Minerals Report - International Seabed Authority

methane from hydrate dissociation during a perturbation in the
pressure/temperature regime. He concluded that these features could be
used as additional exploration indicators.
He noted that features such as mud diapirs, bacterial mats and
methane-derived carbonates have also been reported on the seabed in
hydrocarbon rich sedimentary basins in numerous parts of the world. He
said that these features are considered as evidence of gas seepage activity,
and could also be used as additional exploration indicators in hydrate-rich
regions.
With regard to techniques for exploring for methane hydrates, Dr.
Desa identified two approaches, the indirect geophysical approach, and
non-geophysical approach. As concerns the tools available for data
acquisition he suggested the use of single and multichannel seismic
reflection, stating that single channel seismic reflection is available with
many people. Dr. Desa said that single channel seismic reflection is
adequate for recognizing the presence of BSR and the amplitude-blanking
phenomenon. He said that multi channel seismic data in addition
provides an approximation of velocity that is useful for BSR
characterization and in estimating hydrates concentration. He further
noted that since hydrates and associated gas alter the elastic properties of
sediments, two properties of sediments should be examined. These are the
compressional (VP), and the shear wave velocities (Vs).
Dr. Desa said that the shear wave velocity could be indirectly
estimated from P-wave source experiments of conventional surveys, and
from seismic refraction experiments with Ocean Bottom Seismometers
(OBS) in conventional refraction geometry. He also said that a combination
of conventional multichannel seismic reflection and high resolution OBS
refraction data would be a good way to detail the velocity depth
discontinuities and elastic properties of hydrated sediments.
Dr. Desa emphasized that because the density of sediments
remains about the same, whether it is sediments and hydrates or
sediments with gas, velocity becomes the primary tool for distinguishing
between the two. He said that the tools he had spoken about are those
that are required to make measurements on velocity. He stated that with
accurate measurements of velocity, models could be created to assist in the
evaluation of the deposits. He said that these include models of the
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