A time–distance graph, used in reflection seismology, in which travel times squared (t²) are plotted as a function of the offset squared (x²) to produce straight-line segments each of which represents the reciprocal of the layer root-mean-squared velocity squared (v_rms²), and the intercept represents the depth (z) to the reflector. (v_rms²t² = 4z² + x².) See also t–d curve; time–distance curve.