A stylised image of the world with the continents in white and the oceans in deep blue. Dotted around the land and oceans are nuclear hazard symbols representing nuclear explosions or tests
Case Study Overview

Improving the detection of nuclear explosions and extreme natural events

Prior to the 1990s there were over 2000 nuclear weapon tests - since then there have been only 10. The main reason was the signing of the provisional Comprehensive Nuclear-Test-Ban Treaty (CTBT) in 1996. To ensure adherence, over 300 stations around the globe monitor nuclear explosions, mainly through acoustic, hydroacoustic and vibration measurements. Prior to 2020, ensuring traceability to the SI of such measurements was a challenge.

Challenge

Low frequency sound (below 20 Hz for human hearing) and vibration (0.01-5 Hz) propagating through the atmosphere, oceans and ground are used to detect natural events such as earthquakes, tsunamis and volcanic activity. These frequencies are also indicative of anthropic events including nuclear explosions. To detect these the International Monitoring System (IMS), designed and implemented under the provisional Comprehensive Nuclear-Test-Ban Treaty (CTBT), uses specific instrumentation at over three hundred stations (infrasound, seismic, hydroacoustic, radionuclide) around the globe.

For waves propagating through the atmosphere, each infrasound station uses many low frequency acoustic sensors called microbarometers. Several stations are necessary to triangulate the source of an explosion. As many stations are in remote locations where extremes of temperature, pressure and humidity can affect readings, it is vital that the instrumentation used is accurately calibrated to account for environmental effects. However, as stations need to operate continually to satisfy a high rate of data availability, operational sensors cannot be removed for calibration in a laboratory. Consequently, calibration is performed onsite, using a reference microbarometer connected to the operational measuring chain. This reference microbarometer must also be rigorously calibrated to ensure traceability to the SI.

There was an urgent need for new “transportable standards” to calibrate reference microbarometers to validate the data from IMS network instruments.

Solution

During the Infra-AUV project, the partner CEA (Commissariat à l’énergie atomique et aux énergies alternatives) improved an innovative portable infrasound calibration system they had developed. This instrument, the “Système de Métrologie Infrasonore de Terrain” (SMIT), composed of an infrasound dynamic pressure generator, control electronics and a geophysical field digitiser, was improved by the addition of a commercial microphone previously calibrated at low frequency by LNE, the National Metrology Institute of France. This was made possible due to a new primary infrasound calibration bench, also developed and validated during the project.

The size of the SMIT was reduced to fit inside a suitcase and the weight to 30 kg allowing transportation to remote IMS sites. The instrument was then tested at different locations with varying temperatures (from -2 °C to 40 °C) and altitudes (from 0 m to 4000 m) validating the behaviour of the device in the field. Finally, the SMIT was trialled at the IMS IS26 infrasound station in Germany to calibrate onsite reference microbarometers providing metrological traceability of the infrasound measurements to the SI.

Impact

During the Infra-UAV project the SMIT demonstrated the ability to calibrate on-site reference microbarometers from 0.1 Hz to 100 Hz. The SMIT system allows the calibration of any type of geophysical infrasound sensor at the location where the measurement will be performed. A new version is also planned to further reduce size and weight and increase the range down to 0.02 Hz thus improving the performance and quality of the measurement results.

This is the first time that traceability to SI units has been brought to infrasound sensors operating in the field. This not only improves the comparability of measurements between locations but also allows a sensor array to account for the direction of arrival of the infrasound signal, and its amplitude and velocity to be known with greater precision.

This is important not only to detect and locate any nuclear events that breach the CTBT, but also provides better information on extreme natural events.

Image showing a huge water fountain caused by a below surface explosion

Providing traceability in low frequencies underwater, through the ground and the atmosphere

The Infra-UAV project was the first to address measurement needs across three technologies relevant to the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO).

New calibration capabilities have been set up for all three technologies, validated by intercomparisons.

On-site calibration methods were evaluated at IMS stations in Germany. At IS26 infrasound station, an innovative travelling standard allowed the first demonstration of an onsite calibration of microbarometers.

Other instrumentation was also developed, including a new primary standard for hydrophones in the frequency range 0.5 – 250 Hz, and a carousel system for microphone calibration (0.1 Hz to 5 Hz) which represent the world’s first such capabilities.

The new facilities, methods and instruments will not only support the CTBT, but also environmental and other types of monitoring stations around the world.

Contact

  • Category
  • EMPIR,
  • Environment,
  • EMN Climate and Ocean Observation,
  • EMN Pollution Monitoring,
Element 1 Element 2 Element 3 Element 1 Element 1 Element 1 Element 1 Element 1 Logo-Footer