Effects of migration operator

I first investigated the effects of my downward continuation migration operator on the AVA. I migrated the synthetic data using the correct velocity model and extracted my calculated amplitudes along each of the three interfaces. These results can be seen in Figure 2. In each of the panels, the solid line represents the theoretical AVA values and the dots show the AVA values obtained after migration. The horizontal axis is shown in offset ray parameter (p_h), which is not an intuitively obvious physical unit. Based on the relationship in equation (2), the range of opening angles for the shallowest interface (left panel of Figure 2) is $0^{o}\ -\ 44^{o}$, the range for the second interface (center panel of Figure 2) is $0^{o}\ -\ 46^{o}$, and the range for the deepest interface (right panel of Figure 2) is $0^{o}\ -\ 68^{o}$. Overall, the results for each of the three interfaces are good. The calculated values for the second and third interfaces are not as consistent as the shallowest interface, partly due to the presence of more migration artifacts at depth. In the case of the deepest interface, it is also an effect of the survey geometry: we cannot expect the deep event to have reliable amplitude information at large p_h (larger than an opening angle of 36^o) because the common midpoint and offset ranges of the seismic data are limited, so energy reflecting at large angles from the deep event are lost. However, the overall trends of all of the calculated AVA values are fairly accurate.

 

ava.comp.mig
Figure 2 Amplitude variation with p_h. Left panel shows the results for the shallowest interface as seen in Figure 1, center panel for the second interface, and right panel for the deepest interface. The solid line shows the theoretical value and the dots show the values obtained from the image after downward continuation migration.