HR: 0800h
AN: OS41C-0494    [Abstracts]
TI: Analysis of Sonic Velocity in an Active Gas Hydrate System, Hydrate Ridge, Offshore Oregon
AU: * Guerin, G
EM: guerin@ldeo.columbia.edu
AF: LDEO/Borehole Research Group, 61 Rte 9W, Palisades, NY 10964 United States
AU: Goldberg, D
EM: goldberg@ldeo.columbia.edu
AF: LDEO/Borehole Research Group, 61 Rte 9W, Palisades, NY 10964 United States
AU: Collett, T S
EM: tcollett@usgs.gov
AF: USGS, Box 25046, Denver, CO 80225 United States
AB: One of the best recognized and most intuitive influence of gas hydrate on its host sediment is the change in its mechanical and elastic properties. This is identified through an increase in acoustic velocity, which is partially responsible for one of the most distinct signatures of gas hydrate presence, the Bottom Simulating Reflector (BSR). The unstable nature of gas hydrate makes the in situ recording of their properties by downhole logging the best way to identify and quantify its distribution. During ODP Leg 204 on Hydrate Ridge, offshore Oregon, acoustic logs were recorded in seven holes and vertical seismic profiles (VSP) were acquired successfully in four holes. These data, recorded within a wide range of frequency and scales provide a unique and extensive survey of the acoustic properties of a dynamic gas hydrate system. Because of the poorly consolidated nature of the Hydrate Ridge sediments, automatic picking of velocity was only partially successful and a complete post cruise reprocessing of the sonic waveforms was necessary to draw accurate compressional (Vp) and shear velocity (Vs) logs. Synthetic seismograms generated with the Vp and density logs allow to confirm the nature of the main reflectors identified in a 3-D seismic survey of Hydrate Ridge, such as the BSR and various faults underlying the southern Hydrate Ridge system. Despite the highly heterogeneous distribution of gas hydrate, the Vp logs and interval velocities calculated from the VSP clearly identify the presence of gas hydrate and the eventual presence of free gas directly underneath the hydrate stability zone or within the faults feeding the ridge system. We use various elastic models to try to estimate gas hydrate and free gas saturations from sonic velocity and from bulk moduli. The best agreement with independent estimates derived from resistivity logs and other methods indicate that gas hydrate interact with the host sediment through cementation, which contributes also to a significant energy loss in the recorded waveforms.
DE: 5102 Acoustic properties
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 0915 Downhole methods
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting