In refraction seismic computations, a graph, usually with arrival times of distinctive seismic signals plotted as ordinates and with distances along the surface of the Earth plotted as abscissas. In earthquake studies, the times of arrival of seismic waves at recording stations may be known, but the time of initiation of the waves may be unknown. As data are accumulated from different recording stations, a time-distance graph may be constructed. If it is possible to extrapolate this graph to the origin on the time and distance coordinates, it becomes a travel-time curve.