Synthetic S-wave checkerboard model. The input synthetic model is shown for three depth slices (a)–(c) and three vertical sections (g)–(i). The locations of the three vertical sections are shown in Fig. 8(c). The corresponding images from the inversion of the synthetic data are shown below the respective input model images. The normalized correlation between input and recovered model for the depth slices are 0.75, 0.89, 0.84, 0.77 for 100 km, 300 km, 500 km and 700 km (not shown), respectively; for the vertical sections the correlation is 0.83 at 18°E, 0.77 at 22°E and 0.77 at 48°N. Note that the colour scales used for input and output models differ as the inversion method recovers only about half of the original anomaly amplitude.