Time-distance diagrams obtained from b(k) (top panels) and δb₁(k) (bottom panels) along a selected path, labelled as α, in region A. The left and middle panels display a sample image from the series (I is shown in the left-hand panels). In both panels, the strip around the paths utilised to compute the time-distance diagrams is marked as a rectangle. The filled portion of the rectangle corresponds to the spatial extent of the feature marked in the time-distance diagram of the right panels. Right panel: Time-distance diagram along the selected path. The arrow shows the correspondence between the origin of the time-distance diagram and the first point of the path. A solid line, labelled with the corresponding apparent speed, is drawn over one of the periodic ridges visible in the time-distance diagrams.