Pwave velocity

Because gas hydrate has a higher velocity (3270 m/s; (Waite et al., 1998)) than the pore fluid it likely replaces, P-wave velocity should be a useful indicator of hydrate concentration. Indeed, this is the case on the Blake Ridge: the average concentration of gas hydrate increases towards the crest of the ridge, as does average Vp measured in VSP data (Fig. 9). However, the increase is subtle: a lateral increase in velocity of about 50 m/s corresponds to a lateral increase in hydrate saturation from 2% of porosity to 7% of porosity (Holbrook et al., 1996). Such a subtle change was interpretable only because of the uniform sediments of the Blake Ridge, which make it an ideal "natural laboratory" for gas hydrate studies. In a less uniform environment (e.g., an accretionary margin) a change of Vp by about 50 m/s would likely be indistinguishable from lateral sediment variations.

200-400 mbsf

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