We have established a database of a sequence of more than 100 surface explosions in northern Finland at the site marked as Explosion site on the map below. These explosions, which are carried out annually in August/September, are presumably for the purpose of destroying old ammunition, and generate unusually strong infrasonic signals in addition to seismic signals.
The situation of ARCES and Apatity in relation to the explosion site.
Typical seismic and infrasonic signals generated by these explosions are shown in the figure below. The figure shows waveform data from the ARCES seismic array one minute prior to and 14 minutes after a surface explosion in northern Finland at a distance of approximately 175 km. The top left panel shows waveforms on the central seismometer: the seismic P-phase (A), seismic Sphase (B), and an infrasonic arrival approximately 640 seconds after the estimated origin time (C). The broadband f-k analysis plots (right side) indicate that all of these signals come from a backazimuth of about 173 degrees. The bottom left panel shows P, S and infrasonic array beams steered towards the epicenter, and illustrates the gain in signal-to-noise ratio resulting from the beamforming process.
Waveform data from the ARCES seismic array.
The figure below shows time-aligned signals on the ARA0_sz sensor of the ARCES array for all the explosions at the northern Finland site which took place in the year 2002. The epicentral distance is 175 km. A large amplitude acoustic signal approximately 600 seconds following the origin time is observed for almost all of these events but, unlike the seismic signals which are almost identical for each explosion, the temporal nature and amplitudes of the acoustic signals differ greatly from event to event.
Time-aligned signals on the ARA0_sz sensor of the ARCES array.
The figure below displays a color-scaled indication of an “infrasound detection statistic” for the ARCES and Apatity arrays for a five-minutes long time-window following each event in the database of explosions at the northern Finland site during the years 2001-2006. For ARCES, which has the most complete data, the vast majority of the events register a candidate acoustic phase between approximately 620 and 660 seconds after the event. A smaller number of events also indicate an earlier arrival from approximately 500 seconds. The most common arrivals occur approximately 600 seconds after the event with a superimposed variation which appears to vary quite smoothly over a several days time-scale.
Infrasound detection statistic for the ARCES and Apatity arrays.
|
 |
|
|
|
