Common midpoint stacking

Summary

Common midpoint stacking is the summation of reflection traces that share the same midpoint between source and receiver, after normal moveout correction. For horizontal layers, these traces sample the same reflection point at different offsets, so their reflections line up after the correction and add up. Random noise does not line up, so stacking $N$ traces improves the signal-to-noise ratio by about a factor $\sqrt{N}$. The velocity that makes the stack strongest also gives an estimate of the seismic velocity, so stacking is a core step of reflection seismic processing.