(a) Enlarged view of the configuration of mirror and ghost points within the model depicted in Fig. 4. The coloured surface represents the free surface. Unfilled circles of various colours denote the four layers of ghost points, while filled circles represent their corresponding mirror points. Lines perpendicular to the free surface connect each ghost point to its mirror point. (b) A zoomed-in section of (a) with a spatial discrete grid overlay. Ghost points are located at the integer grid endpoints while mirror points are on the fractional grid within the meshes.