A simple homogeneous layer over half-space model has P velocity a1 and S velocity b1 in the top layer and P velocity a2 in the bottom Assuming a

1. A simple homogeneous layer over half-space model has P velocity a1 and S velocity b1 in the top layer and P velocity a2 in the bottom

Assuming a layer thickness of h and a ray parameter of p , derive an equation for the differential time (Tps - Tp ) between P and Ps at the surface.

How do i get the velocity a2 in the equation?