|
My interest
I am interested in the seismic characterization of gas-hydrate- and free-gas-bearing sediments.
My data come from the Hydrate Ridge, offshore Oregon, NW coast USA (figure 1). This was part of my
PhD research (Funded by the NSF, USA).
Read the May 2006 issue of
The Leading Edge for reviews of Gas hydrate research.
|
Introduction
Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane)
in a lattice of water molecules. It looks like a crystal (Figure 2) when stable (at high pressures
and low temperatures, Figure 3). Gas hydrates are widely present in permafrost and deep oceanic environments
around the world. When hydrate dissociates (due to lowering in pressure and/or increasing temperature),
it produces about 164 times methane gas (in volume)(Figure 2). It means enormous amount of gases are stored
in the hydrate. If we can successfully (safely, economically, and environment friendly) mine gases
from hydrates, that can solve the energy problem for many years. The production technology for gas hydrates
are not well developed (Figure 4).
Seismic signatures
Gas hydrates and free gas are detectable with seismic waves, because P- and S-wave velocities, in general,
increases in the presence of gas hydrates and the P-wave velocity decreases in the presence of free gas, but
there is very small change in the S-wave velocity in the presence of free gas. Gas hydrates are found up to
300m below the seafloor depending on water depth and local geothermal gradient. Small amounts are free gas
are often present below the gas hydrate stability zone (GHSZ). The hydrate-to-gas contact gives a strong
acoustic impedance contrast, which is evident on seismic sections as a bottom-simulating reflectors (BSR)
(Figure 5). BSR is a widely present seismic characterstics representing the base of hydrate stability zone.
|
