Mi, Yanpeng (1998): Seafloor sediment coring and multichannel seismic studies of gas hydrate, offshore Vancouver Island. 115 pp., georefid:2000-064701

Gas hydrate is an ice-like compound in which water molecules form a 3-D network trapping small gas molecules such as methane. Gas hydrates are important because they might be a significant future energy resource and because they may play a role in controlling global climate. The general objectives of this thesis are to determine the lateral variation of gas hydrate saturation in a small region on Cascadia Margin and to further understand the processes related to the formation of gas hydrate and free gas zone. In the first part of this thesis, physical properties of seafloor sediment were measured in a region of hydrate occurrence on the middle continental slope offshore Vancouver Island. Seafloor sediment resistivity, velocity, bulk density, grain density, porosity and water content were measured. The measured physical properties complemented those of sediment cores from Ocean Drilling Program (ODP) Sites 889/890, in which the seafloor sediment is generally missing or incomplete. Porosity was found to vary consistently with resistivity and optimum Archie's Law parameters were used to predict seafloor sediment porosity. These parameters may be used in future resistivity surveys to provide seafloor porosity estimates. Sediment velocity was very close to that of seawater, which indicates that the reflection coefficient (RC) calculated from the seafloor bulk density can be safely used to constrain seismic RC. Measured bulk density appears to be lower in the deepwater region for the shallow sediments. In the second part of the thesis, short-offset multichannel seismic (MCS) data collected in a tightly-spaced grid (400 m line separation) southwest of ODP Sites 889/890 were analyzed and interpreted. A 120 cubic inch airgun was recorded by a 24-channel ITC streamer with offsets ranging from 108 m to 292 m. Amplitudes were converted to RC values using a multiple/primary ratio method. A single conversion factor was calculated from seismic lines over several flat regions that are normally in the small slope basins. The seafloor RC determined from the seismic data was high in the shallow water region (1200 m) and much lower in the deep sedimentary basin (2200 m), consistent with RC values calculated from bulk density values. This difference is mainly due to the tuning effect caused by the thickness variation of a soft mud layer covering the seafloor. Constrained by ground truth from measured seafloor impedance the seafloor RC derived from amplitude scaling can be safely used for transmission corrections in the BSR RC calculations. The base of a gas layer below the BSR was clearly resolved on the western flank of a local topographic high. Forward modeling based on waveform matching showed subtle variations in the velocity and density structure that can generate a similar signature at the BSR and the base of free gas zone. Instead of a negative acoustic impedance boundary, the base of the free gas zone is positive. It is either caused by an interface between normal sediment and overconsolidated sediment or between free gas charged sediment and normal sediment. A clear density enhancement due to overconsolidation was observed below the BSR in core measurements from ODP site 892, where the BSR occurred at 74 mbsf. Unlike Site 892, however, the sediment bulk density below the BSR at ODP Sites 889/890 did not show a similar enhancement. This effect is probably because the BSR depth (225 m) at Sites 889/890 is much larger so that the sediment has lost more pore fluid through normal consolidation than the corresponding shallower sediment. The effect of overconsolidation on bulk density is too small to be resolved. Estimation of the gas saturation remains difficult because the velocity decreases dramatically with only a few percent of free gas. A maximum gas saturation of 5% pore space is suggested. Gas hydrate saturation immediately above the BSR was calculated directly from the seismically derived BSR RC, with the assumption of a background velocity of 1600 m/s and average porosity of 50% at the depth of BSR. For RC ranging from 0 to 0.12, the gas hydrate saturation is no higher than 30%, typically 10% to 15% of the pore space.
West: -126.5253 East: -126.5206 North: 48.4159 South: 48.3945
Expedition: 146
Site: 146-889
Site: 146-890
Data access:
Provider: SEDIS Publication Catalogue
Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=2000-064701 (c.f. for more detailed metadata)
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