A method for estimating sound speed and the void fraction of bubbles from sub-bottom sonar images of gassy seabeds

摘要

There is increasing interest in the effect of bubbles in gassy sediment. This is, first, because ofthe impact those bubbles have on the structural integrity and load-bearing capabilities of thesediment; second, because the presence of bubbles can be indicative of a range of biological,chemical or geophysical processes (such as the climatologically-important flux of methanefrom the seabed to the atmosphere); and third, because of the effect which the bubbles haveon any acoustic systems used to characterise the sediment. For this reason, a range of methodshave been investigated for their ability to estimate the bubble population in the seabed. Withinsuch a range, there will a mix of advantages and limitation to given techniques. This reportoutlines a very basic method by which an observations which have already been taken forother purposes (sub-bottom profiles) may be subjected to a rapid analysis to obtain anestimate of the effect of bubbles on the sound speed in the sediment, and from there toprovide a rapid preliminary estimate of the void fraction of bubbles present (assuming quasistaticbubble dynamics). This approach is not meant to compete with large-scale field trialswhich deploy specialist equipment to monitor gas bubbles in sediment, but rather to provide amethod to exploit archived sub-bottom profiles, or to survey a large area rapidly withcommercial equipment from a small vessel, in order to obtain an estimation of the local voidfractions present, and their location and extent in three dimensions.