I create simple 1-D models for the three schemes of hydrate deposition by
assuming that (1) the hydrated sediment is overlain by brine-saturated
sediment and underlain by gas-saturated sediment or (2) the hydrated
sediment is both overlain and underlain by brine-saturated sediment.
I explore those two possibilities because although there is clear
evidence from the seismic data at the Blake Outer Ridge that there is
free gas beneath the hydrate, the possibility of lateral brine
patches underneath cannot completely be excluded. Therefore, including
both scenarios in the forward modeling will yield the different amplitude
responses connected to either brine or gas saturation.
The elastic sediment properties are calculated based on the actual hydrate
and gas saturations estimated in Chapter 4. I chose one surface position
at about 49 km to represent the model base, and use its hydrate and gas
saturations
to calculate the elastic, saturated sediment properties
using the rock-physics scheme described in detail in Chapter 4.
In this way, I base the modeling on the actual different saturation estimates
connected with the different models.
All sediments are
assumed to have a porosity of about 45%, which approximates the porosity
in the hydrate zone at the surface position under consideration. The
thicknesses of the layers are adjusted to match the zero-offset travel
times of the real reflectivity gather at the surface position of 49 km.
In this way, a
direct comparison between the synthetics and the real gather is possible.
The resulting model is summarized in Table ![[*]](http://sepwww.stanford.edu/latex2html/cross_ref_motif.gif)
.
The properties listed in Table are used to
calculate the actual saturated
P-wave, S-wave and density of the different models of hydrate
depositions. The results for the case of hydrate overlying gas-saturated
sediments can be seen in Figure .
The properties of hydrate model A are represented by the solid line, those
of model B by the dashed line and those of model C by the dotted line.
The calculated saturated sediment properties display the expected
increase in P-wave velocity in the hydrate-bearing zone and the decrease
in P-wave velocity due to the saturation of free gas underneath. The effect
on density is very small for all three hydrate models. The most
pronounced difference is, as hoped, visible in the S-wave velocities of the
models. While hydrate as part of the pore fluid does not affect
the S-wave velocity at the estimated hydrate saturation, hydrate cementing
the frame does significantly increase the S-wave velocity in the hydrate
sediments with respect to the sediments underneath. This already indicates
that this model is not what might apply to the Blake Outer Ridge data since
the AVO analysis discussed in Chapter 3 did not provide evidence of such
a pronounced increase in shear velocity in the hydrate zone. Hydrate
being part of the fluid and hydrate being part of the solid appear
to be seismically indistinguishable from these elastic properties.
When the hydrate becomes part of the frame, it slightly increases the
shear wave velocity in the hydrated zone. This increase, however, is so
minimal that it will not be resolvable from seismic.
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The P-wave velocity, S-wave velocity and density propertied for
hydrate models A, B and C in the case of hydrate being underlain by
brine-saturated sediments can be seen in Figure . The
figure shows that in this case there is a less pronounced P-wave velocity
contrast at the transition from hydrate to the sediments underneath.
Both S-wave velocity and density do not show a strong, visible change
in regard to the previous gas-saturated sediments.
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