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For transmission into salt, $V_{P3}=4000\mathrm{m}{\mathrm{s}}^{-1}$, soThe transmitted P wave bends away from vertical because it enters a faster medium. The incident angle remains below the $30^{\circ }$ P-wave seismic critical angle at this interface.

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.

For a P-to-S converted seismic head wave, set the transmitted S-wave angle to $90^{\circ }$, giving $\sin i_c=V_{P,\mathrm{incident}}/V_{S,\mathrm{transmitted}}$. At the limestone–sandstone boundary, $2500/1200>1$, so no transmitted S-wave head wave is possible for incidence from limestone.

At the sandstone–salt boundary, using the stated interpretation $V_{S3}=3000\mathrm{m}{\mathrm{s}}^{-1}$,The P-transmission seismic critical angle is lower, $30^{\circ }$, but this does not forbid the converted S branch: at $41.81^{\circ }$ the transmitted P field is evanescent while the S field reaches grazing incidence.