## 3 Numerical Experiments
### Imaging Results
#### 3.2.3 Cave-Layer Hybrid Models  
The imaging results of model C-1 are shown in Figure 17. The imaging result of MVSS beamforming is shown in Figure 17b. The shapes and locations of the interfaces and caves can be clearly observed. There were some interferences between the layer interfaces and  
Figure 16: Model B-4b. (**a**) True velocity model. (**b**) Imaging result of MVSS beamforming. (**c**) Imaging result of Kirchhoff migration. (**d**) Imaging result of DAS beamforming.  
Figure 15: Model B-3. (**a**) True velocity model. (**b**) Imaging result of MVSS beamforming. (**c**) Imaging result of Kirchhoff migration. (**d**) Imaging result of DAS beamforming.  
the cave boundaries. On both sides of the layer beneath the shallower interface, some artifacts caused by S-waves can be observed.  
The imaging result of Kirchhoff migration is shown in Figure 17c, indicating that some interferences appeared near (Mark 1) and between (Mark 2) the layer interfaces. The DAS beamforming imaging result is shown in Figure 17d, revealing multiple arcuate interferences between the two interfaces (Mark 2) and above the shallow interface (Mark 3).  
When the three imaging methods are compared, the imaging results of Kirchhoff migration and DAS beamforming still exhibit artifacts of direct waves and surface waves, and there are more interferences between the layers and near the interfaces. Nevertheless, the MVSS method achieved the best resolution and most effectively suppressed the direct and surface waves.  
Compared with those for model B-1, the imaging results for model C-1 display more interferences between the interfaces and on both sides of the caves because of the increased stacking of S-waves from different reflection interfaces. In addition, compared with model A-1, it can be noted that the image of the cave in model C-1 is more precise.  
Compared with the RTM (Figure 17e), in the composed model of cave and fold, MVSS beamforming still made a better vertical resolution. There was a strong artifact caused by a surface wave in the RTM result.  
Model C-2 comprises a combination of a fault, multiple layers, and a cave (model A-1 and model B-2). The imaging results are shown in Figure 18, where Figure 18b shows the imaging result of the MVSS beamforming method. The shallower interface of the layer can be clearly observed, and the cave between the layers can be detected as well. However, the deeper interface is not complete at widths of 10 m to 20 m (Mark 4).  
Comparing the three methods makes it evident that the artifacts caused by surface waves are still noticeable in the Kirchhoff migration and DAS beamforming imaging results. Furthermore, the incompleteness of the deeper interface is as notable as it is in the MVSS beamforming imaging result, while the interferences between the layers and on both sides of the image (Mark 2) are weaker in the MVSS beamforming imaging results. However, the disturbances on the deeper interface (Mark 3) are similar among all three methods.  
Figure 17: Model C-1. (**a**) True velocity model. (**b**) Imaging result of MVSS beamforming. (**c**) Imaging result of Kirchhoff migration. (**d**) Imaging result of DAS beamforming. (**e**) Imaging result of RTM.  
Compared with model B-2, a part of the deeper interface of the layer is blurred in model C-2. There are two main reasons for this blurriness. The structure is particularly complicated in the middle of the area due to the coexistence of the fault and cave, which causes the wave to reflect back and forth between those structures; this produces many multiples. Thus, the reflected waves from the deeper interface are seriously disturbed by multiples from the complex structure in the middle of the model. Moreover, compared with the features of model A-1, the cave's roof and floor are not clear (Mark 1) because the waves are disturbed by the fault.