invert the data from many different tiles to obtain 3-D maps of subsurface flows directly [@2011JPhCS.271a2002F].

In time-distance helioseismology, travel times of waves that propagate beneath the surface between pairs of surface points are estimated by cross-correlating the observed oscillations between surface points. The cross-correlation function derived by cross-correlating the oscillations at two surface points A and B, say, exhibits a number of “wave packets”. In a ray-theoretic interpretation, these packets correspond to waves that travel between A and B along different ray paths, either traveling directly without intermediate surface bounces, or bouncing at the surface one or more times before arriving at the point of observation. In practice, it can be necessary to average the cross-correlations between many similarly separated pairs of points before the wave packets are clearly visible . Partly for this reason, the observations are typically filtered in some way before the cross-correlations are made. The most common filters are a phase-speed filter or a filter that aims to isolate oscillations corresponding to a particular radial order $n$. Also, usually it is only the first-arrival wave packet – corresponding to propagation between A and B with no intermediate surface bounces – that is used in the 