Reference cross-covariance functions for exemplary distances. We show disk-averaged mean cross-covariance functions from the simulation (blue) and the zero-order cross-covariance (red dashed: matched; green dotted: unmatched), which is used as reference cross-covariance function in the model. The perturbation to this reference in the presence of flows is modeled by the Born approximation. The vertical gray lines indicate the windows used for the GB04 travel-time fit. The whole displayed range was used for the Gabor fit.