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Ocean Bottom
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)
JGI, Inc,
4-component ocean bottom seismometer (OBS) 
data
were acquired in
the Nankai Trough, offshore 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.