismometer belonging to the Geoscope seismological program has been set up at Rivi貥 de l'Est.
Digitised data from Schlumberger and Lennartz stations enable real-time earthquake processing, by using a HP computer system developed at IPGP that automatically pick P and S-wave arrival times.
2. Differential magnetism, with a 8-station array that has been set-up since 1985. These stations operate with ELSEC proton magnetometers, that measure the absolute value of the magnetic field with a precision of 0.25 nT. To eliminate the main part of the time-varying transient field, differences between values at each station and at a reference station are calculated. Data are telemetered each minute to the Observatory, where they are stored and processed in real time with a on-line PC computer.
3. Ground deformations, with a 9-electronic tilt station array, a 19 dry tilt-station network, one permanent GPS mine, 2 automatic EDM systems, four 3-component strainmeters, a geodetic network, a levelling network and a bench-mark gravimetric network. Permanent monitoring concerns electronic tilt, extensometry, GPS, and automatic EDM.
Electronic tilt stations are located in the central area (except two distant stations) and are composed of 2 perpendicular electronic titlmeters, radial and tangential respectively to the summit crater. Sensors are silica horizontal pendulum designed at IPGP, sealed to the rock by means of a silica cone. Such an apparatus allows both good mechanical amplification and thermal inertia. Tiltmeters operate by measurement of the motion of a light spot on a cell, with detection limits of 0,1 microradian and very low electronic drift. Data are sampled at each station every minute by a Canon microcomputer, and consist in the values of the radial and tangential sensors, of the external and rock (5 cm depth) temperatures, and of electronic parameters allowing the real time check of the good working of the station. Blocks of 5 successive data are teletransmitted every 5 minutes through relay stations, and are therefore available in real time at the Observatory. In late 1989, an item of software allowing the real-time acquisition and processing of telemetered clinometric data has been developped. It allows to display graphically in real time from the stations located within the Enclos caldera both tilt vectors and the geometric position of the inflation center, this latter being expected to coincide with the surface location of ascending magma. It allows also the visualisation of curves of the physical parameters measured. The inflation center is computed as the least square adjustment of the intersections of tilt vectors intersections, and is successively displayed on a map on the screen during magmatic intrusions.
Automatic EDM monitoring is performed by two systems measuring each about 15 reflectors along lines on the flanc of the volcano with a periodicity of 1 hour.
Continuous GPS recordings are made using a couple of GPS receivers (one on the top of the volcano, the other on the western rim of its caldera) connected one to the other by a radio-link. Periodic transmission of dta (every 6 hours) from the summit station to the other one allows determination of the 3D relative co-ordinates of the antenna with respect to the other. Accuracy of GPS measurements under such conditions is about 1-2 mm in horizontal and 1-2 cm in vertical direction.
Non permanent deformations monitoring is performed mainly with :
- spirit level tilt stations, that are measured with precise optical level WILD NA2, equipped with a micrometer. 7 stations are routinely measured.
- EDM monitoring, with a trilateration network (18 bench-marks), that allows to routinely measure with a Wild DI 2000 geodimeter :
* a surveillance network (about 10 distances between the caldera rim and the central cone),
* a summit network (about 10 distances between bench-marks distributed over the crater rim).
4. Geochemistry, with a 25-station network allowing the measurement of 222-Radon activity in ground. Sensors are silicium photo-diods. Three stations were situated directly in the crater Dolomieu, ten stations were located on the summit around the craters. Six stations were installed within the caldera on the flanc of the volcano in the N-E rift zone and the S-E rift zone. Finally, two stations were installed in the Rivi de l'Est tunnel drain, and analyse the Rn activity in ground water, and two stations measure the Rn activity in two hydrothermal springs in " Cilaos " at the base of the Piton des Neiges volcano. Nineteen stations have self-contained acquisition, the data of which are collected about once a month by use of a PSION microcomputer in the field. The data of six stations were collected every minute and transmitted every hour by radio to the observatory.
Temperatures of the summit crater fumeroles are continuously controlled by two probes and data are radio-transmitted every minute, moreover temperatures of six points are measured every week manually.
5. Gravimetry, with a gravity network of 46 gravity bench-marks, that has been measured in 1984-1985 and remesured in 1991 and in 1993.