Example realizations of the noise ϵ(x) from our noise model on the kernel (left) and MURaM grids (middle), and synthetic noisy travel times τ(x) (right). The noise has a standard deviation of 10.3 s and the travel-time signal in the right panel of 1.9 s, which compares well with the data (Hanasoge et al. 2012). Left panel: can be compared to Fig. 5 in the supplementary material of Hanasoge et al. (2012). Figure 2 shows the contribution of the travel-time signal to the right panel in more detail.