Effect of explosion energy and depth on the nature of explosion cloud: A field experimental study

Field explosion experiments using dynamite were carried out to investigate the effect of explosion energy and depth on the nature of explosion cloud. The scaling law of explosion was established among the energy, depth, and the nature of explosion cloud; shape, height and duration of flow out. The shape of explosion cloud changed systematically from funnel type via elongated to short types with increasing the scaled depth of explosion. At the shallow scaled depth, both scaled height and scaled duration increased with increasing scaled depth. Both values reached the maximum at 0.003–0.004 m/J1/3 of cube-root scaled depth, and then decreased with increasing scaled depth. If the cube-root scaled depth was over about 0.01 m/J1/3, we could not observe any explosion cloud on the ground surface. These experimental results should be applicable to the understanding of volcanic explosions if the buoyant effect of internal heat in the explosion cloud was negligible and ground medium is identical. We applied these results to the explosion clouds of Usu phreatic explosion at 14:53 h on 17 April 2000, and estimated that the explosion energy and the depth were 4×109 J and 6 m, respectively. Errors caused by uncertainty of scaling factors are less than one order of magnitude for the energy and depth.