A seismic/acoustic study of low altitude air blasts at Ft. Devens, Massachusetts

On 5 May 1986, 3 packages of TNT were detonated 100 ft in the air above the Turner Drop Zone at Ft. Devens, Massachusetts. The aerial explosions were recorded by seismic and acoustic sensors over 2 linear arrays that covered the distance range from 0 to 1000 meters. One array was installed over flat topography, while the other was installed across a hill with 80 ft of relief. The goal of the experiment was to examine the response of the ground and the air to a unit impulse. Although recording of the higher frequencies generated by the blasts was limited by the sampling rate capability of the equipment (100 and 200 samples/sec) it was clear that the hill produced a sheltering effect. Considerable transverse motion appeared in the records. Reduced time plots revealed a propagation speed of 0.35 km/sec corresponding primarily to an airwave induced seismic signal, although some seismic precursors are evident. The ratio of peak seismic velocity to pressure ranged from 10-4 cm/sec/Pa for the line that transversed the hill to .00001 cm/sec/Pa for the flat array. The experiment should be repeated in the same area during different seasonal coverings (snow and ice) and extended to areas with more complex topography and different vegetation. In future experiments the position of the air blasts should be varied to facilitate discrimination of paths to the receivers. The results could be extrapolated to an extended, continuous source such as a low flying aircraft or cruise missile.