Travel-time measurement geometries. a) Measurement geometry sensitive to the horizontal component of the flow divergence. Travel times are measured between a central point r and the average over a surrounding annulus with radius Δ as introduced by Duvall et al. (1993). b) Proposed new measurement geometry sensitive to the vertical component of flow vorticity. Travel times are measured sequentially along neighboring pairs of points r_i and r_{i + 1} located on a closed contour. In this example, n = 6 points are used, forming a regular hexagon. c) As b), but the hexagon is rotated by an angle β around r.