## 4 Field Data Application Results and Discussion
After separating the upgoing P-wave from Figure 14c, the residual wavefield was obtained as shown in Figure 14e. The comparison between Figure 14d,f shows that when all the P-wave data were subtracted, the energy spectrum of the downgoing S-wave appeared, as shown in the red box of Figure 14f. Then, the downgoing S-wave was subtracted from Figure 14e and the energy spectrum of the upgoing S-wave appeared (Figure 14h). In the above process, the wavefields were subtracted in the order of the energy spectrum from strong to weak (downgoing P-wave, upgoing P-wave, downgoing S-wave, upgoing S-wave). With this process, after the former wavefield is subtracted, the energy spectrum of the later wavefield appears. In order to verify the accuracy of the results, the methods of high-resolution Radon transform and static time shifting (traditional median filtering) were used to separate the original data. We compared the separation results for our method with the high-resolution Radon transform method and static time-shift separation results at 5000-6000 ms, as shown in Figure 15. The contrast indicated that parts of the events of the deep reflection wave were broken by the static time-shift median filtering method, which was obviously due to the inaccuracy of the slowness value that resulted in the misalignment of events. However, the median filtering method based on dynamic alignment, because the event is aligned according to the actual accurate slowness value, basically did not reproduce the event faults. The results of the high-resolution Radon transform showed better continuity for some local events, but this method destroyed the variation of the amplitude and presented characteristic bifurcation patterns. Figure 13: Wavefield data after successive wave separations and the slowness spectrum. (**a**) Original VSP data; (**b**) Slowness spectrum of (**a**); (**c**) Residual wavefield after subtraction of the downgoing P-wave in (**a**); (**d**) Slowness spectrum of (**c**); (**e**) Residual wavefield after subtraction of the upgoing P-wave in (**c**); (**f**) Slowness spectrum of (**e**); (**g**) Residual wavefield after subtraction of the downgoing S-wave in (**e**); (**h**) Slowness spectrum of (**g**). Figure 14: Results for the separation of the VSP wavefield. (**a**) Original VSP data; (**b**) Downgoing P-wave; (**c**) Downgoing S-wave; (**d**) Upgoing P-wave; (**e**) Upgoing S-wave; (**f**) Residual wavefield after subtraction of four wavefields. Figure 15: Cont. As shown in Figure 16, we compared the residuals of the three methods to analyze whether there is important energy left. In the red and green quadrilaterals, there are several obvious downgoing wave events in Figure 16d but not in Figure 16b,c. In addition, the blue quadrilaterals in Figure 16c show a weak downgoing wave event but there are no events left in the corresponding position in Figure 16b. Besides, the slowness spectrum of residual wavefield using our method is the weakest of Figure 16i-k. This indicates that the effective energy left in residual wavefield using our method is less than those of other methods. In the slowness spectrum, the upgoing and downgoing waves can be distinguished according to the positive or negative characteristics of the slowness with the proposed method. The P-waves and S-waves can be distinguished by the magnitude of the absolute value of the slowness because of the differences in the velocity and incidence angle. According to the results of this study, although the slowness of the same wavefield was variable with the intercept time, the quantity of the variation was not enough to exceed the slowness difference between the P-wave and S-wave, and so in the slowness spectrum, the P-wave and S-wave could be distinguished. Because of the differences in the energy spectrum between different wavefields, the four wavefields were separated successfully according to the priority ranking based on the order of the energy spectrum from strong to weak.