Left: one example of an f-mode wave packet measured at a receiver located at x_r = 30 Mm for the supergranule SG1. The extent of the window function is denoted by vertical dashed lines. Right: relative error in estimated travel-time shifts using wave packets corresponding to radial orders f and p₁ (indicated by different colors) as a function of different degree of truncation of Eq. (A.2) (indicated by different symbols), for different supergranule models. Truncating Eq. (A.2) to quadratic order results in a linear relation between travel-time shift δτ and wave displacement ξ^iter (plotted with diamonds).