datum
1. n. [Geophysics]
An agreed and known value, such as the elevation of a benchmark or sea level, to which other measurements are corrected. In seismic data, the term refers to an arbitrary planar surface to which corrections are made and on which sources and receivers are assumed to lie to minimize the effects of topography and near-surface zones of low velocity.
See: benchmark, datum correction, elevation correction, receiver, source
Diagram of a datum and hydrophone array.
2. n. [Well Completions]
A depth reference point, typically established at the time the well is completed, against which subsequent depth measurements should be corrected or correlated.