 Hyperbolic interface collimation of P-waves

A solid-to-fluid interface can turn spherical source P waves into parallel vertical rays if its height $Z(R)$ satisfies the displayed travel-time identity, with $r=\sqrt{R^2+Z^2}$. The positive branch through $Z(0)=z_0$ is a hyperboloid when $\alpha >{\alpha }_f$. Differentiating the travel-time identity enforces Snell law for elastic and acoustic waves. Travel time above the interface is then $z_0/\alpha +(z-z_0)/{\alpha }_f$, independent of cylindrical radius. The leading wavefront is planar although its wave amplitude need not be uniform across that plane.