Observatoire Volcanologique et Sismologique de Guadeloupe
(Institut de Physique du Globe de Paris)
French West Indies
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Roberto Moretti- Head of Soufri鲥 volcano Observatory at Guadeloupe
Observatoire Volcanologique et Sismologique in Guadeloupe (OVSG) is one of the three French volcanological observatories with Montagne Pel饠(Martinique) and Piton de la Fournaise (R鵮ion) observatories. It is in charge of the monitoring of La Soufri貥 volcano and regional seismicity around the Guadeloupe archipelago. The head of the French volcanological observatories is at the Institut de Physique du Globe de Paris.
A basic geophysical laboratory was created in 1948 in Saint-Claude on the flanks of Soufri貥 volcano responsible for installing and maintaining a handful of seismic stations. Upgrade and modernization of the volcano monitoring network began with the onset of significant seismic unrest in 1975 which turned out to be premonitory to the violent and long-lasting phreatic eruption of 1976-1977. As a result of this eruption whose consequences could have been much more severe, a substantial program of basic volcano research as well as a comprehensive multidisciplinary volcano monitoring network has been supported since then by the French government.
During the 1976 eruption, seismic monitoring equipment was moved to a crisis site near the sea in the thick protective walls of the XVIth century Fort Delgres, while administrative staff mainly stayed in Saint-Claude. The Observatory was finally moved in 1993 to a single operational site on top of the extinct Hou묭ont volcanic plug at an elevation of 430 m and at a distance of about 8 km from the active volcano summit. The current modern Observatory houses all monitoring and data processing installations, a chemistry analytical lab, offices, a small library, a survival crisis cellar, observational tower, as well as technical lab space and living quarters for visiting scientists. Current staff (about 10 resident and visiting) include researchers, electronic, chemical, and computer engineers, technicians, and administrative all part of the French ministry of Education and Research.
The Soufri貥 of Guadeloupe Volcano Observatory is responsible for operating and maintaining the surveillance network, and for data processing, analysis, and archival. These data is available to IPGP and other collaborators. The Observatory is also in charge of informing the official authorities and elected officials in Guadeloupe concerned by risk prevention, mitigation, and emergency planning and management, as well as informing the population via a public monthly information bulletin and regular media communications. The Observatory also actively participates in research programs, collaborative monitoring programs in the Caribbean (Montserrat, Dominica), and training activities related to dissemination of scientific volcano-related information and applied aspects of volcano hazard preparedness and prevention. The recent increase in fumarolic activity and low-energy shallow level seismicity since 1997 prompted a significant upgrade in the monitoring network, development national research programs, and public information as well as renewed interests by Civil Protection authorities to set up contingency mitigation and emergency plans.
The Soufri貥 monitoring lies on several scientific disciplines lasting from geophysics and geochemistry to geological observations. The current network includes about 70 telemetered permanent stations with continuous or semi-continuous recording, and about 250 sites measured manually in the field with varying acquisition periods (see network maps below).
The seismic networks set up on the Guadeloupean islands are composed of 39 stations, which are designed to record seismic activity directly linked to the Soufri貥 volcano but also to the subduction of the Atlantic plate beneath the Caribbean one, which can give rise to major earthquakes (1843, M ~ 8). Usually it records every year between 500-1000 regional and local earthquakes related to the subduction zone, about 10 of them are felt by guadeloupean population. On November 21th 2004, an intraplaque shallow event of magnitude 6.3 occured between Les Saintes Archipelago and Dominica. One child was killed, about 45 people injured and several public building and houses were damaged in Guadeloupe. The event was followed by more than 23,000 aftershocks (the biggest one on February 14th, 2005) and it still continues on August 2005.
a) Volcanological network
For the permanent monitoring of the volcano, 18 permanent stations have been installed on and around it in a polygon of 100 km² on the Basse-Terre island. The stations belong to four types:
- 10 analog short-period one-vertical-component stations (Mark Product L4C), telemetered to the observatory by voice-grade radio after amplification and frequency modulation of the seismic signal (UHF/VHF),
- 1 analog short-period three-component station (Mark Product L43D), with frequency modulation of the amplified signals and mixing of the carriers by Lennartz ampli-modulators and Lennartz radio links to the observatory. This station which was the first one with a triaxial sensor of the network and is installed on the site of the previous St-Claude observatory, 5 km away from the summit of the volcano,
- 3 digital short-period three-components stations (Mark Product L43D and Lennartz LE 20-s). These stations use Lennartz PCM 5800 digital instruments, which by gain ranging have a dynamic range of 120 dB, with digital wide band telemetry to the observatory. One of these stations has been operating since 1993 1-km from the summit to constrain the focus depth by unclipped S-readings on horizontal components. A second one was installed directly on the top of the volcano to help detection and location of the low energy activity at the summit. The third one was installed at the observatory, 8.5 km from the volcano summit.
