a) Example HMI τ^oi travel-time map for one f-mode-filtered 8 h dataset centered at the solar equator and annulus radius Δ = 10 Mm. The coordinate x is in the west direction and y in the north direction. The travel times have been measured using the linearized definition in Gizon & Birch (2004). The color scale has been truncated to increase the contrast. The minimum and maximum values are −152.4 s and 117.3 s. b) Example τ^ac travel-time map for the same dataset as in a) with Δ = 10 Mm and n = 6, averaged over the four angles β. The minimum and maximum values are − 149.7 s and 144.8 s. c) Power spectrum of τ^oi for both HMI data and noise model (Gizon & Birch 2004), averaged over azimuth and 336 × 8 h datasets (size about 180 × 180 Mm²) centered at the solar equator, plotted versus the product of horizontal wavenumber k and solar radius R_⊙. The thickness of the lines denotes the 1σ error. d) Power spectrum of τ^ac for both HMI data and noise model, averaged over azimuth and 336 × 8 h datasets. The τ^ac maps were averaged over four angles β before computing the power.