# Imaging Top of Volcanic Mounds Using Seismic Time- and Depth-Domain Data Processing
## 3 Results of Seismic Data Processing
### Depth-Domain Processing Using the MVA Technique  
Depth-domain data processing flattened the CRP gathers, which proves that the updated velocity is correct. The MVA technique is a data-processing method that can obtain a reasonable velocity estimation using tomography or a wave equation for the complex subsurface structures. Then, this velocity is used to generate a subsurface image in the depth domain. In this study, a depth-domain data-processing workflow (Figure 3b) using a tomography-based MVA technique [18] was applied. In order to obtain high  
Figure 5: (a) Stack section after applying only the preliminary processing in time-processing workflow. (b) Stack section after applying the entire time-processing workflow (Figure 3a). quality depth-domain results, e.g., seismic sections and velocities, the input data were used with data (Figure 4b) whose signal-to-noise ratio was improved after applying the signal processing in the time domain. The depth-domain interval velocity, converted from the time-domain RMS velocity by the Dix equation [25], was used as the initial velocity model. Next, the loaded time-domain data were sorted by the common offset data. After calculating the travel time [26] using the eikonal solver, we performed Kirchhoff pre-stack depth migration (PSDM) [27]. The migration results were sorted as CRP gathers, and the residual move-out (RMO) technique [28] was applied. Finally, ray-based tomographic velocity analysis [18] was applied to obtain the updated depth-domain velocity. In this study, depth-domain processing using the MVA technique (Figure 3b) was repeated three times. By comparing the depth-domain initial velocity (Figure 6a) converted from the RMS velocity using time-depth conversion and the final velocity (Figure 6b) obtained after MVA at the third iteration, we were able to confirm that the overall subsurface velocity structures after applying MVA were greatly improved. In particular, it can be seen from the velocity differences in Figure 6c that the velocity was significantly updated in the region of interest (0.9 to 1.2 km), which corresponds to the depth of basalt flow. Figure 6: Cont. In general, the CRP gather is analyzed to verify whether the velocity is correct. CRP gathers are signals reflected from the same reflection point, so if the velocity is correct, the CRP gather should be flat [28]. Therefore, we compared CRP gathers before (Figure 7a) and after (Figure 7b) MVA to confirm that the final depth-domain velocity was reasonable. From Figure 7, we can observe that the CRP gather became flatter after the MVA was applied. Since the CRP gather was generated depending on the velocity, the depth of the reflection events in the CRP gather was corrected using the updated velocity after applying the MVA. In addition, in order to quantitatively confirm the accuracy of the updated velocity, the velocity error curve according to the number of iterations is shown in Figure 8. In general, the iteration number is determined by considering the trade-off between accuracy and computational cost. It can be seen that the velocity error was reduced to about 3.8% in the third iteration. From the error curve, we judged that the velocity error sufficiently converged in the third iteration, and thus the MVA process was stopped. Therefore, it was verified that the final depth-domain velocity obtained after applying the MVA was reasonable. Finally, to confirm the effectiveness of the depth-domain data processing, we compared the stack sections before (Figure 9a) and after (Figure 9b) the MVA was applied. The red and green lines represent the top of the basalt before and after MVA was applied, respectively. In Figure 9b, the difference in depth between the two basalt tops ranges from 32.5 to 60 m. The 3D depth-domain stack sections after applying MVA are also shown in Figure 10. From the 3D results, the horizons between the in-line and the cross-line agree well with each other, and the spatial depth variance of the basalt top before and after the MVA application can be observed.