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

Abstract

There is increasing interest in the effect of bubbles in gassy sediment. This is, first, because of
the impact those bubbles have on the structural integrity and load-bearing capabilities of the
sediment; 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 methane
from the seabed to the atmosphere); and third, because of the effect which the bubbles have
on any acoustic systems used to characterise the sediment. For this reason, a range of methods
have been investigated for their ability to estimate the bubble population in the seabed. Within
such a range, there will a mix of advantages and limitation to given techniques. This report
outlines a very basic method by which an observations which have already been taken for
other purposes (sub-bottom profiles) may be subjected to a rapid analysis to obtain an
estimate of the effect of bubbles on the sound speed in the sediment, and from there to
provide a rapid preliminary estimate of the void fraction of bubbles present (assuming quasistatic
bubble dynamics). This approach is not meant to compete with large-scale field trials
which deploy specialist equipment to monitor gas bubbles in sediment, but rather to provide a
method to exploit archived sub-bottom profiles, or to survey a large area rapidly with
commercial equipment from a small vessel, in order to obtain an estimation of the local void
fractions present, and their location and extent in three dimensions.

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Identifiers

ORCID for T.G. Leighton: orcid.org/0000-0002-1649-8750

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