Passive acoustic quantification of underwater gas seepage

In this paper the design, methodology and the experimental results obtained through a novel passive-acoustic based device for quantifying the gas flux from bubble plumes and marine seeps are presented. In order to estimate gas flux variations a gas capture buoy is designed. The seep environment is simulated by forcing compressed air through a submerged tube. Different flow rates are generated by changing only the pressure of the supplied air. The assembly collects gas and redirects it through a nozzle mounted on top of a collector, where the sound of the gas leaving the nozzle is recorded through a single hydrophone. A quantitative gas flux estimate is made using a Morlet wavelet based analysis of the recorded sound signals series. The data from the acoustic measurement is compared with the data from the visual measurements, as well as with the measurements at a gas flow meter. The accuracy of the acoustic approach is found to be in the range of ~3% for the high air fluxes and up to 0% for the low fluxes. The technique elaborated in this paper will be employed in the volume gas flux measurements in seep active areas. Some sources such as marine seepage are poorly quantified due to a lack of consistent measurement techniques. Quantifying the bubbling flux of methane and other gasses through the hydrosphere and into the atmosphere is important for estimating global budgets