Can acoustic emissions be used to size bubbles seeping from a sediment bed?

Experiments on bubble formation from a granular medium are presented. The granular medium consisted of immersed stones giving a sediment bed thickness of 10 cm. High speed photographic and acoustic passive wireless techniques were employed to obtain the bubble size. The power spectra of the acoustic data for bubble production of 0.5 bubbles/s showed the existence of two principal peaks which are correlated with two discrete events. Firstly, a signal peak at 0.497 kHz, representing asymmetric bubble detachment, and secondly, a peak at 2.070 kHz, which corresponded to interactions of the granular media as inter-particle gaps advance, representing elastic resistance to orifice creation. Provided the signals were windowed to the bubble-detachment event only, the classical bubble-acoustical Minnaert relation for the bubble size agreed with optical data within about 10%. Since seepage generates discrete bubble pulses, appropriate acoustic analyses could both count and size the bubbles formed. The results of this study lead to the proposition that underwater acoustic sensors could warn of critical changes in bubble-seep behavior over time. This could lead to the possibility of remote early warnings, because shifts in production-rate regimes from seeps may herald alterations in the progression of global warming, or impending earthquakes and tsunamis.