Seismic Velocities

Using the different schemes of hydrate deposition, it is possible to calculate the effect these models will have on the actual velocities and thus on velocity contrasts from hydrate-bearing sediments to the underlying sediments. The theoretical calculations are based on the rock-physics theory discussed in detail in Chapter 4. The calculation parameters, i.e., the sediment mineralogy, is, as before, 60% clay, 35% calcite and 5% quartz. The actual calculation parameters are given in Table 4.1.

I determine the effect of increasing hydrate saturation on the sediment elastic properties, i.e., P-wave velocity and S-wave velocity. The resulting velocities for hydrate deposition models A, B, and C can be seen in Figure . They are determined by assuming a porosity of 45% and a BSR depth of approximately 500 m, which is consistent with the seismic data. The left side of Figure represents the saturated sediment velocities for model A, the middle shows the velocities for hydrate model B, and the left shows those of model C. Maximum hydrate saturation is about 50%. It is obvious, that model A and B affect the P-wave and S-wave velocities similarly at small hydrate saturations. In both models, the presence of hydrate increases the P-wave velocity of the sediment slowly with increasing hydrate saturation. Since model B affects the stiffness of the sediment frame, its effect on the P-wave velocity becomes stronger than that of model A. Whereas the P-wave velocity increases, the S-wave velocity slightly decreases with hydrate saturation for model A. It is practically independent of hydrate saturation. While the S-wave velocity produced by model B also does not change much for small hydrate saturation, it is clearly increasing with increasing saturation. By becoming part of the solid, the hydrate not only affects the P-wave velocity but also, slightly, the S-wave velocity. In the cementation model, the hydrate acts to considerably increase both P- and S-wave velocities. This effect is well-pronounced already at very small hydrate saturations.

 

velhyd
Figure 2 P-wave (solid line) and S-wave velocities (dashed line) for hydrate models A, B, and C versus hydrate saturation.

The increase in P-wave velocity in all three models is in good agreement with the existing seismic velocity analysis performed in Chapter 2. This impact of hydrates on the acoustic velocity was used in Chapter 4 to invert for the amount of hydrate present in the pore space. While a sufficient increase in velocity can be obtained due to only very small amount in the cementation model, both model A and model B require larger amounts of hydrates to obtain a similar P-wave velocity increase.

Since the P-wave velocity cannot be used to discriminate uniquely between the different hydrate deposition models, the S-wave velocity becomes important. The different effects of the models on the S-wave velocity might be detectable in AVO analysis and thus allow a discrimination of the different models and their resulting hydrate saturation estimates.