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Multi-frequency acoustic data (deeptow, surface towed) on Hydrate Ridge: First results
Guidard, S.; Klaeschen, D.; Versteeg, W.; Vanneste, M.; Henriet, J.-P.; Bialas, J.; The SO150-Shipboard Party (2001). Multi-frequency acoustic data (deeptow, surface towed) on Hydrate Ridge: First results. J. Conf. Abstr. 6(1): 153
In: Journal of Conference Abstracts. Cambridge Publications: Cambridge. ISSN 1362-0886, meer
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Documenttype: Samenvatting
Trefwoord
    Marien/Kust
Auteurs  Top 
  • Guidard, S.
  • Klaeschen, D.
  • Versteeg, W., meer
  • Vanneste, M., meer
  • Henriet, J.-P., meer
  • Bialas, J.
  • The SO150-Shipboard Party
Abstract
    During the SO150 expedition within the HYDGAS project framework on Hydrate Ridge, multi-frequency seismic profiles (sparker, watergun, GI-gun, airgun array, bolt gun) have been recorded with both deep-towed and surfacetowed receivers in the immediate vicinity of ODP-site 892 (Leg 146) in order to characterise the acoustic signatures related to gas hydrate and free gas accumulations in the sediments. The data encompasses a total frequency bandwidth from 20 to 1500 Hz. The analogue signal detected by the deeptow streamer was transmitted via a 8000 m long cable before being digitised in the acquisition unit. The submersion depth was typically 500 m (offsets 850-900 m, vessel’s speed about 3 kt). Despite the geometrical corrections inherent to the deep-tow system, advantages of such records over surface-towed data are the improvement of resolution (both lateral and vertical) and enhanced signal/noise ratio, as is evidenced by comparing the simultaneously recorded deeptowed rs. surface-towed acoustics sections. Next to that, data with larger offset can emphasise the presence of shallow gas accumulations (AVO-effect).At present, data are being analysed in GEOMAR and RCMG. First results indicate that the continuity and reflection amplitude of the BSR is observed to decrease with increasing source frequency, a feature explained in terms of vertical and horizontal resolution. GI-gun data give the best quality records with continuous high-amplitude BSR features in the entire area. On the very-high-frequency sparker profiles, the commonly-expected BSR is often replaced by a series of enhanced reflections, thought to be caused by gas saturation. This suggests that (1) gas migration and accumulation is stratigraphically and lithologically controlled and (2) the BSR as evidenced on lowerfrequency data originates at the top of the free gas layer rather than at the base of the hydrate stability zone.
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