*Appendix A.3. Image Processing and Stacking*

The result obtained by the beamformer is the imaging result in the time domain. We estimated the depth of the reflection interface based on the background velocity and the wave travel time, which is a conversion from time domain to depth domain. For each shot, an image under a particular illumination angle is obtained. The final image is obtained by stacking the amplitudes of each shot, where *N* is the total number of shots, *Id* is the final image with all shots stacked together, and *c* is the estimated background velocity:

$$I\_d = c \sum\_{N=1}^{N\_{\text{max}}} I\_t(N) \tag{A17}$$
