After having derived the different models for possible hydrate formation in the pore space, they can be used to calculate the hydrate and gas saturations along the seismic section shown in Figure 1. This is done by formulating an ``inverse'' problem: we forward model the saturated sediment moduli until we obtain a good fit with the seismic interval velocities, which were obtained using stacking velocity analysis Ecker (1997). The interval velocities, which can be seen in Figure 3, display an increase in the hydrated sediment section above the BSR and a decrease underneath due to the presence of free gas. To perform the forward modeling, we assume a sediment mineralogy of 60% clay, 36% calcite and 5% quartz based on drilling results from the Blake Ridge Region Matsumoto et al. (1996). The actual calculation parameters for the sediment mineralogy and the pure hydrate and methane gas can be found in appendix B.
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The forward modeling also requires knowledge of porosity since porosity and saturations cannot be solved for simultaneously. Without a-priori information, any brine, gas or hydrate saturation can fit the seismic velocities at a certain porosity. Therefore, we first calculate a baseline porosity section by fitting the seismic velocities assuming that all of the sediment is fully brine saturated. This results in anomalies where the assumption breaks down. The porosity is underestimated in areas of hydrate saturation, while it is overestimated when free gas is present. Three baseline porosity curves are shown in Figure 4. The first one, taken at a lateral distance of 15 km, represents a part of the seismic line without prominent BSR. Showing no apparent porosity anomaly, the brine saturated assumption appears to be true in this region. The middle panel is the baseline porosity at the beginning of the BSR at a distance of about 34 km. It displays a negative anomaly that can be explained by the presence of hydrate. The last panel shows both a negative and a strong positive anomaly, indicating the presence of hydrate and free gas.
In order to relate these anomalies to hydrate and gas saturations, we need to determine the anomaly residuals by subtracting a normal (without gas hydrate and gas) profile from the baseline porosities. We offer two methods of determining this normal porosity profile. In the first one, we determined the average normal porosity trend from the part of the seismic line which does not have a BSR. In the second one, we calculate the normal porosity on a trace-by-trace basis.
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