First-break data and the refraction wavetrains that follow the first breaks usually are so strong that they have to be excluded from the stack to avoid degrading the quality of shallow reflections. This is done by muting that involves arbitrarily assigning zero values to traces during the period when the first breaks and following wavetrains are arriving. Thus the multiplicity of a stack increases by steps, the shallowest data often being a twofold stack, slightly deeper data being a fourfold stack, and so on until the full multiplicity of the stack is achieved after the muted events have passed beyond the most distant geophones.