Minerals Report - International Seabed Authority

sediment and to provide an indication of the presence of underlying free
gas zone.
c) Wave- form inversion
It must be appreciated that AVO analysis alone is not sufficient to
distinguish different structural models across the BSR, nor can the AVO
technique define the upper and lower boundaries of the free gas zone 49,54 .
Identification of the top and bottom of the low velocity zone underlying
the BSR can provide information about the thickness and lateral extent of
the free gas layer associated with the formation of gas hydrates. Waveform
inversion of seismic reflection data can be used to distinguish the models
and provide tight constraints on the thickness of a gaseous zone beneath
the BSR. With the waveform inversion technique, a velocity model can be
created such that the synthetic data over all offsets fits the real data at the
BSR.
5.2. Non-Geophysical Proxies
It is not always possible to observe natural gas hydrates directly
because they decompose during recovery of samples from the seafloor.
There are four groups of proxies providing more or less reliable evidence
of hydrate presence in cores. In order to study these proxies, surficial and
shallow sub-surficial sediment samples are needed. Since the sediments
and water samples are analysed for proxies for gas hydrates, they need to
be stored under in situ conditions for later analyses in the laboratory.
These sediment samples are normally collected using piston or gravity
corers, techniques with limitations in maintaining the in situ
characteristics. An advanced technique of sampling is through specially
designed pressure core samplers (PCS). The core samples collected
through piston/gravity corers are collected in transparent PVC core liners
so that the presence of gas pockets in the sediments can be visually
inspected. The gas bearing sections of samples should be stored either in
liquid nitrogen containers or kept in Nalgene bottles prior to storing them
at –80 degrees C for subsequent gas analysis.
In sediments, methane occurs as dissolved gas in the pore fluids
and to some extent, adsorbed on the surface of particulate matter. Thus
improper thermal or mechanical handling or storage of the sample can
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