Fig. 15

a Schematic map of the geometry of stations. The star symbols show the station locations at \({{\textbf {r}}}_1\) and \({{\textbf {r}}}_2\). The open circle shows the location of a phase velocity anomaly at \({{\textbf {r}}}_3\). b Elliptic curves with constant \(|r_{13}-r_{23}|\), and hyperbolic curves with constant \(|r_{13}-r_{23}|\)