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BSR clearly has the opposite polarity to the seafloor reflection. For
normal-incidence reflections the P-wave reflection coefficient is given by Eq. (4.62).
There is an abrupt increase in impedance at the seafloor due primarily to the
increase in density on going from sea water to the very porous sediment; this gives
rise to a positive reflection. At the base of the hydrate stability field the impedance
falls abruptly. This fall in impedance, due to the decrease in seismic velocity and
partly density, gives rise to a negative-polarity reflection. Synthetic seismograms
calculated from the impedance model match the reflection data well.
Figure 4.43.
The bottom-
simulating reflector (BSR)
on a seismicreflection line
on the continental slope
west of Vancouver Island,
Canada, part of the
Cascadia margin where the
Juan de Fuca plate is
subducting beneath the
North American plate.
(From Hyndman
et al.
Geophysical studies of
marine gas hydrates in
northern Cascadia.
Geophysical Monograph
124, 273-95, 2002.
Copyright 2002 American
Geophysical Union.
Reprinted by permission of
American Geophysical
Union.)
Figure 4.44.
Detail of the
seafloor and BSR (Bottom
Simulating Reflector)
reflections from Fig. 4.43.
The seafloor reflection has
positive polarity (+); the
BSR has negative polarity
(
). The impedance model
(Eq. (4.87)) and synthetic
reflections calculated from
the impedance model
match the data well. (From
Hyndman
et al.
Geophysical
studies of marine gas
hydrates in northern
Cascadia. Geophysical
Monograph 124, 273-95,
2002. Copyright 2002
American Geophysical
Union. Reprinted by
permission of American
Geophysical Union.)
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