Acoustic signals of underwater explosions near surfaces

Abstract

Underwater explosions are conventionally identified and characterized by their seismic and/or acoustic signature based on spherical models of explosion bubbles. These models can be misleading in cases where the bubble is distorted by proximity to the free surface, the bottom, or to a solid object. An experimental and numerical study of the effects of various nearby surfaces on the bubble’s acoustic signature is presented. Measurements from high-speed movie visualizations and acoustic signals are presented which show that the effect of proximity to a rigid surface is to increase the first period, weaken the first bubble pulse, and affect significantly the second period, resulting in a peak value at standoffs of the same order as the maximum bubble radius. These results are compared to results under a free surface, over a bed of sand, and over a cavity in a rigid surface. In all cases, the first period is increasingly lengthened or shortened as the motion of fluid around the bubble is increasingly or decreasingly hindered. The effect of bubble distortion is to weaken the first bubble pulse and increase the bubble size and the duration of the second cycle.