*3.1. Theoretical Model*

A theoretical model with four interfaces is designed to synthesize seismic data and test the method. The depth of the model is 5500 m and the sampling interval is 5 m. The width of the model is 1000 m, and 100 CDP points are set. The theoretical model of interval velocity is shown in Figure 4. CDP gathers are obtained through forward modeling, with the time length of 1 s and the maximum offset of 3100 m. Figure 5a–c are the CDP gathers at positions of CDP 30, CDP 50, and CDP 70, respectively. For each CDP gather, NMO removal processing is performed according to the root mean square velocity of 1500 m/s and a velocity spectrum is generated. Figure 6a–c are the velocity spectra of CDP 30, CDP 50, and CDP 70, respectively.

**Figure 4.** Theoretical model of interval velocity.

**Figure 5.** CDP gathers at different positions. (**a**) CDP 30; (**b**) CDP 50; (**c**) CDP 70.

**Figure 6.** Velocity spectrum. (**a**) CDP 30; (**b**) CDP 50; (**c**) CDP 70.
