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1 Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
2 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
3 Institute for Geophysics, University of Texas, Austin, Texas 78759, USA
New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermo-dynamic regime where it should be trapped as hydrate, suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean. In the study area, none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor; instead they terminate at the base of a highly reflective, unfaulted capping layer. Seismic inversions of anomalous regions show (1) increased velocities beneath the hydrate stability zone, suggesting less gas, and (2) increased velocities within the hydrate stability zone associated with observed low-amplitude chimneys and bright spots, indicating increased hydrate concentrations. These observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate. The observations strongly imply that given appropriate permeable pathways, free gas can escape into the ocean. Even in a low-flux environment, the hydrate stability zone is not an impermeable barrier to free-gas migration.
Key Words: methane hydrate • methane flux • gas dynamics • seismic reflection • waveform inversion
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