Seismic attenuation from VSP data in methane hydrate-bearing sediments*
Jun Matsushima7-3-1 Hongo Bunky-ku, Tokyo, 113-8656, Japan. Email: jun-matsushima@geosys.t.u-tokyo.ac.jp
Exploration Geophysics 38(1) 29-36 https://doi.org/10.1071/EG07001
Submitted: 13 November 2006 Accepted: 10 December 2006 Published: 5 April 2007
Abstract
Recent seismic surveys have shown that the presence of methane hydrate (MH) in sediments has significant influence on seismic attenuation. I have used vertical seismic profile (VSP) data from a Nankai Trough exploratory well, offshore Tokai in central Japan, to estimate compressional attenuation in MH-bearing sediments at seismic frequencies of 30–110 Hz. The use of two different measurement methods (spectral ratio and centroid frequency shift methods) provides an opportunity to validate the attenuation measurements. The sensitivity of attenuation analyses to different depth intervals, borehole irregularities, and different frequency ranges was also examined to validate the stability of attenuation estimation. I found no significant compressional attenuation in MH-bearing sediments at seismic frequencies. Macroscopically, the peaks of highest attenuation in the seismic frequency range correspond to low-saturation gas zones. In contrast, high compressional attenuation zones in the sonic frequency range (10–20 kHz) are associated with the presence of methane hydrates at the same well locations. Thus, this study demonstrated the frequency-dependence of attenuation in MH-bearing sediments; MH-bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range. As a possible reason why seismic frequencies in the 30–110 Hz range were not affected in MH-bearing sediments, I point out the effect of thin layering of MH-bearing zones.
Key words: methane hydrate, attenuation, VSP, free gas, Nankai Trough.
Acknowledgments
This study was carried out under the MH21 Research Consortium in Japan. The author thanks METI (Ministry of Economy, Trade and Industry) and JOGMEC (Japan Oil, Gas and Metal National Corporation) for permission to publish this paper. The author also thanks two anonymous reviewers for their constructive reviews.
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* Part of this paper was presented at the 8th SEGJ International Symposium, 2006.