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In a seismic exploration program, the problem of converting travel-times to subsurface depths is of fundamental importance. The necessary computational procedures for this conversion are considerably simplified if the assumption of a linear distribution of velocity with depth can be used. Consequently, one of the first problems to be solved in such a program is to determine whether an assumption of this type should be adopted; and, if so, what values of Vo and a are to be used.

Whether or not the hypothesis of a linear variation of velocity with depth may be a good approximation to the actual situation in any region can often be surmised from the lithology. One of the first items on the agenda of a seismic crew should be the “shooting” of a sufficient number of refraction profiles, laid out as ideally as possible on a flat surface and, it is hoped, over a “normal” subsurface.* This latter requirement can be attained by laying out a number of profiles. If the time-distance curves Ior two or more profiles in the area are substantially alike, the conclusion that the subsurface sections are “normal” and “flat” is almost certain to be a valid one.

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