Gas Hydrates Research: ODP Leg 164
Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflector (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.
Fig. 1 Location of ODP Leg 164 (box), Blake Ridge. Bathymetric contours are shown at 500-m intervals.
Fig. 2 Single-channel seismic data on Blake Ridge (14), depth-converted using velocity-depth functions (right panel) determined from traveltime inversion of vertical seismic profile data. Solid lines show positions of drill holes 994D, 995B, and 997B. Two highly reflective zones are observed: one at 0 to 150 mbsf, and the other at and beneath the BSR [strong reflection at 3220 to 3250 m below sea level (mbsl)].
Fig. 3 Comparison of seismic velocity from VSPs, chlorinity ([Cl]), CaCO3 content (as weight percent of total matrix), and vertical-incidence seismic reflection data from (A) Site 994, (B) Site 995, and (C) Site 997. Anomalous chlorinity values indicate that methane hydrate is present between 220 and 450 mbsf at all three sites. Seismic reflections correlate with vertical lithological changes in the upper 150 mbsf and with low seismic velocities indicative of free gas (bright spots) beneath the hydrate stability zone. The low reflectance above the BSR is the expected response of lithologically uniform sediments.
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