Details of Grant 

EPSRC Reference:	EP/D000580/1
Title:	The application of non-linear theory and novel acoustical techniques for the quantitative examination of bubble populations in marine sediments
Principal Investigator:	Leighton, Professor T
Other Investigators:	Best, Professor AI Humphrey, Professor V Dix, Professor J
Researcher Co-Investigators:	Dr GBN Robb
Project Partners:	National Physical Laboratory NPL
Department:	Sch of Ocean and Earth Science
Organisation:	University of Southampton
Scheme:	Standard Research (Pre-FEC)
Starts:	11 July 2005	Ends:	10 April 2009	Value (£):	292,663
EPSRC Research Topic Classifications:	Acoustics Numerical Analysis
EPSRC Industrial Sector Classifications:	Environment
Related Grants:
Panel History:
Summary on Grant Application Form
The physical nature of the seabed is becoming increasing important to a number of industries, owing to an increased use of the world's oceans. One of the major problems to these industries is the presence of gas bubbles (principally methane) within the sediments, the presence of which have numerous effects. Firstly, gas bubbles have a dramatic effect on the sediment's physical properties, greatly reducing its ability to support loads. This can result in submarine landslides and the collapse of offshore structures, e.g. oil rigs. Secondly, gassy sediments can cause blowouts during oil/gas drilling operations, which are costly on both a financial and environmental basis. Thirdly, although methane (a major greenhouse gas) trapped within sediments is a significant supplier of global methane emissions, little is known about the actual quantity which leaks into the atmosphere. Finally, mine and submarine detection techniques require accurate knowledge of how sound travels through both gassy and non-gassy sediments.All of these applications require the ability to measure the number and sizes of bubbles present (i.e. the bubble populations). Such a development would allow the stability of sediments to be assessed, drilling blowouts to be predicted and prevented and the fraction of atmospheric methane arising from sediments to be more accurately calculated (hence improving predictions of future climate change). Although methods of identifying areas of gassy sediments are available at present, the sole technique available for measuring bubble populations (the X-ray scanning of pressurised cores) is expensive/labour intensive and can only measure bubbles with radii greater than 0.5 mm. There is therefore a need to develop a new technique for measuring the bubble populations on the seafloor (i.e. in situ). Although sound waves (acoustic signals) are commonly used to identify the presence or absence of gas bubbles in marine sediments, the existing theory describing this relationship is limited to linear conditions (those generated by low-energy signals travelling through a low concentration of gas bubbles) and to single frequency signals. Gassy sediments frequently contain high concentrations of bubbles, and so require high-energy signals to pass through them (i.e. non-linear conditions), while the signals used for investigation are multi-frequency. Hence, a new theory is required, the successful creation of which will enable us to develop new acoustic techniques for measuring bubble populations in situ. The lead researcher on this project has already developed a non-linear, multi-frequency theory for bubbly liquids, whilst the co-researchers have undertaken extensive research into the passage of acoustic signals through gassy and non-gassy marine sediments, both in the laboratory and in situ. We therefore aim to combine this knowledge to:1. Develop a new theory to describe the effects of gassy sediments on acoustic signals. This will adapt the corresponding theory for bubbly liquids and be relevant for non-linear and multi-frequency signals. 2. Develop a new technique to measure acoustic signals passing through gassy materials. This will be capable of undertaking two separate measurements of how acoustic signals are modified by the sediment. Firstly, the change in velocities and energy losses of the sound passing through the gassy sediment will be measured and secondly, signals scattered from a much more focused volume of gassy material will be recorded. Both of these will be converted to bubble populations using the new theory.3. Test this new approach, firstly in the laboratory by passing sound through both a bubbly gel and artificial gassy sediment with known bubble populations. Finally, a marine site that contains gassy sediment but with an unknown bubble population will be examined, with the results validated by a cross-comparison between each set of measurements and with X-ray scans of a single pressurised core.
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