Seismogram & free surface

Fire a source and watch the wavefield (top) while a shot gather builds up at the surface receivers (bottom). The gather shows the direct wave, the reflection hyperbola from the interface, and — with the free surface on — the surface multiple.

Toggle Free surface and re-fire: the extra multiple event appears or vanishes. This is why marine field data must be modeled with a free surface — the recorded data already contains ghosts and surface multiples.

The free surface is a pressure-release condition (p = 0) at the top boundary; the other three sides are absorbing.