Why is a seismic reflection a stratigraphic time line?
Most seismic reflections occur at a more-or-less flat interface between two stratigraphic units with differing acoustic impedance (velocity times density). The “more-or-less-flat” (but not necessarily level) limitation is because complicated surfaces are very difficult to image. Gently curving surfaces produce reflections; rough surfaces usually do not, because energy is scattered instead of being reflected. The most common geological feature which is a smooth, flat (or gently curved) surface marking a change between two materials with differing acoustic impedance is the boundary between two beds of different sedimentary lithologies: between a shale and a sandstone, between shale and limestone, between coal and shale, etc.
