One of the fundamental assumptions which has been used to convert the results of a reflexion seismograph survey into a subsurface structural picture has been that the vertical distribution of the velocity remained constant over the area of the survey, and that any differences in reflexion times to a given layer could be interpreted as indicating differences in depth to that layer. Fortunately, this assumption has been quite satisfactory in many cases, especially when the object of a survey has been to locate fairly pronounced structures. The actual velocity law used was sometimes just an asusmption, or may have been based on results of refraction shooting, or computed from the Δt's of the reflexion survey itself. It was early recognized that such velocity information was not entirely satisfactory, and well geophones were designed which made it possible to carry out velocity surveys in wells. As it becomes increasingly important to map structures with small closure, it becomes increasingly necessary to know the actual velocities which should be used in seismic interpretations.

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