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.
If a thin layer produces first arrivals which cannot easily be identified on a seismogram, the layer may never be identified. Thus, another layer may be misinterpreted as incorporating the hidden layer. As a result, layer thicknesses may increase. References Cited These sources offer excellent discussion of theory, derivation of formulas, and practical examples of refraction seismology. Burger, H.R. 1996.