Slices of the point-to-point 3D travel-time difference kernel for u_θ (left) and u_ϕ (right) along the plane indicated in Fig. 1. The 3D kernel for u_r is mostly zero within this plane. The kernel is computed with r₁ and r₂ separated by 42°, with mean latitude 40°. The green cross indicates the intersection of the ray path and the image plane. The dashed black line shows the image plane of Fig. 1.