Rationale and Objectives
Why The Blake Ridge?
The Blake Ridge is probably the best area in the world
to address our scientific objectives, for two reasons:
- The wealth of existing data on the Blake Ridge provides
an outstanding basis for siting our proposed study and for interpreting
the results. These data include site-survey and regional seismic
reflection profiles, ocean-bottom seismometer data, zero-offset
and walkaway VSP data, sonic and resistivity logs, and lithological,
chemical, and paleontological data down to >200 m below the BSR
from Leg 164.
- The extraordinarily homogeneous
lithology and
index properties of the sediments below about 150 mbsf provide
an ideal tabula rasa against which anomalous
physical or acoustic properties -- such as the P-S converting
boundaries, velocity variations, and bright reflectivity beneath
the BSR -- can be interpreted in terms of hydrate and free gas.
Why Now?
We are motivated by three
surprising observations that came out of seismic work
during Leg 164:
- The unexpectedly thick free gas zone beneath the BSR discovered
during Leg 164. Zero-offset vertical seismic profiles (ZO-VSP) acquired
during Leg 164 showed that low velocities indicative of free
gas persist to at least 250 m, and perhaps deeper, beneath the
BSR. The Leg 164 result implies
that the free gas zone plays a greater role in the hydrate/gas
system than previously appreciated. Moreover, the
combination of low velocities, high reflectivity, and uniform
lithology implies that sub-BSR reflections are bright spots
that mark gas occurrence [16] . Seismic imaging thus
provides a tool for creating direct, detailed maps of gas
accumulation and migration pathways.
- The lateral increase in
hydrate concentration toward the crest of the Blake Ridge.
This change in hydrate concentration was interpreted from three
independent data sets: the chloride
content of pore waters, downhole well logs, and
VSP data. This result demonstrates that seismically
resolvable differences in hydrate concentration occur on the
scale of kilometers on the Blake Ridge. The MCS data set
we propose will enable interpretations of hydrate content
over a 255 sq. km. area on the Blake Ridge, as well as the
establishment of relationships between hydrate content and
the distribution of gas in the underlying free gas zone.
- The identification of
converted shear waves on an ocean-bottom seismometer
(OBS). (See Figure)
A shear-wave arrival on the NE part of the profile
indicates that the BSR provides effective P-to-S conversion.
These data suggest that converted S-waves
can be used to map the extent and properties of the top of the
hydrate-bearing zone, the BSR, and the free gas zone.
What Scientific Questions Do We Hope to Answer?
- What is the lateral and vertical distribution of hydrate?
- What is the spatial distribution of methane gas: Where
does gas form, how does it migrate, and where does it accumulate?
- Do gas-charged sediments represent a zone of weakness?
- Does hydrate cement sediments?
- Can free gas exist within the hydrate stability zone?
- Does the BSR "step down"?
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Last Revised: January 27, 1998
