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Infrasound in the Zone of Silence
SOUTHERN METHODIST UNIV DALLAS TX
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Two controlled source experiments were conducted to study infrasound signal propagation at distances less than 300 km from the source. This area around a source is sometimes referred to as the zone of silence because, based on ray theory, no infrasound energy is expected to return to the ground. However, many observations of returned infrasound energy have been made at these distances. In 2006, the site at 76 km recorded both tropospheric and stratospheric arrivals, while at 108 and 157 km only stratospheric arrivals were recorded. In 2007, the site at 157 km recorded both tropospheric and stratospheric arrivals, while at 288 km both stratospheric and thermospheric arrivals were recorded. While stratospheric arrivals appear most frequently, atmospheric modeling with the InfraMAP software, did not predict these arrivals. Current modeling efforts focus on the tropospheric arrivals. Amplitude variations at NVIAR for sources with the same yield varied more than an order of magnitude on two consecutive days. The site located 157 km from the source observed variations by a factor close to five. We therefore attempt to estimate the yields of the explosions using the predominant frequency content of the signals. The physical basis for such a method is found in an increased acoustic transit time of the explosion blast radius with increased yield. Past formulas were developed for old nuclear atmospheric explosions, but no extrapolation to the lower yields was ever performed. In the current work we use controlled sources with explosive yields of 2000-4,000 lbs. in order to verify the applicability of such a scale to lower yields. Preliminary results show that this is possible.
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