Frequency differences as a function of frequency. The difference between the frequencies of the optimal model including a correction of the surface effects following Ball & Gizon (2014) (BG14 in Table 3) and that of the model without a correction for the surface effects (X₀ = 0.70, Z₀ = 0.017, and α_ov = 0 in Table 3) is shown in dark blue. The difference between the same two models (BG14 and X₀ = 0.70, Z₀ = 0.017, and α_ov = 0) but using the frequencies of the corrected optimal model prior correction is shown in light blue. The light brown markers correspond to the magnitude of the surface correction term.