Carbonate reefs usually appear as homogeneous bodies surrounded by sediment layers in seismic reflection data, but in the figure below the high amplitude reflection from the top of sediment (layer 2 at 0.3-0.34 seconds) is not horizontal yet the geological interface is horizontal.
This phenomenom is called velocity pull-up. The seismic waves that reflected off the top of Layer 2 beneath the carbonate layer have also travelled through the carbonate. Because carbonate (at least in this case) has a faster P-velocity than the surrounding shale, waves that have travelled through the carbonate will return to the surface receivers in a shorter amount of time, relative to waves that only travelled through the shale. Thus the carbonate/Layer 2 interface appears at an earlier two-way-time relative to the Layer 3/Layer 2 shale interface.