“…In the following, we assume that the geometry of the elastic wave-motion problem as well as the eventual boundary conditions are invariant along the z-direction, thus, without loss of generality, when a source depends on an e−jαo z factor, also the total field depends on the same factor, i.e. bold-italicψt=bold-italicψtfalse(x,y,zfalse)=ffalse(x,yfalse) e−jαo z, see for instance [17] before (2.8). Of course, the same behaviour can be obtained by applying a Fourier transform also along the z-direction and assuming an incident plane wave with a particular skew direction that yields e−jαo z.…”