Averaging kernel for the PtQ-2 inversion for the surface longitudinal flow. Top row: surface cuts. Bottom row: vertical cuts at y = 0. Columns: localisations of longitudinal flows, latitudinal flows, vertical flows, and sound-speed perturbations (from left to right). The red and the green full and dotted contours correspond to the half-maximum of the target function and the half-maximum and the minus half-maximum the of averaging kernel, respectively. The full and the dotted blue contours correspond to +5% and −5% of the maximum of the averaging kernel. We note that the cross-talk contribution from the perpendicular horizontal component (second column) is greatly minimised as compared to Fig. B.2.