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This paper deals with an approximation modification of F. W. Hales'1 graphical method and its execution by a computer program for relatively small dips.

From available velocity information, “paper plots” of time-distance and raypath-distance are made to determine the optimum shot-to-cable location for a particular refractor. These results are tested by field shooting. After determination of the “window” for shooting, a program of data lines is shot and recorded on magnetic tapes which are playback processed; tilted, filtered, and phase corrected.

Data analysis includes the necessary corrections to datum and the analysis of all reversed time ties and subsequent listing on “shift-plot” sheets. Data, times, and distances from these sheets for forward and reverse shots are input independently into the electronic computer program whose output is a scaled vertical section of both sets of data and their average. The program is versatile in that the refractor may be solved as a two-layer problem or the shallower horizons may be stripped off prior to solving for the particular horizon. Model studies show the results for an asymmetrical anticline, a fault example, and a multilayer problem.

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