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  • 1
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    Gas-controlled seafloor doming (2015)
    GSA (Geological Society of America)
    In:  Geology, 43 (7). pp. 571-574.
    Details
    Publication Date: 2019-10-24
    Description: The upward migration of gas through marine sediments typically manifests itself as gas chimneys or pipes in seismic images and can lead to the formation of cold seeps. Gas seepage is often linked to morphological features like seabed domes, pockmarks, and carbonate build-ups. In this context, sediment doming is discussed to be a precursor of pockmark formation. Here, we present parametric echosounder, sidescan sonar, and two-dimensional seismic data from Opouawe Bank, offshore New Zealand, providing field evidence for sediment doming. Geomechanical quantification of the stresses required for doming show that the calculated gas column heights are geologically feasible and consistent with the observed geophysical data. The progression from channeled gas flow to gas trapping results in overpressure build-up in the shallow sediment. Our results suggest that by breaching of domed seafloor sediments a new seep site can develop, but contrary to ongoing discussion this does not necessarily lead to the formation of pockmarks.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1130/G36596.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Processing of multibeam water column image data for automated bubble/seep detection and repeated mapping (2017)
    American Society of Limnology and Oceanography
    In:  Limnology and Oceanography: Methods, 15 (1). pp. 1-21.
    Details
    Publication Date: 2020-02-06
    Description: Water Column Imaging Multibeam Echosounder Systems (MBES) are effective and sensitive tools for investigating free gas (bubble) release and its rise through the water column. The main advantages of MBES are the detection range and lateral coverage in the water column and at the seafloor; furthermore, they are becoming increasingly available on research vessels worldwide. However, high noise levels and systematic artefacts due to side-lobe induced signal interference degrade MBES Water Column Images (WCIs) and hampered automated bubble detection and related gas seepage investigations. We present a new technique advancing automated detection of bubble streams and moving toward a quantitative gas-release assessment. It is shown that bubble streams can be detected reliably by their spatio-temporal behavior even when they are discontinuous in WCI data. Using assumptions about the bubble rising trajectories, bubble release spots at the seafloor can be traced even if the source location is obscured by acoustic noise or unwanted acoustic targets. A map with acoustic response and source locations of bubbles being released can be produced and serves as a starting point for more detailed quantitative analyses. The efficiency of the method has been assessed at a methane seep site in the Dutch North Sea. Multiple survey lines are merged to a detailed acoustic map of the area. Processed results are in good agreement with manual investigations of the WCI data as well as ROV-based video analysis.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/lom3.10138
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Acoustic imaging of natural gas seepage in the North Sea: Sensing bubbles under control of variable currents (2010)
    American Society of Limnology and Oceanography
    In:  Limnology and Oceanography: Methods, 8 . pp. 155-171.
    Details
    Publication Date: 2018-08-15
    Description: Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo-spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship-born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long-lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.4319/lom.2010.8.155
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability (1998)
    GSA (Geological Society of America)
    In:  Geology, 26 (7). pp. 647-650.
    Details
    Publication Date: 2019-10-24
    Description: Authigenic carbonates are intercalated with massive gas hydrates in sediments of the Cascadia margin. The deposits were recovered from the uppermost 50 cm of sediments on the southern summit of the Hydrate Ridge during the RV Sonne cruise SO110. Two carbonate lithologies that differ in chemistry, mineralogy, and fabric make up these deposits. Microcrystalline high-magnesium calcite (14 to 19 mol% MgCO3) and aragonite are present in both semiconsolidated sediments and carbonate-cemented clasts. Aragonite occurs also as a pure phase without sediment impurities. It is formed by precipitation in cavities as botryoidal and isopachous aggregates within pure white, massive gas hydrate. Variations in oxygen isotope values of the carbonates reflect the mineralogical composition and define two end members: a Mg-calcite with δ18O =4.86‰ PDB and an aragonite with δ18O =3.68‰ PDB. On the basis of the ambient bottom-water temperature and accepted equations for oxygen isotope fractionation, we show that the aragonite phase formed in equilibrium with its pore-water environment, and that the Mg-calcite appears to have precipitated from pore fluids enriched in 18O. Oxygen isotope enrichment probably originates from hydrate water released during gas-hydrate destabilization.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1130/0091-7613(1998)026〈0647:ACFTCS〉2.3.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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