Overburden effect

We used model 1 to understand the effect of the overburden in the resulting amplitudes. We compared the amplitudes of the 2-layer case to the amplitudes in model 1, which includes an overburden zone with 2 flat layers and a target zone encased in shale (see top of Figure 1). The picked amplitudes at the top of the target zone (2 km depth) are plotted in Figure 9; note the good agreement with the results illustrated in Figure 6 corresponding to the 2-layer case. The amplitudes in model 1 are slightly higher, which can be expected because in this case the reflector appears at a higher time and the spherical divergence correction that we are applied increases the amplitudes more at a higher time (without the velocity taken into account). To confirm this, we picked the amplitudes before any preprocessing and they are identical at the near offset. We also expected some differences at the far offsets due to the AVO effect of the additional layers in model 1 (overburden effect).

Using Equation (2), we calculated the intercept (A) and gradient (B) attributes from the picked amplitudes in model 1 by fitting the amplitude versus $\sin^2\theta$ values to a straight-line approximation using a least-squares curve fitting method. Figure 11 shows the crossplot of the resulting intercept and gradient attributes; note the good quantitative agreement with Figure 10, which illustrates the theoretical values from Shuey's approximation.

 

model1pick Figure 9 Picked amplitudes from model 1

 

ABshuey Figure 10 Intercept versus Gradient crossplot from Shuey's approximation of P-wave reflection coefficient

 

ABmodel1 Figure 11 Intercept versus Gradient crossplot from picked amplitudes in data model 1 before migration