Conformable sequences of sedimentary rock may form planar boundaries. However, erosion and uplift easily produce irregular boundary contacts. More sophisticated algorithms can process refraction surveys where irregular interfaces might be expected. Profile length and source energy limit the depth penetration of the refraction method. Typically, a profile can only detect features at a depth of one-fifth survey length.
Refraction depends on layers to increase in velocity with depth.
In the hidden slow layer senario, a buried layer is overlain by a faster layer. No critical refraction will occur along the boundary interface. Thus, refraction will not detect the slow layer. All is not lost since reflection seismology could detect the slower layer. Seismograms require careful analysis to pick first arrival times for layers.