Spatial variation of mid-mantle transition zone structure. We apply the ‘sequencer’ to detect the main trend seen in inverted high-quality reflectivity-profiles of the structure between 465 and 640 km depth (normalized to the 410-discontinuity depth; cf. text for details). (a) Contour plot (shaded green in background) and average (colour-coded) of the sequenced reflectivity profiles. (b) Visualization of the spatial pattern of the sequenced profiles using the same colourmap as in (a).