The free surface condition is an approximation for the surface boundary condition. (A more sophisticated boundary condition is obtained by using a homogeneous acoustic half-space to model the atmosphere above the ocean. The impedance contrast is, however, so large that there is no practical need for modeling the atmosphere.) For an acoustic medium this yields,

while for an elastic medium both stresses must vanish:

Thus, two linearly independent solutions can be obtained using,

From the definitions of ** y**
in Eq. () we obtain the
following boundary conditions,

Tue Oct 28 13:27:38 PST 1997