In general the operator is a matrix operator but if the wavefield, P is expressed as a vector of amplitudes of the different wavetypes the operator A is diagonal. The different wavetypes are the eigensolutions of the Christoffel equation (see appendix) so the operator is diagonalized. For each of the six wavetypes the extrapolation in depth can be expressed as,
Thus, once I have calculated the wave amplitudes at the top of any layer I can calculated the amplitudes at the bottom of the layer by phase shifting the data. The value of pz is the same for all frequencies but the phase shift through the layer for each frequency is given by.
This calculation is different for each frequency but on a vector computer it is fully vectorized over frequency.
If I am ``turning off'' propagation of a particular wavetype it is at this stage that I do it. I can simply zero the amplitude of the wave instead of applying the shifting operator.