1. n. [Geophysics]

A map that represents the change from one map to another, such as a reservoir map of an area made from two different seismic surveys separated in production history (one possible product of 4D seismic data), or an isochron map that displays the variation in time between two seismic events or reflections.

See: four-dimensional seismic data,  isochron map,  seismic survey