Observatories and Research Facilities for EUropean Seismology
Volume 5, no 2 September 2003 Orfeus Newsletter


EVO (Evora, Portugal) a broad-band station and GRC (Garchy, France) a broad-band observatory
two stations of the GEOSCOPE network

Geneviève Roult and the GEOSCOPE team

Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 PARIS, France

Introduction - EVO - GRC - References

Introduction

The implementation in the nineties of the new generation of digital broadband seismic stations on the global scale (GEOSCOPE in France, GEOFON in Germany and IRIS in USA) and on a regional scale (MedNet in Italy, Pacific21 in Japan, CDSN in China, CNSN in Canada ...) had a fundamental and positive scientific impact. These stations faithfully record the whole spectrum of seismic signals without distortion and the intensive use of these broadband data have provided remarkable images of earth's deep interior by seismic tomography from the surface to the center of the Earth. These tomographic models are now routinely used in various domains as geodynamics and geochemistry. These data are used both for earthquake focal mechanisms studies (Centroid Moment Tensor solutions) and make it possible to obtain the source time function of their rupture or complex time histories of earthquake faulting, related to tectonics.
The GEOSCOPE program was launched in 1982 by the National Institute of Sciences of Universe (INSU) at the French National Center of Scientific Research (CNRS), at the instigation of the Institute of Physics of the Earth. The purpose of the GEOSCOPE program was the installation of about 25 stations well distributed worlwide (in particular in the southern hemisphere), in the standard configuration defined by the FDSN (very broad-band 24bit, continuous recording at 20 samples/s). In terms of siting locations, the aim of the GEOSCOPE program is almost fulfilled. We plan to install one new station in Russia, in a northern site at very high latitudes, near Vorkuta, in collaboration with our colleagues of IIEPTM (Moscow), a new station in Patagonia (Cohyaique, Chile), a new joint Concordia (PNRA, Programma Nazionale di Ricerche in Antartide) station in Antarctica (DCC, Dome C) in cooperation with GEOSCOPE, and a joint IRIS/GEOSCOPE station on Tristan Da Cunha Island (Figure 1). In Europe two stations, which have been operating in the past few years, are now being closed for different reasons: EVO in Portugal and GRC in France.


Figure 1. Geographical distribution of the GEOSCOPE network, with information on the stations.

EVO station (Evora, Portugal)

Its geographical coordinates were 38.532 N, 8.013W

This station has been installed by GEOSCOPE and CGUL (Centro de Geofisica de la Universidad de Lisboa). It was equipped in 1992 with three STS1 seismometers, in cooperation with EOST (Ecole et Observatoire des Sciences de la Terre) of Strasbourg in France, and the University of Lisbon in Portugal. Locally, the station was maintained by the Physics Department of the University of Evora. The 3 STS1 sensors were in the old broad-band configuration. The datalogger was a 20bit-digitizer called "Cantin" digitizer, built by Michel Cantin at EOST. The corresponding transfer function corresponded to a flat response in acceleration from 3600s to 600s, and a flat response in velocity from 20s to 1s. The data corresponding to this initial period is not available at the Geoscope Data Center in Paris. February 1996 this station was upgraded and transformed into a Very Broad-Band configuration, corresponding to a 3-components BH channel recorded at 40 samples/s and an LH channel recorded at 1 sample/s. The corresponding transfer functions are plotted on Figure 2.


Figure 2. EVO station: transfer function of the different channels (BH, LH) in very broad-band configuration (after February 1996) for the 3 components (vertical, North-South, East-West).

The available data correspond to the period February 1996 to July 1997 and are freely accessible from the GEOSCOPE Data Center in Paris through the common ways (Web, NetDc procedure) described in Roult et al. (2000) or in Roult (2001). The description of the site and the acquisition chain, the plots of the transfer functions are visible on the Geoscope Web site. These data correspond to two different network codes of the FDSN (Federation of Digital Seismographic Networks), GEOSCOPE (G) and the University of Lisboa (LX).

EVO was close to another station, EVOP (latitude = 38,5283N, longitude = 8.1236W, elevation of sensor = 100m) installed by the French military agency CEA/DASE (Departement d'Analyse et de Surveillance de l'Environnement). The sensors are DASE long period seiosmometers. This station located at Santos Brissos, (Valverde, Alentejo) is operated by the Department of Physics of the University of Evora. The data are presently made available through Lisboa.
As EVO and EVOP, 2 "long-period" stations, are so close together, the actual institute decided to move the EVO station to Chaves, a site at the north of Portugal, with the same STS1 seismometers but with now with a Quanterra Q330 datalogger and its baler. This new station will become part of the new broad-band seismic network under the name ULISSEIS (University of LISbon SEISmic network), with network code LX.

GRC (Garchy, France) an experimental site

The GEOSCOPE program claimed for ten years the absolute necessity for installing everywhere various sensors and transforming all the Geoscope seismographic stations in multiparameter observatories. The new generation networks will have to be composed of multiparameter continental or oceanic stations including at least broadband seismometers, environmental sensors (microbarometers, thermometers) and eventually other sensors (electromagnetic sensors, strainmeters, GPS, ...). The design of the complete chain of acquisition, from the sensor to the distribution of data, will imply to integrate all the technical progresses made in micromechanics, electronics, computers, space science, and telecommunication systems. The improvement obtained after correction for the atmospheric pressure is well known now (Figure 3). For scientists who work at long periods and use free oscillations at low frequency (lower than 2mHz), this effect is significant, but can be easily removed if the installation of the station included such a measurement (Beauduin, 1996; Beauduin et al., 1996; Roult & Crawford, 2001). The observation of the interesting phenomenon of free oscillations of the Earth (Nawa et al., 1998; Tanimoto et al., 1999) even in periods seismically quiet can be significantly improved with such recordings.


