Abstract
Methane gas hydrates are attracting the attention of the
scientific community because of their potential as a future
energy resource, their possible role in climate change, and
their potential involvement in slope failure. The Gulf of
Mexico is increasingly recognized as a separate end-member
for gas hydrate systems: a focused-methane-flux environment,
in which faulting from salt tectonics provides numerous
fluid migration paths. Close to faults, gas hydrates often
crop out at the seafloor. Unlike in most low- and high-flux
environments, gas hydrates in the Gulf of Mexico are
generally not marked by bottom simulating reflections
(BSRs). Gas hydrate outcrops have been studied in recent
years, however, we know very little about the extent of the
gas hydrates below the seafloor, about gas hydrates, if
present at all, away from the faults, and about possible
free gas beneath the base of gas hydrate stability. In a
joint project, the U.S. Geological Survey, University of
Mississippi and Department of Energy conducted a
high-resolution seismic study to shed light on the Gulf of
Mexico gas hydrate environment. Both single-channel-seismic
(SCS) and coincident ocean-bottom-seismometer (OBS) data
were acquired. The OBS data quality is very high, including
substantial 3-D ray coverage and identifiable P-to-S
converted waves. First results already show indications for
free gas beneath a gas-hydrate-bearing mud diapir, which is
surprising given the lack of BSRs. We will now develop and
apply new techniques to fully exploit the wealth of
information in these data in order to address the following
questions of gas hydrates in a focused-methane-flux
environment: (1) Do the shallow gas hydrates extend
considerably beneath the seafloor?, (2) Does free gas exist
beneath the shallow gas hydrates?, (3) If free gas is
present, what traps it?, and (4) Why are BSRs not observed?
We will optimize an existing 3-D seismic
reflection/refraction tomography method for application to
this type of coincident SCS and OBS data. In particular, we
will incorporate the structural information from the
traveltimes of the SCS data which will add a significant
constraint on the 3-D velocity model. We will also evaluate
Vs from the PS-waves. We will develop a method for 3-D
PS-NMO based on the tomographic inversion. We will then
determine Vs by a combination of 1-D event correlation and
PS-NMO analysis. This study will significantly enhance our
understanding of gas hydrate systems in the focused-methane
flux environment of the Gulf of Mexico, as well as provide
new tools for the evaluation of coincident multi-component
OBS and high resolution SCS data.
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