Sensitivity of the mean travel time, [τ(r₁,r₂) + τ(r₂,r₁)] /2, to relative sound-speed perturbations δc/c in the interior, with observation point r₁ located on the polar axis and observation point r₂ at latitude of 45°. Panel a): cut at r = 0.95 R_⊙ through the sound-speed kernel. Panel b): slice through a plane containing the two observation points and the center of the Sun. The green line indicates the position of the ray path. Panel c): slice in a plane perpendicular to the ray path connecting the two observation points (green square) at equal distance from the two observation points. In both slices b) and c), the black arc of a circle locates radius r = 0.95 R_⊙ which corresponds to panel a).