Oscillation of bubbles released from needles in natural and artificial sediments

Abstract

The acoustical characteristics of bubbles differ significantly in sediments as compared to those in sediment-free water. To examine this effect, an experiment has been designed by releasing bubbles from a needle/hydrophone assembly inside saturated natural and artificial sediments. Observations show that the resonance frequency of these bubbles released from needles in the sediment decreases and their total damping coefficient increases significantly compared to bubbles released from needles in sediment-free water. For example, a bubble with a radius of 2.5 mm in water has a resonance frequency of approximately 1.3 kHz and a quality factor of approximately 30. These parameters reduced to approximately 800 Hz and 2 for a similar bubble in a natural sediment with a mean particle size of 30 μm. Theoretically the total damping coefficient of an oscillating bubble in water consists of the thermal, acoustic and viscous contributions. Another dissipation factor, the Darcy damping, is found to be dominant in acoustic emissions of bubbles in sediments. This term accounts for the interaction forces between the liquid and the porous skeleton. The theoretical predictions of the total damping coefficient of bubbles in sediments (in the rigid matrix approximation) are discussed and the results are compared with experimental observations.