Figure 3. Daily power spectrogram at TAM (Tamanrasset, Algeria) from 1994 to 1998, in the frequency range 2-4mHz. The vertical straight lines correspond to the spheroidal eigenfrequencies of the Earth (Roult and Crawford, 2000).
Top) before atmospheric pressure correction
Bottom) after atmospheric pressure correction

The GEOSCOPE network has two experimental sites in France, one close to Strasbourg (ECH, Echery), used by the Geoscope team of EOST in Strasbourg, and the other one close to Paris (GRC, Garchy) used by the IPGP team. GRC Observatory has been operating from June 21th 2000 to December 19th 2002, in the Massif Central, an ancient geophysical site presently definitively closed by our academic authorities. It is situated in a fractured limestone with hydrological fracturation. A broad-band STS2 seismometer was installed in a seismic vault at the end of a 10m long tunnel, on a concrete floor. The datalogger was a prototype built by a french company (AGECODAGIS in Toulouse) and planned to provide a new generation of digitizers called Geoscope2000, with 6 channels in 24bits and 16 channels in 16 bits. The corresponding transfer functions are plotted in Figure 4.


Figure 4. GRC station: transfer functions of the different seismic channels (BH 20sps; MH 5sps; LH 1sps; VH 0.1ps)for the 3 components (vertical, North-South, East-West).

The six 24-bit channels were devoted to the 3 components of the seismometers (sensitivity of 1.74884e+09 counts/m/s) and to the POS channels or boom positions (sensitivity of 4.033994e+09 count/m/s).
The 16 other auxiliary channels were devoted to various environmental parameters (microbarometers inside and outside, thermometer, rain gauge..) or magnetic variometers, tiltmeters. The sampling rate of all these continuously recorded channels is 0.625 sps. The distinction between the different instruments are designed by their location code (00, 10 etc).

  • two different thermometers: one PT100 and one PT1000
  • five microbarometers: 3 located at quite the same place and 2 others at 10m, in a triangle configuration:
    • one built by Streckeisen , MIBA with a very good resolution
    • one of EFFA-type with a very good resolution
    • three VAISALA-type.
The purpose of the installation of so many different sensors in the vault was to obtain the gradients of temperature and pressure, for a better estimation of their effect on the seismic recordings. The different sensibilities of all these auxiliary sensors are summarized in the following table.

LKI00 3.276800e+01 count/degree C PT100
LKI10
 
3.276800e+01 count/degree C
 
PT1000
 
LDI00 1.024000e+00 counts/Pascal Effa
LDI10 3.276800e+00 counts/Pascal Streckeisen
LDI20 6.291456e-01 counts/Pascal Vaisala
LDI30 6.291456e-01 counts/Pascal Vaisala
LDI40 6.291456e-01 counts/Pascal Vaisala

References

  • Beauduin, R., 1996. Etude du bruit de fond sismique à l'aide des données GEOSCOPE et des données de l'expérience OFM/SISMOBS, Thesis IPGP, University Paris VII.
  • Beauduin, R., P. Lognonné, J.P. Montagner, S. Cacho, J.F. Karczewski, M. Morand, 1996. The effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station, Bull. Seismol. Soc. Am., 86, 1760-1769.
  • Ekström, G. et al., 2001. Harvard Seismology Research: Seismic noise, Time evolution of long-period noise levels at FDSN stations: IRIS GSN (Global Seismographic Network) and additional stations (http://www.seismology.harvard.edu/~ekstrom/Projects/WQC/temporal.html).
  • Montagner, J.P., P. Lognonné, R. Beauduin, G. Roult, J.F. Karczewski and E. Stutzmann, 1998. Towards multiscalar and multiparameter networks for the next century: the French efforts, Phys. Earth Planet. Int., Special Issue I.O.N., 108, 155-174.
  • Nawa, K., N. Suda, Y. Fukao, T. Sato, Y. Aoyama, K. Shibuya, 1998. Incessant excitation of the Earth's free oscillations, Earths Planets Space, 50, 3-8.
  • Roult, G., J.P. Montagner, E. Stutzmann, S. Barbier and G. Guiveneux, 1999. The GEOSCOPE Program: its Data Center, Phys. Earth Planet. Inter., 113, 25-43.
  • Roult, G. and W. Crawford,.2000. Analysis of "background" free oscillations and how to improve resolution by subtracting the atmospheric pressure signal, Phys. Earth Planet. Inter., 121, 325-338.
  • Stutzmann, E., G. Roult and L. Astiz, 2000. The seismic noise level at GEOSCOPE stations, Bull. Seism. Soc. Am., 90, 3, 690-701.
  • Tanimoto, T., J. Um, K. Nishida, N. Kobayashi, 1998. Earth's continuous oscillations observed on seismically quiet days, Geophys. Res. Lett., 25 (10), 1533-1556.

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