Travel times δτ^(p) as defined in Eq. (89), computed from the difference between cross-covariances C^(p)(Θ,ω) computed using polynomials of degrees and a reference cross-covariance C_ref(Θ,ω). Computations with and without flow are shown by the dashed and solid lines, respectively. The vertical line labeled “p = 10” indicates the number of degrees of freedom used in all calculations in this paper.