# AVO-Friendly Velocity Analysis Based on the High-Resolution PCA-Weighted Semblance
## 3 Experiments  
### Synthetic Data  
To analyze the performance of the new method in velocity analysis, we compared and analyzed the resolution of proposed semblance, conventional semblance, and AB semblance with a synthetic CMP gather. As shown in Figure 4a, we generated a CMP gather with the class-II AVO anomalies [28] by the inverse normal moveout, where the AVO anomalies have the polarity reversal seismic amplitudes. We performed NMO corrections with a series of scanning velocities, and calculated the different semblance spectra on the NMO-corrected data. Then, we picked the velocity values corresponding to the peaks in semblance spectra. Figure 4b shows the conventional semblance of the synthetic CMP gather, in which the solid line represents the picked velocities. It can be seen that the energy clusters of conventional semblance were lost in the AVO regions (time 1.5 s to 3.0 s), resulting in inaccurate velocity picking. Figure 4c shows that the energy clusters of the AB semblance were restored in the AVO regions. However, the size of the energy cluster became larger, indicating a drop in resolution. Figure 4d shows that the resolution of our new method is significantly improved, which is of great benefit to avoiding the multiplicity of the auto-picking algorithm, and the energy clusters of new method are also strong in the AVO region.  
Figure 3: Comparison of the semblance of different methods at time 0.24 s. Where the blue curve represents the AB-semblance, the red curve represents the conventional semblance, and the yellow curve represents the proposed PCA-weighted semblance.  
We applied each set of picked velocities to correct the movement of the synthetic CMP gather as a form of quality control. Figure 5 shows the velocity values picked up from different semblances, where the black curve represents the true values of velocities, the green curve represents the picked velocities from conventional semblance, the cyan curve represents the AB semblance, and the red curve represents our new method. It is clear that the picked velocities of conventional semblance had obvious errors in the AVO region. Conventional semblance is an incompetent method for the AVO phenomenon. The results of AB semblance show a significant improvement in dealing with the AVO phenomenon. However, the accuracy of AB semblance is inferior to the conventional method because the trend function of AB semblance is insensitive to the velocity. The red curve reveals that the proposed method has the highest accuracy compared with other methods, and its accuracy in the AVO region is still good. Figure 6 shows the NMO-corrected gathers by the picked velocities from different semblances. It can be seen that the NMO-corrected gather of the conventional method (Figure 6b) suffered from both over-correction and under-correction problems in the far-offset traces, especially for the AVO area. Clearly, the seismic events with the AVO anomalies were better flattened in the AB semblance (Figure 6c) compared with the conventional semblance. In the shallow region of NMO-corrected gather, Figure 6d shows that the seismic events were flattened completely, indicating that the new method can provide more accurate picked velocities to perform NMO correction.  
Figure 4: Semblance spectra of the synthetic CMP gather with the class-II AVO anomalies. From left to right: (**a**) uncorrected CMP gather on which all methods of semblances were applied, (**b**) conventional semblance, (**c**) AB semblance, and (**d**) PCA-weighted semblance.