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Abstract. During Ocean Drilling Program Leg 164, shear sonic velocity and other geophysical logs were acquired in gas hydrate-bearing sediments on the Blake Ridge to characterize the very distinct seismic signature of such formations: anomalous low amplitudes overlying a strong bottomsimulating reflector (BSR). A comparison of the bulk moduli derived from the logs to standard elastic consolidation models shows that the sediments are overconsolidated above the BSR at 440 meters below seafloor (mbsf) because of the presence of hydrates. Below the bottom of the thermodynamic hydrate stability zone at -520 mbsf, the high compressibility of the formation and the attenuation of the monopole sonic waveforms are typical of sediments partially saturated with free gas. Between these two depths, gas hydrate and free gas seem to coexist. Within the Gas hydrate stability zone, we estimate the amount of gas hydrates using different models based on theories for wave scattering in multiphase media and for grain cementation. In close agreement with measurements made on discrete in situ samples, the latter describes most accurately the interactions between the matrix, the pore fluids, and the hydrates. This model indicates that 5 to 10% of the pore space is occupied by hydrates deposited uniformly on the surface of the grains. The comparison with Gassmann's model also show that the amount of free gas below the BSR never exceeds 5 % of the pore space but is high enough to generate the BSR. The coexistence of free gas and gas hydrates below the BSR may be explained by capillary effects in the smaller pores or by remaining crystalline structures after partial hydrate decomposition.

[1] The Mallik 2L-38 research well was drilled to 1150 m under the Mackenzie Delta, Canada, and penetrated a subpermafrost interval where methane hydrate occupies up to 80% of the pore space. A suite of high-quality downhole logs was acquired to measure in situ the physical properties of these hydrate-bearing sediments. Similar to other hydrate deposits, resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework. In addition, sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed. We use monopole and dipole waveforms to estimate compressional and shear attenuation. Comparing with hydrate saturation values derived from the resistivity log, we observe a linear increase in both attenuation measurements with increasing hydrate saturation, which is not intuitive for stiffening sediments. Numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results. We also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydratebearing sediments. We suggest that this model can be improved and extended, allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact.

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International audienceA holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ∼9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation