Confidence of Zero-Offset Reflectivity

In order to conduct AVO modeling of amplitudes, it is important to have a measure of the actual zero-offset reflection amplitude. The prestack migration described in Chapter 2 should have resulted in a good estimate of the subsurface zero-offset reflectivity. This reflectivity is both input into the impedance inversion and the subsequent AVO modeling. The inverted P-impedance contrast is approximately twice the size of the actual zero-offset reflection.

The zero-offset reflectivity can also be obtained from the first multiple of the seafloor. In a layered medium, the reflection coefficient of the seafloor and of every subsequent layer can be calculated after Warner 1990:

$$\begin{eqnarray} &\rm R_{\rm w}& \:=\: {\rm A_{\rm m} \over \rm A_{\rm w}}, \non... ...\rm i}& \:=\: {\rm A_{\rm i} \over \rm A_{\rm w}} \: \rm R_{\rm w}\end{eqnarray}$$ (7)

where $\rm R_{\rm w}$ is the reflection coefficient of the seafloor, $\rm A_{\rm w}$ is the amplitude of the seafloor reflection and $\rm A_{\rm m}$ that of the first seafloor multiple. The reflection coefficient of the ith layer is given by $\rm R_{\rm i}$, and $\rm A_{\rm i}$ is the amplitude of the ith reflection. The original Blake Outer Ridge data was recorded to about 12 s two-way travel time, and thus includes the first seafloor multiple.

Comparison of the reflection amplitudes obtained from both methods will tell me how much confidence to give the reflectivity resulting from the migration and hence the zero-offset amplitudes that are input into the AVO modeling. Because in the case of using the first seafloor multiple the reflection coefficients of all subsequent reflectors are based on the seafloor reflection coefficient, I compare the two results on the seafloor.

 

refl
Figure 11 Reflectivity comparison between the one resulting from migration (left) and the one obtained from using the first seafloor multiple (right).

Figure displays the comparison between the reflection coefficients of the seafloor. The left panel shows the coefficients as obtained after migration, while the right panel displays the reflection amplitude after calibration with the first seafloor multiple. The reflection coefficients after migration appear better constrained than those obtained via multiple calibration. Since the first seafloor multiple occurs very deep in the section (more than 9s two-way travel time), more errors due to decrease in the S/N noise ratio and picking can be expected. The overall trend of the reflection coefficients is, however, fairly similar. Therefore, I conclude that the zero-offset reflection amplitude of my data does indeed yield a good approximation of the subsurface reflectivity.