Experimental observations of acoustic backscattering from spherical and wobbly bubbles

Abstract

Methane bubbles released from the seafloor transport gas through the water column to the atmosphere. Direct or optical methods via underwater vehicles are often used for quantifying methane gas flux in the water column, however these methods are time consuming and expensive. Acoustic measurements, using split-beam and multibeam echo sounders that are readily available on most sea vessels, provide a more efficient method for determining methane gas flux. These acoustic methods typically convert acoustic backscatter measurements of bubbles to bubble size using analytical models of bubble target strength. These models assume that bubbles have uniform shape; however it has been shown that bubbles with a radius greater than 1 mm, which have large Eötvös and/or large Reynolds number, are non-spherical. To investigate the error associated with assuming large bubbles are spherical, a 6 m deep tank experiment was conducted to compare calibrated target strength measurements of both small spherical and large wobbly bubbles to existing acoustic scattering models. Bubble sizes observed in this experiment ranged from a fraction of 1 mm to 6 mm in radius. This experiment used a broad range of frequencies (10-300 kHz) to cover typical echo sounder frequencies utilized in field measurements of natural methane seeps.