Brine Saturated Pore Space

The section created from the original brine-saturated model is shown in Figure 14. The first thing to notice is that the amplitudes for the reflections are not very strong. This is because the impedance contrast between the two is not enough to generate a strong reflection. The reflection present does exhibit the 180^o phase change which is expected from a wave reflecting off a layer with lower impedance Sheriff and Geldart (1995). The reflection from the bottom of the body is also present. This reflection does not show the $180^\circ$ phase change because the lower unit has a relatively high impedance. The offset in the brine gathers, Figure 15, does not show any particular change with offset, except that the expected decrease in amplitude with offset is observed. An equation which relates P-wave reflection amplitude with increasing angle (or offset) is Shuey's approximation:

 

$$R(\Theta) \; \approx \; R_0+\left[ER_0+\frac{\Delta\nu}{(1-\... ...\frac{\Delta V_p}{V_p}\left[tan^2\Theta - sin^2\Theta\right] \;$$ (9)

where $\nu$ is Poisson's ratio, R₀ is the normal incidence reflection coeffecient, and E is a term involving the velocity and density changes Mavko (2000). Basically this equation, and other AVO equations, show that with little change in Poisson's ratio over a contact, the amplitude should decrease with offset, which is observed in the Figure 15.

 

oil_section
Figure 16 Zero-offset section for oil filled pore space.

 

oil_cmp Figure 17 CMP from oil section at midpoint 2100m.