The final fluid substitution was that of brine for gas. The gas is both the less dense and least rigid of all the fluids
used, and thus the impedance contrast between our sand body and the shale units around the body was the greatest
in this model. Figure 18 shows the last section. The amplitudes are the the highest for this fluid
substitution because the reflection coefficient is the largest. Like in the oil section, many interesting effects
of using elastic modeling can be seen, including *S*-wave reflections and what appear to be *S*-wave refractions.

AVO theory, according to equation (9), predicts that for a negative reflection coefficient and a decrease in Poisson's ratio, as in a gas sand below a shale, the amplitude will increase with offset Ostrander (1984). In fact, this is what was noted in the gas saturated model. The CMP in Figure 19 clearly shows that when gas is the constituent of the pore space, amplitude will increase with offset. Also, when there is a positive reflection coefficient, and increase in Poisson's ratio, as in a gas sand overlying a shale, the amplitude will also increase with offset. This is what we see in the bottom reflector. Thus when the sand body is gas saturated, the AVO effect causes increase in amplitude with offset at both interfaces.

9/5/2000