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Nuclear explosion monitoring is undergoing a steady progression toward event detection, location and identification at lower magnitudes. As the threshold for detection and characterization has decreased, the distances at which monitoring takes place have decreased correspondingly from originally teleseismic, to regional (< 2000 km) and now to local (< 200 km) ranges. The consequent challenges to monitoring include far larger numbers of events to process and the need to interpret broader categories of events.
In the context of monitoring smaller seismic events, the numerous man-made mining explosions in various parts of the world become a significant consideration. Such explosions are typically conducted in such a way that they fracture large amounts of rock, while not creating ground motions exceeding a certain safety threshold. The principal technique for conducting such explosions is so-called ripple-firing, whereby a large number (some times several hundred) small individual charges are detonated in a predetermined spatial pattern and with short time delays (of the order of milliseconds) between each charge. The resulting seismic event is typically of Richter magnitude 3.0 or below.
NORSAR is carrying out an ambitious research program in detecting, locating and characterizing mining explosions. Our principal focus region is the Kola Peninsula, which has a large number of mines where mining explosions are frequently carried out. Furthermore, through a longstanding cooperation with the Kola Regional Seismological Center (KRSC), we have access to detailed information (so-called ground truth data) concerning these explosions, and this provides an opportunity to verify the results of the processing against independently obtained information.
The principal mining areas in the Kola Peninsula, together with the ARCES
seismic array in northern Norway and the Apatity seismic/infrasound array
in the Kola Peninsula. The explosion site in northern Finland is used to
destroy outdated ammunition.
The collection of ground truth information for several thousand mining explosions during more than 10 years has provided a unique database. In spite of extensive studies over the years, some of the features are still poorly understood, and will require further research.
Recordings filtered in 5 frequency bands of one underground and one
surface explosion conducted in Khibiny, only 3 minutes apart and located
at a distance of only 300 m from each other. Contrary to expectations,
the surface explosion has order-of-magnitude more high-frequency
signal content than the underground explosion.
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