In refraction shooting it is frequently necessary to make calculations for the thickness of velocity intervals, and estimates of the critical distance for shooting to a particular velocity horizon. In the following paper a set of families of curves are built up based on slope form of the total travel time equation T n = (X/V (sub n+1) ) + Sigma n (sub a = 1) (2ha co phi (sub a, n+1) )/V a . The separate times of travel through any velocity interval may be determined from the curves for the sectional velocity and the refracting horizon velocity. These calculations are possible since the velocities encountered in refraction seismic work cover a finite interval from about 5,000 ft/sec to about 20,000 ft/sec. Finally, with the family of curves calculated above, it is possible to determine rapidly the critical distance of shooting for a particular marker horizon if the sectional velocities and their intervals are known approximately. The curves are used to solve the critical distance equation to a particular horizon of the formEquation

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