*4.2. Depth Domain Imaging Matching and Velocity Model Building*

The depth domain velocity model of the P-wave and SH shear wave have been established, respectively, with a velocity model building workflow including layer- and grid-based tomography. It is shown in Figure 6 with the depth domain velocity models of the P-wave and SH shear wave, respectively, that they have a certain similarity in the spatial variation of the velocity field; the velocity of the P-wave is relatively low at the position of CMP (common middle point) 1200–2500, and it is relevant to the gas enrichment in this area, which can also be seen in the pre-stack depth migration results shown later.

**Figure 6.** (**a**) The depth domain velocity model of the P-wave with a velocity model building workflow including layer- and grid-based tomography. (**b**) The corresponding depth domain velocity model of the SH shear wave.

The imaging of the reflector of the P-wave and the strong reflector of the SH shear wave should be located at the same depth because the pre-stack depth migration images show the wave impedance interface of subsurface media. It is very difficult to match the depth domain imaging positions of the P-wave and SH shear wave in practical applications due to the low signal-to-noise ratio of P-wave data, the possibility of anisotropy, and various uncertainties in the area. A relatively easy-to-implement process has been designed to attempt to match the imaging depth of the P-wave and SH shear wave. Our idea is to constrain VP and vs. because the most significant factor affecting the imaging depth is the velocity field. The specific method is that a set of strong reflectors in the survey are selected; they are picked up in the migration images of the P-wave and SH shear wave, respectively; and their average velocities are calculated from the surface to the reflectors. The specific formula is *V* = <sup>Δ</sup>*<sup>Z</sup>* ∑ *ti* , in which, for ∑ *ti* = ∑ *i* 2Δz*<sup>i</sup> Vi* , <sup>Δ</sup>*<sup>Z</sup>* = <sup>∑</sup> <sup>Δ</sup>*zi*, similar calculations for both the P-wave and SH shear wave have been made, the imaging position of the strong reflector in the depth domain is determined by this average velocity, and the average velocity ratio obtained should also be consistent with the P-wave and SH shear wave velocity in the survey [40]. The existing depth domain velocity can be corrected if the velocity ratio of SH shear wave and SH shear wave in the space of the survey is obtained. The pre-stack time migration images have been used and the corresponding reflectors have been picked up to solve this problem, the pre-stack time images of the P-wave and SH shear wave approximately represent the vertical two-way travel time of the P-wave and SH shear wave with zero offset, and the travel time ratio is obtained to further constrain the VS/VP ratio.

Back to the first step, a corresponding proportional constraint has been made on the P-wave velocity field with a correction of about 1–3%, and the weak anisotropy parameter is utilized to solve the residual moveout of imaging caused by this correction. In fact, the numerical model verifies that isotropic and anisotropic imaging profiles are close to each other under weak anisotropy (<5%); therefore, anisotropy is ignored in this study [41]. In Figure 7, the horizon selected for the P-wave and SH shear wave in the time domain is used to calculate the P-wave and SH shear wave velocity ratio. Figure 8 shows the P-wave and SH shear wave velocity ratio.

**Figure 7.** Pre-stack time migration (PSTM) sections: (**a**) P-wave and (**b**) SH shear wave.

For ease of understanding, we built a flow chart of velocity modeling, as shown in Figure 9. Figure 9 is about the P wave and SH shear wave velocity modeling process in this paper. In this figure, for the convenience of display, the S wave means SH shear wave. We can see that, after the preliminary modeling, we need to go through layer-based tomography and grid-based tomography, and the velocity ratio is obtained simultaneously with the velocity modeling. After the velocity ratio is obtained, it is used to constrain the velocity model of P wave to get the accurate velocity model.

**Figure 8.** VS/VP ratio in the survey.

**Figure 9.** Velocity modeling flowchart.
