Minerals Report - International Seabed Authority

zone of gas hydrate stability. On the other hand, the bottom-simulating
reflector is the region of impedance contrast in the sediment column at
the base of the zone of gas hydrate stability that gives rise to the
anomalous reflection on seismic records.
Several different criteria have been used to characterize the
anomalous reflections inferred to be a BSR and to help differentiate these
reflections from other unusual ones.
These criteria 37 are
i) Reversed polarity of the seismic wavelet relative to the
reflection from the sea floor (equivalent to a negative reflection
coefficient) at the BSR.
ii) Frequent crosscutting of the seismic reflections from bedding
planes by the BSR, indicating that it is not a bedding plane
reflection.
iii) Close mimicking of the sea-floor topography by the BSR.
A BSR-like reflection may not necessarily arise from gas hydrate
formations. In some areas, these acoustic features may also result from
temperature-controlled diagenetic effects. During the Deep Sea Drilling
Project (DSDP) Leg 19, seismic reflections akin to BSR in some Bering Sea
sediments draped on the Umnak Plateau were observed (Kvenvolden 5
and references therein). The drilling at one of these sites (site 185)
however indicated the presence of only methane in the sediment with no
evidence of the existence of gas hydrates. In view of this, some
researchers (cf. Kvenvolden 5 ) attributed those BSR-like seismic
reflections to a lithologic transition from hemipelagic diatom ooze to
indurated claystone. Thus, there are two types of BSRs, one indicating
the base of the gas hydrate zone and the other signalling a diagenetic
boundary (cf. Kvenvolden 5 ). Since the BSRs represent an acoustic
velocity contrast in the sediment, it is not necessary that all BSRs reflect
the existence of hydrates; similarly a BSR may not always be discernible
when a hydrate is present 38 .
Nevertheless the coincidence in depth of the BSR with the
theoretical, extrapolated pressure and temperature conditions that define
the hydrate phase boundary and sampling of hydrate above BSRs give
confidence that this seismic indication of hydrates is dependable 4 .
Therefore, gas hydrate areas are generally inferred when the so-called
INTERNATIONAL SEABED AUTHORITY 529