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Figure Captions
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Geochemical observations will be made within
seafloor sediments, at the sea floor, and in the lower water column.
Geochemical sensors include:
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Pore water samplers
to measure salinity and concentrations of hydrocarbon gases at
various depths below the seafloor.
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Chimney samplers to
measure the composition and quantity of gas passing through the
seafloor.
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Arrays to measure
conductivity, temperature, density, and composition of dissolved gas
at various levels in the lower water column.
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A mass spectrometer will perform chemical
analyses on the seafloor.
The microbial observatory will monitor various
aspects of:
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Microbial activity
including abundance, diversity, temporal variability, and dynamics
of microbial communities.
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Rates of methane
oxidation and sulfate reduction.
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Relationships between
microbial products and hydrate formation.
Monitoring sensors will include retrievable,
pressurized seafloor test cells and bioreactors, high-surface-area
sampling plates of different materials, low-light digital cameras, and
devices to retrieve samples under in-situ conditions.
Research goals include:
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Documenting the
stability and persistence of gas hydrate outcrops.
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Determining whether
methane oxidation and sulfate reduction occur within hydrates
without dissociation.
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Characterizing the
structure and functions of microbial communities.
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Identifying
biochemical controls on, and ecological roles of, certain bacterial
mats.
Fine-grained magnetic sulfides that originate
with magnetotactic bacteria and then accumulate in gas hydrates will be
monitored as possible locators of hydrate deposits.
 Seismic Data
Seismic data will be collected with six linear
sensor arrays: two vertical and four horizontal. One vertical array of
hydrophones, inclinometers and compasses will extend from the seafloor
to a height of 200 meters into the water column. A second vertical array
of hydrophones and 3-component (3-C) accelerometers will be in a
borehole and extend 150 meters below the seafloor.
Each horizontal array of hydrophones and 3-C
accelerometers will be 400 meters long. Four horizontal arrays will be
deployed in an orthogonal cross to create arm lengths equal to water
depth (~800 meters).
The monitoring station is being deployed near
a hydrate mound in water more than 800 meters deep in Block MC 118 (Figure
2). The mound is located inside the distorted bathymetry contours in
the lower part of
Figure 2.
The Minerals Management Service has reserved a
large portion of Block MC 118 (the area inside the “MMS Reserve
Boundary” in
Figure 2) for exclusive use of the
monitoring station and associated research.
Pre-installation surveys began in January,
2005. The first observing systems, a pore-fluid sampler and an array of
sub-bottom thermisters were installed in May, 2005. Installation is
continuing in stages until the monitoring station is complete.
Completion is expected in 2007.
Seismic Monitoring
When fully operational, the observatory will
generate about nine gigabytes of data per hour. Almost all of this data
flow will come from seismic sensors operating in continuous acquisition
mode.
A conventional image of the mound will be
created by inverting data acquired using conventional seismic sources.
Monitoring will consist of comparing this conventional image to
subsequent images obtained using ambient noise as the energy source.
A procedure for imaging the mound using the
noise of nearby ships is under development. This technique will utilize
the station’s hydrophone data and is based on an established technique
known as Matched Field Inversion.
Attempts will be made to obtain images using
other types of ambient noise, particularly the noise of wind-driven
waves at the sea surface and the background noise of micro-seismic
events. The mound will be re-imaged with conventional seismic sources
from time to time.
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