Abstract:
Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the
Hikurangi Margin, east of the North Island of New Zealand, and is an area of active
seafloor methane seepage associated with an extensive gas hydrate province.
Methane seep sites on the Hikurangi Margin are characterised by localised buildups
of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor
otherwise characterised by soft, muddy sediments and provide a unique window
into the workings of the gas hydrate system. Seafloor methane seeps sites
on Omakere Ridge have been successfully imaged using three newly-acquired
acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60
Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom
profiler dataset (4 kHz).
Seafloor seismic amplitude and similarity maps have been derived from a
preliminary shipboard post-stack migrated data cube. A pronounced acquisition
artifact is manifest in the seafloor horizon slice as high- and low-amplitude
stripes that alternate periodically in the crossline direction. This artifact has been
removed from the seafloor horizon slice using 2D spatial frequency filtering, followed
by direct sampling and stochastic removal of the very-low-frequency components
in the spatial domain. The seismic amplitude map has then been transformed
into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics
have been created after correcting for beam pattern effects and angular variation
in backscatter after taking into account the bathymetry. Several backscatter
mosaics were incorporated into a stacked mosaic over the study area to attenuate
random noise. The ParasoundTM sub-bottom profiler data were processed to display
instantaneous amplitude and separated into 43 lines over the study area.
Comparison of 3D seismic attributes, multibeam backscatter intensity and
shallow subsurface reflection characteristics provides new insights into the previously
unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge.
Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter
intensity are interpreted as carbonate formations of at least 6–7 m thickness,
while areas exhibiting low seismic reflection coefficient and moderate/high sonar
backscatter intensity are interpreted as areas where the carbonates are less developed.
Anomalous high-amplitude subsurface reflections beneath the seeps in the
ParasoundTM data are interpreted as buried carbonates and may indicate a previously
unknown earlier phase of seepage at Omakere Ridge, but could also be
caused by gas or gas hydrates. The extent of authigenic carbonates is directly
related to the duration of seepage and thus provides a new proxy for the chronology
of seepage at Omakere Ridge, which has proved consistent with an existing
hypothesis based on the abundance of deceased and live chemosynthetic fauna
at the seep sites.
