1. n. [Geophysics]

The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections.

See: base of weathering,  check-shot survey,  static correction,  suppression,  vertical seismic profile