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Sensors and networks |
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The CEA's environmental assessment and monitoring Department (Département analyse, surveillance, environnement, DASE) has designed and developed sophisticated and often unique devices and techniques adapted to the phenomena or elements to be detected and measured (seismic events, infrasounds, radionuclides). This scientific and technical expertise is in high demand and widely recognized, particularly in view of the development of the International Monitoring System (IMS) being set up by the Comprehensive Test-Ban Treaty (CTBT). |
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 Seismic event detection |
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The waves generated by a seismic event are detected and measured by seismometers.
IMS seismic stations
170 of the 321 IMS stations will be seismic stations. 50 of these, known as 'primary' seismic stations (including the Tahiti station), will be continuously transmitting real-time recordings. The 120 other 'secondary' seismic stations (including the New Caledonia and Guiana stations), will transmit their recordings on request.
A seismic station can be equipped with several seismometers. Three types of seismometers exist: |
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Short-period seismometers (recorded wave period under 2 seconds) mainly used to study local and regional seismicity and body waves of teleseisms. |
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Long-period seismometers (period up to several hundred seconds) mainly used for studying surface and body waves for large earthquakes. |
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Broadband seismometers used to record in a 100-0.02 s band. These sensors are generally less sensitive than short- or long-period seismometers. |
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The seismometers also differ in terms of detected wave components, and include:
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Vertical seismometers that detect the vertical component of seismic waves. |
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Horizontal seismometers that detect 1 of the 2 horizontal components of seismic waves. |
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Three-component seismometers equipped with 1 vertical sensor and 2 horizontal sensors in the same frame. |
IMS hydroacoustic stations |
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Hydroacoustic stations are either located underwater or along the coast. They detect all the hydroacoustic waves ducted through the SOFAR channel. The distinctive feature of this low-velocity channel (well known to submariners and, depending on the latitude, situated at a depth ranging between a few hundred meters and 1200 m) is that it acts as an underwater waveguide. |
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Hydroacoustic stations rely either on hydrophones or on one or more T-phase seismic stations situated near the coast, preferably on islands. Hydrophones are sensitive to the waves propagating in the water, particularly those caused by blasts. T-phase seismic stations use the steep slopes of islands as receivers of hydroacoustic waves that are then converted into seismic waves.
11 hydroacoustic stations are planned for the IMS: 6 stations equipped with hydrophones (including the Crozet station) and 5 seismic stations (including the Guadeloupe station). |
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Radionuclide detection |
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Radionuclide detection stations
2 types of station are used to detect radionuclides in the atmosphere: |
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Principle: a sensor continuously takes in the ambient air and channels it through a filter that traps the radionuclides. The filter is then removed and compacted to optimize element detection by gamma spectrometry. This type of detection station relies on human intervention and on the presence of an analytical laboratory nearby. |
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Principle: these stations, known as Spalax, are fully automated. Air is collected, then elements such as water, oxygen, carbon dioxide and radon are eliminated, and the remaining xenon is concentrated to identify its 4 radioactive isotopes by spectrometry.
IMS radionuclide stations
The IMS will include 80 radionuclide monitoring stations based on either aerosol or gas technology. The treaty plans to set up 6 radionuclide stations in the French Overseas Departments and Territories of Tahiti, Guadeloupe, Réunion, Kerguelen, French Guiana and Adélie land. |
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Infrasound detection |
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Infrasonic waves are measured by a highly sensitive pressure sensor, or microbarometer.
IMS infrasound stations
The CTBT International Monitoring System will ultimately include 60 infrasound stations.
Each of these stations includes at least 4 sensors, 3 of which are located 1.5 to 3 km apart in an equilateral triangle formation. This spacing improves detectability and enables propagation speed and direction to be deduced through signal processing.
The sensors set up as part of the CTBT have unusual features. The barometer's 4 air inlets are generally supplied by series of 8 air inlets. The purpose of this configuration is to reduce the level of wind-related noise. Thus the collected air flow average is calculated, which limits the effect of turbulence and facilitates desired signal recovery.
Station signal transmission
All monitoring station data are transmitted by satellite to the International Data Center (IDC) in Vienna (Austria).
In the case of radionuclide detection stations, the analysis results are sent every 24 hours.
Advanced research and development activities
For each technology used, the DASE engineers and technicians have developed their own detection networks and reliable sensors with extremely low detection thresholds.
The ZM500 seismic sensor detects ground displacement velocity measured in nanometers per second, i.e. to within one billionth of a meter per second.
The MB-2000 microbarometer can take very low-frequency acoustic measurements with a precision of one thousandth of a Pascal (Pa), i.e. 100 thousandths of a millibar.
On completion of a campaign involving comparison with other detectors of the same type, the Spalax gas radionuclide detection station was recognized as one of the most effective for analyzing xenon isotopes (particularly xenon 133). |
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