Minerals Report - International Seabed Authority

zone with a view to ascertaining whether the methane migrates to the
deposit. He noted that although it is known that methane comes from
biogenic production and there is a common belief that the pore waters of
associated sediments should be saturated, but there is an ongoing debate
about how saturated they should be. He pointed out that methane could
migrate in different forms, as bubbles, as rich fluids, and by molecular
diffusion. He also pointed out that hydrates occur in many different ways,
including disseminated bits of hydrates within pore spaces, as fine layers,
as nodules, and as massive hydrates.
With regard to free gas, Dr. Desa said that it is formed from those
gas molecules that are neither bound to other hydrocarbon molecules nor
bound as hydrates. He also said that the presence of free gas could be
determined through acoustic techniques.
At this point in his presentation, Dr. Desa turned his attention to
the geophysical indicators and non-geophysical proxies that could
facilitate prospecting/exploration for hydrate deposits, as well as to gas
escape features on the seafloor.
In relation to geophysical indicators, Dr. Desa said that three
phenomena are associated with gas hydrates in seismic reflection profiles.
He described these as:
1. A reflection known as the “bottom simulating reflection,” or
“BSR”, a very strong reflection that occurs at the base of the zone of
gas-hydrate- cemented sediment;
2. A reduction in acoustic velocity at the base of the gas hydrate zone,
such that high-velocity deposits occur over low-velocity deposits
(known as “polarity reversal” or “inversion”), and
3. A reduction in amplitude of reflections within the gas hydrate
zone, known as blanking.
Dr. Desa said that the first gas hydrates were identified on the
Blake Ridge on the basis of bottom simulating reflections. Their
manifestation from seismic profiles was that the BSR had a high reflection
for polarity reversal. Dr. Desa described a high reflection for polarity
reversal, a large reflection coefficient and increasing sub bottom depth
with increasing water depth as different criteria that characterizes a BSR.
INTERNATIONAL SEABED AUTHORITY 563