Since the amplitude of converted S-S waves ( S1-S2 or S2-S1 ) is usually low we can use total energy in the X'-y' and Y'-x' sections (the horizontal components on the rotated coordinate frame) as a measure of the separation. This is the method that Alford(1986) introduced; the reference frame of the source and receiver components is rotated until the energy in the converted S-S wave sections is minimized. The estimation can be performed on shot gathers or on CMP-stacked data. If shot gathers are used the estimation can be contaminated by noise and unwanted wavetypes (e.g. S-P waves) in the horizontal component shot gathers. By stacking with a suitable S-wave velocity function the S-wave signals are chosen in preference to other arrivals. After suitable rotation angles have been estimated using the stacked data, the rotation can be applied to the shot gathers in preparation for the next stage of the separation algorithm, the P-S wave separation.