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| | Abstract | | It has been known that velocity is valuable information to find the methane hydrate-bearing sediments. On the other hand, the research interests in the correlation between seismic attenuation and hydrate saturation have been growing because recent seismic surveys have shown that the presence of methane hydrate in sediments has significant influence on seismic attenuation. The purpose of this study is to verify the method to be applied for estimating attenuation by use of monopole and dipole waveforms in sonic logging. Firstly, we verified the median frequency shift method for calculating attenuation using sonic waveform data. The results without correction are relative due to dependence of optional reference value. We compared two different correction methods. Secondly, we calculated 1/Qp and 1/Qs from monopole and dipole waveforms at Nankai Trough area based on the median frequency shift method. We obtained a moderate positive correlation between attenuation of sonic data and hydrate saturation. Finally, we classified attenuation results along the depth into several groups using Self-Organizing Map to detect hydrate-bearing sediments. Data of gamma-ray, resistivity, velocity, and attenuation were used for input parameters of Self-Organizing Map. |
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