Meridional slice through the sound-speed kernel (x, r_​s) computed in Model S at a single frequency of 3 mHz, in units of 10⁻³⁴ kg m⁻³ s⁻³. Both the real (left panel) and imaginary (right panel) parts of the kernel are shown. The scatterer at z_s = 0.7 R_⊙ is indicated by the crosses. The observation pupil P is a polar cap of full angular size 120°. We notice the “ghost values” at the antipode.