Phase difference (colour) and coherence (greyscale) spectra estimated from the (a) velocities measured using the COG shift of Fe I λ 5434 Å and Fe I λ 5576 Å and (b) velocities measured using the line-minima shift of Fe I λ 5434 Å and Fe I λ 5576 Å. The grey curve marks the dispersion relation of the surface gravity waves. The blue curves mark the propagation boundary of the acoustic-gravity wave corresponding to the approximate formation heights where the COG (dashed) and line-minima (solid) velocities are measured.