A plot of arrival time against the source-to-geophone distance. Used in velocity analysis and in interpreting refracted events (head waves). The slopes of segments of the curve give the reciprocals of the apparent velocities for various refractor beds.
See also normal-traveltime curve, reduced traveltime, and Figure T-8. Time-distance curves are sometimes composited from measurements made at shorter offsets (see Figure T-8d).
FIG. T-8. (a)
Refraction time-distance curves RA
=reflections from A,B
=head waves at A,B
=wide-angle reflections from A,B
Wavefront diagram showing first arrivals. (c)
Wavefront diagram showing first arrivals from an idealized salt dome. (d)
Composite time-distance curves
(dashed lines) constructed from arrival curves from sources 1, 2, 3, 4, and 5 (solid lines). Refraction portions from sources 2, 3, and 4 have been displaced upward to show the ‘‘equivalent arrival times’’ as if the profiles from these sources had been longer. Such displaced curves should overlap for confidence and show the same reciprocal time tr
. Actual time-distance curves from these sources might have been different if another refractor had been reached (e.g., dotted lines).
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Time-distance curve (T-X curve)