Seismometer Wide-Angle Reflection Study of Gas Hydrate Accumulations in Nankai Trough, Offshore
Asakawa, Eiichi1, Peter Ward1,
Maarten Vanneste2, Stephanie Guidard2,
Juergen Mienert2, Tatsuo Saeki3 (1)
4-component ocean bottom seismometer (OBS) data were acquired in
the Nankai Trough, offshore
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.