- 4 digital broad-band three-component stations (Guralp CMG-40T). Installed in 2003 to monitor shallow hydrothermal activity, these stations use GeoSIG GSR-24 digital converters and transmit 24-bits data in real-time through radio-modem in the 2.4 GHz band. Sensors have been installed at 2-m depth with a thorough thermal isolation system.
b) Regional network
21 other stations are installed to cover the Guadeloupe archipelago: Grande-Terre, La D鳩rade, Marie-Galante islands but also some surrounding islands like Montserrat, Antigua, and Dominica. These data increase the detection capacity of the networks but also the accuracy of the location for earthquakes occuring along the subduction slab, mostly eastward of Guadeloupe, and in the Pointe-itre Bay. The equipment belong to three type:
- 8 analog short-period one-vertical-component stations (Mark Product L4C),
- 1 digital short-period three-components station (Mark Product L43D),
- 17 digital three-component accelerometers (Kinemetrics EpiSensor FBA-EST), part of the French National GIS-RAP network or CDSA (Antilles Seismological Data Center, project with local BRGM and University), with triggered detection and phone cabled modem transmission.
Some stations operated by Seismic Research Unit (Trinidad) on other islands are also used for regional studies (Nevis), as well as stations of Montagne Pel饠Observatory in Martinique (IPGP).
c) Recording and processing
Except for the accelerometer network in trigger mode, all seismic signals are transmitted continuously and in real time toward the observatory, some through radio repeaters. Records are in continuous mode, 100 Hz sampling, using IASPEI SUDS format and GEOSIG native format, 2 or 5-min duration files, and saved every day on two full CD-R (with automatic process).
Between 1975 and 2003, two five-channels stripchart paper recorder (Sefram) were used to display continuously at 2.5 mm/s the sismograms of five stations of both networks (volcanologic and regional). They are now replaced by computer processes using Matlab routines and HTTP interface with automatic alarms on saturated events (possibly felt by population). The records are daily read by an operator. Location of hypocenters and usual parameters of seismicity are locally computed. Data are archived and automatically processed with preset time scales of 24h, 30 days, 1 year and 10 years comprehensive graphics for simulated helicorders (one per component), bulletins (number, energy, Guttenberg-Richer diagrams) and hypocenter maps and profiles at different geographic scales. Regular bulletins are monthly issued by the observatory; further analysis are carried out at IPGP.
Since the end of 2004, maximum horizontal ground accelerations (PGA) are estimated for the main cities of Guadeloupe immediately after a seismic event.This simulation is based on a local attenuation law named "B-Cube" [Beauducel et al., 2004], that took advantage of the numerous strong-motion records associated to Les Saintes crisis.
The observation of the ground deformation of Soufri貥 began in 1976. The networks have been progressively improved and are now using different scale and sensitivity instruments:
- 5 sites using 20 silica pendulums (Blum type) with control of rock temperature, are telemetered to the observatory. This network was progressively expanded and improved, especially adding 2 redundant components to each site (at few meters of distance),
- 2 permanent three-component crackmeters have been installed since 1980 on the south and the north flanks of the dome. They allow to observe open cracks on the volcano,
- 18 permanent one-component long-base extensometers have been installed since 1995 on the historically active fractures of the dome (up to 20-m width). They are manually measured every 3 months,
- a levelling network, on a road located at the southern flank of the volcano has been regularly surveyed since 1978,
- a GPS network (of about 35 points) has been measured every 2 years since 1993,
- 2 permanent GPS stations have been installed since 2000, using Ashtech Z12 receivers and digital radio-modem transmitters 2.4 GHz. Acquisition rate is 30 s and data are processed using 8-hours sessions,
- 10 prism reflectors on the dome and surrounding are measured using a permanent laser-based distance (EDM) system from the observatory (Leica TM1100). This is mainly for landslide survey and possible flank collapses monitoring.
All these data are processed using real-time automatic graphic routines showing parameter trends with preset time scales of 24h, 30 days, 1 year and 10 years. Manual measurements are integrated into the database using HTTP forms.
Fluid circulations within the volcanic edifice are monitored with monthly thermo-mineral springs and gas fumaroles sampling, in order to evaluate the composition of the volcanic fluids:
- 6 hot springs have been monitored since 1979, 2 others since 1992. In addition to temperature, pH, electrical conductivity, and flow rate, samples are locally analysed using Ion Chromatograph (Dionex) then sent to Paris for further analyses,
- depending on the degassing activity, 1 to 4 fumaroles are sampled every month using P2O5, NaOH or void flasks, then analysed using Gas Chromatograph (Varian) and Mass Spectrometer (Pfeiffer QS422). Samples are then sent to Paris for isotopic analyses (especially Oxygen and Chlorine),
- also 5 fumaroles have been equiped with KOH solution (4N) recording continuously acid components of gases (so called "japanese box"). These are replaced and analysed every month,
- 5 sites have been equipped with permanent Radon sensors (Cliperton and Barasol) for diffuse soil degasing monitoring on historically active fumarolle zones around the dome, they are complemented with periodical soil measurements with portable IR gas spectrometer (Geotechnical),
- in 2003, the observatory has acquired jointly with Antilles-Guyane University a FTIR gas spectrometer (Midac), which is used periodically to measure the main gas flux at the summit.
