Measuring marine gas flux via passive acoustic signals—New field and lab observations

Abstract

In recent years there has been a growing need to refine methods for quantifying the volume of gas released into the water column from marine sediments via seeps/vents. Traditional methods of flux quantification are flawed, being energy intensive techniques that provide single snapshot measurements. As a result, emission estimates across larger areas are very poorly constrained, often ranging across several orders of magnitude, are rarely cross validated and almost never account for temporal variation. Passive acoustic flux inversion techniques, whereby the acoustic signal of such seeps can be used to measure the gas flux, have been identified as a key tool for overcoming these limitations. Several field studies have already demonstrated the potential of such techniques, granting fresh insights into the variability of gas release with tides, surface seiches and even day/night temperature cycles. Here, we discuss our work to further advance passive acoustic flux inversion in order to enable large scale non-specialist adoption. Presenting field observations and our recent work studying the excitation of gas bubbles and the acoustic signature of gas propagating through unconsolidated sediment. Achieved with a combination of high-speed photography, CT imaging, radiography, geophones, fibre optic cables and traditional marine hydrophones.