Ocean Bottom Seismometer Wide-Angle Reflection Study of Gas Hydrate Accumulations in Nankai Trough, Offshore Tokai, Japan

Asakawa, Eiichi¹, Peter Ward¹, Maarten Vanneste², Stephanie Guidard², Juergen Mienert², Tatsuo Saeki³ (1) JGI, Inc, Tokyo, Japan (2) University of Tromso, Norway (3) Japan Oil, Gas and Metals National Corporation, Japan

4-component ocean bottom seismometer (OBS) data were acquired in the Nankai Trough, offshore Tokai, Japan. We analysed these data in combination with multi-channel seismic (MCS) data to study the distribution and characteristics of methane hydrates in the sediments. Although the seismic data reveals a very complex and dynamic sedimentation history, the bottom-simulating reflector (BSR), indicative for the presence of gas hydrates, is easily recognized as a laterally-variable amplitude reflection.

We applied an imaging technique to the OBS reflection data and obtained results that have good agreement with the MCS section. We also applied modelling and inversion pro­cedures to reveal the detailed structure. The OBS data allowed us to construct a 9-layer model for the uppermost 700m of sediments in the eastern Nankai Trough. Travel-time inversion gives elevated P-wave velocities, reaching values of up 2100m/s. Such high veloc­ities can be explained by a partial hydrate saturation of pore space of up to 20%. The BSR at ~320mbsf coincides with a significant drop in P-wave velocity to values between 1580 and 1750m/s. This lower-velocity layer is ~80m thick.

The S-wave velocities were subsequently derived by event correlation, time picking and forward modelling of the wide-angle data, and are up to 700-750m/s in the hydrate zone. We observe a small decrease of S-wave velocity underneath the BSR. This might indicate a weak hydrate cementation of the sedimentary matrix. In the deeper section however, the high P­and S-wave velocities indicate over-consolidated sediments. The velocities show good cor­respondence with well-logging data from nearby boreholes.