All these data are entered into the database using HTTP forms and processed using real-time automatic graphic routines showing all parameter trends with preset time scales of 1 year and 10 years.
During 1993 a bidirectional radio-linked magnetic network has been settled on Soufri貥 volcano, extended in 2000 and complemented with a ground self-potential station:
- 6 magnetic field permanent stations located around the dome, recording the Earth magnetic field with an accuracy of 0.1 nT. A reference station located 5-km apart is used to compute short and long term differences,
- One permanent station of ground self-potential measurements is installed across the August 30, 1976 major fault on a 400-m long baseline, recorded with 1-min sampling and direct digital transmission to the observatory.
All these data are processed using real-time automatic graphic routines showing absolute and relative parameter trends with preset time scales of 24h, 30 days, 1 year and 10 years.
OTHER NETWORKS AND SURVEY
- A microgravity network (of about 35 points located on the volcano edifice) is measured every other year at the same time as GPS,
- 3 multi-parameter sensors (temperature, pressure, water level, acoustic noise) are installed in two 100-m deep boreholes on the west and the south part of the volcano, and a last one in a 120-m deep acid crater lake at the summit of the volcano. The aim is to access the thermal change of the dome after the 1976 activity,
- A complete autonomous weather station at the volcano summit, complemented by a network of 6 rainmeters jointly operated with M鴩o France, allows to evaluate water input and to correct measurements from atmospheric effects,
- Phenomenology survey is performed by systematic detailed visual observations of modifications in superficial activity with photo and video support.
Thanks to IPGP and collaboration with other universities, a team of about 50 scientists and technicians affiliated with the Office of Volcano Observatories participates actively in the development of new monitoring equipment, processing and analysis of the data. In this framework, frequent surveys and experiments in geophysics (electrical and seismic tomography, .), chemistry, and volcanology (deposits and other geological mapping, .) are carried on to complement the permanent monitoring network.
DATABASE AND REAL-TIME GRAPHIC PROCESSING
The Observatory maintains an extensive computer network with automatic data processing, storage and transmition to IPGP for all of the telemetered and manual data. A recent development of automated processing routines allows immediate access via an internet server to the status of the monitoring network, to complete numerical and graphical data set, as well as to key quantitative indicators of volcano activity. This original system has been effective since 2001 and was adapted for Montagne Pel Observatory in 2003. It consists in a single Linux server (dual processors 2.4 MHz, 1.5 Mb RAM and 150 Gb SCSI-W3 disks) and home-made scripts written in Matlab, Perl and Shell. Main characteristics are the following:
a) Data bank
- telemetred data are temporary stored on dedicated acquisition computers in files with native format, then copied on a single server disk using network connection;
- manual data are entered using HTML forms and stored in text files;
- network characteristics (stations and sensors) and maintenance chronology are stored using HTML forms and simple text files; every monitored sites are included (an automated station or even a simple GPS benchmark), in order to keep historical detailed or basic characteristics of each site of interest (this leads to more than 300 data sheets).
b) Data processing
- routines written in Matlab are specific to each network and allow to import data into homogeneous time-referenced matrices;
- they produce preset moving time windows (day-, month-, year-scale, ...) extended graphics of three types: 1 graph per station (original data), 1 per network (processed/validated data), 1 per discipline (first order modelling, under construction);
- user defined graphic requests can be performed using HTML form (choice of date intervals and graphical options).
c) Acquisition control
- script shells on the server allow basic tests on each acquisition computers (network connection, computer clock, date and time of last recorded data);
- any problems on acquisition computers are automatically sent via Windows pop-up on several computers, e-mail and SMS.
d) Routine schedule
- Every minute: test of acquisitions, process of continuous seismic data (digital seismograph on the last 36 hours), process of user graphic request, update control screens;
- Every 20 minutes: (depending on the present server capabilities) transfer of new data files from acquisition computers to server (copydir update), launch Matlab graphical routines for all networks (preset time-scales);
- Every day: update some graphics ("all data" time scale) and network maps based on DEM and orthophotos (1 per network, 1 per discipline, 5 integrated network maps at different geographical scales, detailed station location); burn 2 CD-R for continuous seismic data archive; automatic mirror of the server disks with a central server in Paris.
e) Web interface
Inputs and outputs of this system is managed through a restricted access Web site with a fixed architecture and dynamic pages generated by the Matlab routines themselves or Perl CGI scripts. The Web site contains hundreds pages and graphics, and thousands cross-links. It leads to several usefull tools for observatory managment:
- Access to monitoring data: near real-time and immediate access (since graphics are automatically made and not processed on request); numerical and graphical data presentation; original, validated and processed data; associated technical information (link from the graph to the station characteristics); global overview (all networks, all stations on the same time scale);
- Technical network maintenance: automatic control of acquisition (accessible from anywhere including alarms); detailled following of technical information and interventions;
- Scientific and technical collaborations: access to a unique level of information, strong support for discussion and data exchange (especially between observatory and distant collaborators).