Publication Date:
2019-02-01
Description:
The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic
velocities that overprint the background lithological velocities of the sediments themselves. Much previous
work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional
velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate
province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for highresolution
velocity analysis that delivered detailed velocity models of shallow marine sediments and their
coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by
free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits.
For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation
that provides important information about the distribution of free gas and gas hydrate. By combining the
velocity information from the seismic transect with geologic samples of the seafloor and an understanding of
sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within
coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere
along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations
away from the channel.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1190/INT-2015-0042.1
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