HR: 0830h
AN: OS51C-0877    [PDF]
TI: Gas Hydrate Distribution From Acoustic Waveform and Electrical Image Logs in Accretionary Complexes, ODP Legs 201 and 204
AU: * Guerin, G
EM: guerin@ldeo.columbia.edu
AF: Borehole Research Group, Lamont Doherty Earth Observatory, Palisades, NY 10964 United States
AU: Goldberg, D
EM: goldbeg@ldeo.columbia.edu
AF: Borehole Research Group, Lamont Doherty Earth Observatory, Palisades, NY 10964 United States
AU: Collett, T S
EM: tcollett@usgs.gov
AF: USGS, Denver Federal Center, Denver, CO 80225 United States
AB: During the summer of 2002, ODP Leg 204 drilled 9 sites on Hydrate Ridge, offshore Oregon, to investigate a particularly rich marine gas hydrate deposit. Earlier that year, Leg 201 had drilled one site (1230) in the Peru trench to study the relationship between gas hydrate formation and subseafloor microbial activity. All of these sites were drilled in active continental margin sediments with variable lithology and degrees of deformation. One clear observation can be drawn when comparing these sites - the distribution of gas hydrate is highly heterogeneous, with significant lateral differences between adjacent holes and vertical differences within individual holes. At some of the sites (i.e. Site 1230 in the Peru Trench and Sites 1247 and 1252 on Hydrate Ridge), only a few gas hydrate samples were recovered. However, downhole measurements such as wireline logs and core-scanning infrared camera images indicate a more widespread hydrate distribution. We use sonic waveform logs and downhole electrical images to discriminate the fine scale variability and the 3D distribution of gas hydrate at these sites. In Hole 1230A and in most Leg 204 sites, cm-scale electrical images (FMS) and monopole and dipole sonic waveforms at various frequencies were recorded simultaneously, allowing accurate correlations between individual measurements. Sonic and electrical logs are sensitive to the presence of hydrate because they record the induration and insulation generated by the solid ice-like compound replacing the pore fluids in marine sediments. The presence of gas hydrate increases electrical resistivity and sonic velocity, and decreases sonic waveform amplitudes. Elastic-wave modeling shows that this effect is dependant on hydrate concentration and on the nature and frequency of the sonic source. Together with FMS images, the analysis of acoustic logs allows for precise identification of gas hydrate occurrences that were not recovered in cores. In addition, the combination of the high-resolution azimuthal images with the sonic data provides insight into how the presence of gas hydrate in the pore space may be affecting the sonic waveform amplitudes.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
DE: 3094 Instruments and techniques
DE: 5144 Wave attenuation
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting