tokens: good for travel distances of about 8°–20° with measurement errors increasing with increasing travel distance. At very low travel distances (about Δ 6°), it can be seen that the forward-modeled travel times do not fit the measured ones. The reason for this effect lies in the fact that the simulated data only incorporates modes with harmonic degree l ≤ 170. This cut-off acts as an additional filter at a harmonic degree at which the filter with the lowest phase speed (L170) is centered. The modes selected by the filter thus do not form a proper wave packet, which may lead to artifacts in the cross-correlations (see, e.g., Zharkov et al. 2006 ). Additionally, it is noteworthy that the quality of the match of the power spectrum (compare matched versus unmatched) does not have a large effect on the travel times at higher travel distances (Δ > 8°). At smaller travel distances (Δ 8°), however, one can observe a substantial effect of the quality of the match of the power spectrum on the travel times. We note here that this conclusion may be different for other flow models. We also note that the absence of a proper averaging of multiple kernels in distance and latitude has a considerable effect for smaller travel distances (Δ 10°) but seems to vanish for the largest distances in the case of the flow model considered here. For comparison, forward travel times using ray kernels are also shown in Figures 5 and 6 . It can be seen that the magnitude of the ray kernel travel times is, in general, very similar to those obtained using Born kernels. However, Born kernels seem to better reproduce some features in the measured travel times, e.g., jumps in the magnitude of the travel times from one filter to another such as from Δ = 221 to Δ = 241 (see the middle panel in Figure 6 ). Because the given flow model varies on rather large length scales, a relatively good performance of the ray kernels is expected. This may be different for meridional flow models, which vary on smaller length scales, see Section 1 . We also note here that the measured travel times obtained using the filter with the highest phase speed (L035) encounter a spurious constant offset, see Figure 7 . This offset was corrected for by substracting the mean travel time at each distance. Such an offset is not present in real solar data (Kholikov et al. 2014 ; Jackiewicz et al. 2015 ). In Jackiewicz et al. ( 2015 ), this offset was not noticed in the simulated data, as a few distances had been dropped from the analysis, which resulted in an equivalent correction. The origin of this offset is not completely understood but may be connected to the relatively coarse spatial resolution of the simulated data (256 pixels on 180°). Figure 7. Measured travel times, without offset correction. Left and middle panels: same travel times as displayed in Figure 5 (top left and top middle panels). Right panel: measured GB04 and forward-modeled travel times from the Born kernels for all travel distances, at
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