 Solution

A transmitted P-wave seismic head wave occurs when its refraction angle reaches $90^{\circ }$, requiring $\sin i_c=V_{P,\mathrm{incident}}/V_{P,\mathrm{transmitted}}\le 1$. At the limestone–sandstone boundary the ratio is $2500/2000>1$: there is no P-wave seismic critical angle for incidence from limestone. A slower lower layer cannot carry that transmitted head wave.

At the sandstone–salt boundary,The head wave travels along the faster salt boundary and radiates energy back into the slower overlying medium. Reversing the direction of incidence would change which interfaces admit such a head wave.