time–distance curve

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time–distance curve 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.