HR: 1340h
AN: OS23A-1055    [Abstracts]
TI: Analysis of Wireline Acoustic Logs, India National Gas Hydrate Program (NGHP) Expedition 1
AU: * Guerin, G
EM: guerin@ldeo.columbia.edu
AF: LDEO/Borehole Research Group, 61 Rte 9W, Palisades, NY 10964, United States
AU: NGHP Expedition 1 Scientific Party
AB: The Indian National Gas Hydrate Program (NGHP) Expedition 1 was designed to study the occurrence of gas hydrate along the east and west coast of India and near the Andaman Islands. In the spring and summer 2006, the expedition discovered gas hydrate in sand, silt, and clay dominated sediments. One of the best recognized influences of gas hydrate on the host sediment is a change in mechanical and elastic properties, typically an increase in sonic velocity, but also a measurable increase in attenuation. Recognizing the strong influence of gas hydrate and free gas on the propagation and amplitude of acoustic waves, sonic waveforms were recorded in multiple modes and frequencies in the ten sites where wireline logs were acquired. To complete the characterization and integrate the drilling data with the regional seismic surveys, vertical seismic profiles (VSP) were acquired successfully in six holes. These data, recorded with a wide range of frequency and scales provide an extensive survey of the acoustic properties in very diverse gas hydrate systems. Because of the poorly consolidated nature of the sediments in some east coast sites, automatic picking of velocity was only partially successful during the expedition, and a complete post cruise reprocessing of the sonic waveforms was necessary to draw accurate compressional (Vp) and shear velocity (Vs) logs in these holes. Synthetic seismograms generated with the Vp and density logs confirm the depth and nature of the main reflectors in the seismic surveys that were used to select the sites, in particular the BSR marking the deepest occurrence of gas hydrate. Despite heterogeneous distributions, the sonic logs clearly identify the presence of gas hydrate in very distinct intervals, and the eventual occurrence of free gas underneath. In addition to providing Vp and Vs logs, the amplitude of the waveforms offers a complete insight into the distribution of gas hydrate in a rich and deformed lithology. The dissipative influence of gas hydrate on acoustic waves is still poorly constrained and strongly dependent on the host sediment, but empirical relationships derived from sonic attenuation, hydrate saturation and gamma ray logs provide independent measures of the diversity of gas hydrate distribution in the sites visited.
DE: 3004 Gas and hydrate systems
DE: 3036 Ocean drilling
DE: 5102 Acoustic properties
DE: 5144 Wave attenuation
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting