HR: 0830h
AN: OS51C-0864    [PDF]
TI: Detailed Seismic Studies of Gas Hydrate Deposits Offshore Sabah and Costa Rica
AU: * Buttkus, B
EM: buttkus@bgr.de
AF: Bundesanstalt f�r Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, 30655 Germany
AU: Behain, D
EM: d.behain@bgr.de
AF: Bundesanstalt f�r Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, 30655 Germany
AU: Boennemann, C
EM: boennemann@bgr.de
AF: Bundesanstalt f�r Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, 30655 Germany
AU: Mueller, C
EM: c.mueller@bgr.de
AF: Bundesanstalt f�r Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, 30655 Germany
AB: Methane hydrates are discussed to hold a high potential as geohazard and future energy resource. In this context, the German Federal Institute for Geosciences and Natural Resources (BGR) has recently carried out a number of marine seismic cruises on active and passive continental margins were gas hydrates occur. BGR's activities concentrate on reservoir investigations, structural studies, and comparative studies to understand the origin of the gas. Latest results from offshore Sabah and Costa Rica will be presented. Gas hydrate occurrence off Sabah appears to be linked to structural and tectonic units and to be focused mainly in the folded, thrusted, and uplifted structures. The bottom simulating reflectors (BSRs) occur mainly in the hanging walls of the individual thrust sheets which form anticline-like structures. Due to the tectonically controlled morphology of the seafloor the distribution of BSRs appear mainly as elongated bodies which run parallel to each other. The convergent continental margin of Costa Rica is an area with large occurrences of heterogeneously distributed gas hydrates. Southwest of Nicoya Peninsula, continuous BSRs are observed between the upper and lower continental margin. North of the so called fracture zone trace, in the area of ODP-Leg 170, no BSRs are observed, presumably related to the unusually low heat flow in this area, whereas southeast of Nicoya Peninsula BSRs are characterized by patchy occurrence with strong lateral variation in reflection amplitude, indicating focussed fluid migration along deep-reaching faults. In both survey areas BGR has acquired long-offset seismic data for high-resolution seismic studies and pre-stack amplitude variation with angle (AVA) analyses. The implementation of an effective medium theory accounting for the local properties of shallow marine sediment allowed to model synthetic AVA responses as a function of gas hydrate concentration and free gas saturation at the BSR. Results indicate the possibility to detect free gas, and to quantify free gas at low saturations after careful data processing. Areas with strong BSRs are thus characterized by bulk free gas saturations of not more than five percent. Furthermore, the results indicate that the contribution of gas hydrate to the observed angle-dependent reflection coefficient is negligible and can hardly be resolved and quantified from AVA.
UR: http://www.bgr.de
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3025 Marine seismics (0935)
DE: 3210 Modeling
DE: 5102 Acoustic properties
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting