A graph showing the arrival times of seismic waves as a function of shot–geophone offset distance. In seismic-refraction surveys it is also called a t–d curve or t–x curve. In seismic-reflection surveys, travel times squared (t²) are plotted against the square of the offset distance (x²) to produce a t²–x² graph. In both sets of graphs, the inverse gradient of straight-line segments provides information about the seismic velocity for the appropriate layer.