A multitechnical analysis of the shallow gas blanket in Southampton Water

Robb, G.B.N (2000) A multitechnical analysis of the shallow gas blanket in Southampton Water. University of Southampton, School of Ocean and Earth Science, Masters Thesis, 52pp.

Abstract

The presence of bubbles of biogenic gas in marine sediments has a dramatic effect on the acoustical properties of the bulk sediment, and can act as an acoustic blanket through which acoustic pulses cannot penetrate. A combination of bore hole data, chirp profiling and forward modelling was used to examine the intertidal sediments in Dibden Bay, Southampton Water. The sediment consisted of stratified Holocene clays, Pleistocene sands and Eocene Bedrock and satisfied necessary conditions to allow the production of biogenic gas. A gas blanket, which dipped in an offshore direction, lay between 1 and 2 m below the seabed. This covered the majority of the bay, with some small gas free windows present. Results suggested that factors controlling the lateral variations in the depth of the gas blanket could be split into two categories: those which control variations on large scales, including pressure, salinity, temperature and depth of the sulphate reducing zone; and those which control variations on smaller scales, i.e. sedimentary boundaries. In Dibden Bay lateral variations in sedimentary boundaries appeared to control the depth of the gas blanket.

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