The porosity residuals obtained by both the averaging method or the trace by trace method can now be mapped into hydrate and gas saturations. Negative residuals are considered to be caused by the presence of hydrate, positive ones by the presence of free gas. Starting from full water saturation, the hydrate and gas saturations are increased until they minimize the residuals within the described uncertainty levels and reproduce the seismic velocities. The results of both the average trend approach and the trace-by-trace approach are shown in Figure 9. The left panel shows the saturation results for hydrate models 1 to 3 using the average method. The right panel shows the results for the trace-by-trace method.
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The averaging method shows maximum hydrate saturations above the BSR between 40 and 50 km. Hydrate model 1, where hydrate is part of the pore fluid, results in a maximum hydrate saturation of about 19%, while hydrate model 2 (hydrate becomes part of the solid) results in a more conservative estimate of about 14%. In the case of hydrate cementing the sediment grains, less than 1% hydrate saturation would be required to satisfy the seismic data. The gas saturation beneath the BSR appears to be approximately 2%. Several other areas throughout the section show possible hydrate saturations. This is due to the uncertainties of the hydrate residuals being hardly separable from those caused by lateral brine porosity variations.
The trace-by-trace method results in significantly cleaner saturation maps since the hydrate and gas anomalies were clearly larger than those caused by the misfits of the brine porosities. All three hydrate model show hydrate saturations both between 32 km and 38 km and between 40 km and 50 km. Model 1 gives a maximum hydrate saturation of about 33 %, model 2 of about 24%, and model 3 again less than 1%. Gas saturation is rather stable at about 2% of the pore space. In the area between 30 and 38 km, the hydrate saturation estimates are about 10% less than those between 45 and 50 km. This area of hydrate saturation can not be clearly distinguished from the averaging method because of the considerable contamination of the section with artifacts.
Comparison of the trace-by-trace and averaging method shows that the latter yields hydrate estimates that can be up to 14% more conservative. Both methods result in maximum lateral hydrate saturations in the area between 40 and 55 km. The averaging method is more successful in recovering hydrate saturations at the end of the section (between 50 and 55 km) where the lack of background reflections disturbs the trace-by-trace method. The trace-by-trace method, on the other hand, results in clearly distinguishable hydrate anomalies throughout the section. Combining both methods is probably a good method of obtaining a possible range of the amount of hydrate present in this region.