## 4 Field Data Application Results and Discussion  
In order to test the application effect of this method, we used the original VSP data of the Z component in the Tuoputai area, Tarim Basin. The geophones were located from 3500 m (level 1) to 3750 m (level 26) in the TP327 CH well at a spacing of 10 m. The target layer was located at the Middle Ordovician Yijianfang Formation (O2 yj) at 6807-7378 m in the well. The reservoir space was mainly composed of fractures and pores with large scales and general connectivity. The reservoir lithology was dominated by pelsparite. The depth of the target layer was deeper than that of the geophones, and so the separation and imaging of the deep wavefield in the seismic records was very important for reservoir predictions. Figure 13a illustrates one of the common shot point gathers. In the seismic records, the upgoing and downgoing waves were mixed with each other. First, the modified Radon transform was performed using Equation (7), and the results are shown in Figure 13b. Because the downgoing P-wave energy spectrum was the strongest, the region in the yellow box of Figure 13b could be preliminarily determined as the P-wave energy spectrum. By linear fitting the position of the extreme  
Figure 12: Results for the separation of the VSP wavefield. (**a**,**e**) Downgoing P-wave; (**b**,**f**) Downgoing S-wave; (**c**,**g**) Upgoing P-wave; (**d**,**h**) Upgoing S-wave; (**a**–**d**) Results for our method; (**e**–**h**) Results for the high-resolution Radon transform method. point in the region of the longitudinal wave energy spectrum, we get \(a=37.48\), \(b=0.0075\). According to Equations (11) and (12), the downgoing P-wave can be obtained by aligning and median filtering, and the results are shown in Figure 14b. The residual wavefield is shown in Figure 13c, and the modified Radon transform results are shown in Figure 13d. The red box area of Figure 13d was negative, which was determined as the upgoing P-wave, and after fitting the extreme value points, \(a=-36.91\), \(b=-0.01089\) were obtained. According to Equations (11) and (12), the upgoing P-wave can be obtained, and the results are shown in Figure 14c. Analogously, the residual wavefields (Figure 13e,g) and their modified Radon transform results (Figure 13f,h) were obtained. The upgoing P-wave (Figure 14d) and the upgoing S-wave (Figure 14e) were obtained by fitting the slowness points and median filtering. Figure 14a illustrates original VSP data. Comparing the separation results (Figure 14b,e) with the original data (Figure 14a), it is clear that there is not any wavefield interference in the separated upgoing and downgoing P-waves and S-waves from the proposed method. From Figure 14f, we can see that the residuals from the wavefield separation are mainly noise. In the process of wavefield separation, the upgoing and downgoing waves can be easily distinguished by the positive and negative slowness. The key is in how to distinguish the P-wave and S-wave. Equation (1) shows that the slowness is determined by the direction and velocity of wave propagation. The S-wave has a smaller projection angle, reflection angle, and velocity than the P-wave, which makes the absolute slowness value of the S-wave greater than that of the P-wave. For the same type of wavefield, the angle \(\varphi\) may be changed, and so it should not be ignored for the accuracy of the separation results. However, the variation of cos\(\varphi\) is much smaller than the velocity difference between the P-wave and S-wave. Therefore, the energy spectra of the P-wave and S-wave on the slowness spectra can be separated. For comparing Figure 14a,c, the downgoing P-wave including the first arrival of the P-wave was subtracted. The energy spectrum positions in the yellow box in Figure 14b correspond to the events of the downgoing P-wave in Figure 14a. By comparing Figure 14b,d, one can see that the upgoing P-wave energy spectrum of the remaining wavefield appeared after subtraction of the downgoing P-wave.