To illustrate the processing of infrasonic signals from stations in the Nordic network, we show data for two of the stations (Kiruna, Sweden and Apatity, Russia) for two surface explosions in NW Russia near the Norwegian border. These explosions were presumably conducted for the purpose of destroying old ammunition. We record a number of such events each year from this site.
Kiruna-data for two surface explosions.
Apatity-data for two surface explosions.
Closer view of the Kiruna and Apatity data for the two surface explosions.
Although the Kiruna signals are clipped due to the limited dynamic range of the digitizer, we have found that reliable azimuths can be easily estimated using either f-k analysis or a cross-correlation technique (bottom left). The fact that the array is very small (75 meters across) does not seem to be a disadvantage at the frequencies of interest for this type of events (2-8 Hz). For the Apatity array, the dynamic range is much larger, and the true signal can therefore be recorded. The cross-correlation analysis (bottom right) provides a sharper peak than for Kiruna, due to the greater size of the array (500 meters across). It is noteworthy that such small arrays are capable of consistently providing reliable azimuths.
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