A computational system is described that detects seismic reflections recorded digitally on magnetic tape, performs migration if desired, and plots the information in cross-section form. The detection or “picking” philosophy is based on the selection of the time positions of either all peaks or all troughs in the set of traces which constitutes the seismic section of interest. A lateral continuity search “correlates” these peaks or troughs across the traces in order to define reflection segments. All reflection segments which span at least three consecutive traces are stored in digital format on magnetic tape. They are given a grade which is the product of their spatial span and mean amplitude. A time-varying grading threshold is employed to limit the number of reflection segments which are output to an automatic plotter or carried for subsequent processing such as computer migration. Several examples of these automatically derived cross sections are shown and compared with the original variable-area record sections. This system is readily extended to multiline or grid data. Thus it is feasible to generate and display automatically three-dimensional interpretive aids such as isochrons and isopachs.