Shear velocity logs in gas hydrate-bearing sediments were acquired for the first time along a transect of three ODP holes across the Blake Ridge. Shear wave data at Sites 995 and 997 indicate an increase in Vs in the presence of hydrate. Irregular borehole conditions at Site 994, however, did not allow Vs to be extracted using standard techniques. In this study, we use a statistical approach to identify shear waves by the local minimum of each waveform energy envelope. The resulting Vs profile is very similar to those at Sites 995 and 997, extending the observed increase in formation rigidity that is associated with the presence of hydrate beyond the lateral extent of the seismic BSR. In addition, we use these results to generate a 2-D tomographic inversion of the waveforms and determine the shear modulus distribution as a function of distance into the formation. The drilling process often influences measurements around a borehole due to invasion of fluids into the permeable formation. In the presence of hydrate, which is only stable under limited thermodynamic conditions, invasion is commonly thought to dissociate hydrate into free gas. At Site 994, however, the inversion suggests that hydrate does not decompose significantly due to fluid invasion into the formation. These results confirm that Vs logs can accurately measure the in situ properties of hydrate-bearing sediment and that hydrate deposits are not confined by the lateral extent of a BSR.

West: -75.3245 East: -75.3245 North: 31.4708 South: 31.4708

Expedition: 164

Site: 164-994

Provider: SEDIS Publication Catalogue

Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=2003-023442 (c.f. for more detailed metadata)

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