Expulsion of Shallow Gas in the Skagerrak — Evidence from Sub-bottom Profiling, Seismic, Hydroacoustical and Geochemical Data

Hempel, Peter, Spieß, V. and Schreiber, R. (1994) Expulsion of Shallow Gas in the Skagerrak — Evidence from Sub-bottom Profiling, Seismic, Hydroacoustical and Geochemical Data Estuarine, Coastal and Shelf Science, 38 (6). pp. 583-601. DOI 10.1006/ecss.1994.1040.

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Abstract

A combined high resolution seismic, sub-bottom profiling, and multi-beam echo-sounding survey in the Skagerrak (Danish sector of the North Sea) together with gas analyses at a station along the profile exhibit the expulsion of gas (mainly methane) and the presence of gas-charged sediments at shallow depth. The echo-soundings yield detailed insight into the distribution and shape of typical sea-floor features associated with gas seepage, such as pockmarks. The pockmarks reach dimensions of 800 m in length, 300 m in width, and 15 m in depth, with the long axis running parallel to the slope of the Norwegian Trench. Processing of the multi-channel high resolution seismic data and the digitally recorded sub-bottom profiler signals indicate an internal compressional velocity of about 1050 m s-1 within the gas-charged sediments reaching from the sea-floor to a sub-bottom depth of about 23 m. Using the lateral distribution and thickness of the gas-charged sediments in conjunction with a mean concentration of gas of 3000 ppb, the present amount of trapped gas is estimated to be 6·45 × 1011 g CH4. The flux of methane through the sea-bed into the water column appears to be 7·2 × 1010 g CH4 per year. To explain the small difference in size between the methane pool in near-surface sediments and the annual flux through the sea-bed, a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active.

Document Type: Article
Keywords: gas seeps; methane; pockmarks; seismic processing; North Sea; sediments; acoustics; Skagerrak
Research affiliation: OceanRep > Geomar Research Center for Marine Geosciences
Refereed: Yes
DOI etc.: 10.1006/ecss.1994.1040
ISSN: 0272-7714
Date Deposited: 23 Jun 2016 12:27
Last Modified: 23 Jun 2016 12:27
URI: http://eprints.uni-kiel.de/id/eprint/33208

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