Rock-Physics Models

In this section, I describe the rock-physics models which are used to determine the internal structure of the hydrated sediment and the amount of hydrate present in the sediment. These physical rock models link the elastic wave velocities in high-porosity sediments to density, porosity, effective pressure, mineralogy and water, gas and hydrate saturation.

 

hydrate
Figure 1 Hydrate models used in this study. Model A represents hydrate being part of the fluid. In model B, hydrate becomes part of the solid frame. Model C assumes that hydrate cements the grains evenly

In order to estimate the amount of hydrate present in the sediment, I examine three different models of possible hydrate deposition in the pore space (Figure ). In the first model, I assume that hydrate is suspended in the water, thus contributing only to the bulk modulus of the pore fluid (Figure A). In the second model, hydrate becomes part of the solid (Figure B). This causes a reduction of porosity and an additionally weak stiffening of the sediment structure. The third model assumes that hydrate cements grain contacts (Figure C), significantly changing the rock stiffness and again reducing the porosity. This last model is probably not likely to apply physically to the Blake Outer Ridge, since the sediments in this region are highly unconsolidated Matsumoto et al. (1996). However, since I want to examine the effect of the different micro-models on the saturations and since this third model (C) might be applicable to regions other than the Blake Outer Ridge, I include it in this study. Strictly speaking, the cementation theory is only valid for porosities less than 40%. However, it can be used to approximately estimate the elastic properties of granular aggregate of higher porosities.