Use overlapping receiver spreads, see
http://rayfract.com/help/overlap.pdf. Shots should be positioned inside combined geophone layout (all overlapping spreads combined), and be distanced from first/last geophone position of whole profile by max. 1 station spacing (Header|Profile).
If shots are positioned 2 station spacings away from first/last profile receiver position then this will work most of the time but not always, depending on topography and subsurface geology.
We recommend using at least 24 receivers per spread and not only 12, for more reliable interpretation.
See also
http://rayfract.com/help/rayfract.pdftopics "Station numbers and spread types" and
"Overlapping receiver spreads".
and sample tutorials
http://rayfract.com/tutorials/riveral8.pdfhttp://rayfract.com/tutorials/EJEMPL3.pdfhttp://rayfract.com/tutorials/tra9002.pdf