HR: 0800h
AN: MR41A-0908    [Abstracts]
TI: Seismic wave attenuation in methane hydrate-bearing sediments: vertical seismic profiling data from the Nankai Trough exploratory well, offshore Tokai, central Japan
AU: * Matsushima, J
EM: jun-matsushima@geosys.t.u-tokyo.ac.jp
AF: The University of Tokyo, Department of Geosystem Engineering, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
AB: Data from two vertical seismic profiles (VSPs) from the Nankai Trough exploratory well, offshore Tokai, central Japan, are used to estimate compressional attenuation in methane hydrate (MH)-bearing sediments at seismic frequencies of 30-110 Hz. We compare spectral ratio and centroid frequency shift methods for measuring attenuation. To isolate intrinsic attenuation from total attenuation, attenuation is computed from multiples using one-dimensional synthetic VSP data from sonic and density logs. The use of two different measurement methods and two VSPs recorded at just 100 m separation provides an opportunity to validate the attenuation measurements. Macroscopically, the peaks of highest attenuation in the seismic frequency range correspond to low-saturation gas zones. This interpretation is supported by raw-level data such as the centroid frequency. No significant compressional attenuation was observed in MH-bearing sediments at seismic frequencies. In contrast, high compressional attenuation zones in the sonic frequency range (10-20 kHz) are affected by the presence of methane hydrates in the same well locations. Thus, this study demonstrated the frequency-dependence of attenuation in MH-bearing sediments.
DE: 0689 Wave propagation (2487, 3285, 4275, 4455, 6934)
DE: 0738 Ice (1863)
DE: 0915 Downhole methods
DE: 3004 Gas and hydrate systems
DE: 5144 Wave attenuation
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005