When dealing with converted waves, the reflection point is known as the conversion point, since is where the P energy is converted into shear energy.
This reflection point is no longer at the midpoint between the source and receiver (x/2). The true reflection point is depth dependent. This means that traditional procesing techniques must be modified for this new geometry.
In converted wave acquisition, the source is usually a conventional seismic energy source, either explosive or vibroseis.
4.5 Hz vertical component shot record
 
4.5 Hz radial component shot record

From Margrave, G.F, Estimates of the signal band of the Blackfoot broadband data, CREWES Research Report Vol. 7, 1995, ch39
Above are whoen PP and PS foldmaps from the Blackfoot 3D3C survey, shot in the summer of 1996.
The Foldmaps show the number of times that a given area of the subsurface will be the reflection in a seismic survey. High fold will minimize the noise present in the data, and an even fold distribution minimizes the tracetotrace variation and the normalization required to correct amplitude variations when the data is stacked.
Since the PS conversion point varies with depth, the PS foldmap also varies with depth.
The subsurface fold is determined by the geometry of the shots and receivers on the surface.
From Lawton, et. al., Design review of Blackfoot 3C3D seismic program, CREWES Research Report Vol. 8, 1996