Collaborative Research: Shear Wave Studies of Hydrate Ridge, Oregon Continental Margin, During Planned ODP leg 198

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Pecher 00-02487Trehu 00-02410Recommended project is for a series of seismic experiments to be conducted from boreholes during Ocean Drilling Program (ODP) Leg 198 to map occurrences of gas hydrate and free gas in sediments on Hydrate Ridge offshore Oregon. These experiments will geophysically extend observations from direct core description, direct core measurement, and borehole logging into the surrounding sediments, thereby provide 3-D coverage of inferred gas hydrate and free gas occurrence over a much larger sediment volume. The experiments include: 1) constant-offset vertical seismic profiling (VSP) with a fixed source receiver, coordinated with zero-offset VSP's, which will result in vertically continuous shear-wave velocity (Vs) profiles; 2) walkaway VSP's, acquired by shooting over the borehole with a second ship to a receiver at fixed depth within the borehole, thereby acquiring high-quality Vs and compressional wave (Vp) data to calibrate rock physics models of gas-hydrate bearing sediments; 3) deployment of ocean bottom seismometers in a dense array around one of the boreholes for dense 3-D seismic coverage to examine an area of inferred focused fluid flux and its relationship to a gas hydrate cap; and 4) using the ODP Advanced Piston Corer (APC) as a seismic source during piston deployment for a larger array of measurements using the OBS array. These geophysical results will be directly tied to drilling results to constrain relationships between fluid flux, free gas, and hydrate formation in a convergent margin setting.***