Left panel: image intensity |𝔼[δI_αβ(x)]| at a single frequency of 3 mHz, as a function of position x along the z-axis. The sound-speed perturbation (see Eq. (54)) is placed at two different positions along the z-axis, z_s = 0.7 R_⊙ (red) and 0.9 R_⊙ (blue). The standard deviation of the noise is given by the black curve. We note that the jagged aspect of the curves is not due to numerical inaccuracies. Right panel: Image intensity and noise level after averaging over 101 frequencies uniformly distributed in the interval from 2.75 to 3.25 mHz. The frequency resolution is 5 μHz, implying an observation duration of T = 55.5 h. A horizontal line segment is plotted at each depth to mark half of the local wavelength.