Traditionally, input acquired in the field consisted of the original paper records; output submitted to the client consisted of structural sections and depth-contour maps of selected interfaces. Before the introduction of magnetic recording, it was common practice to do the conversion in the field office. Tools for this conversion ranged from slide rules and desk calculators to wavefront charts. These tools were based on the geometry of rays in media where velocity is a function of depth only. The detailed algorithms underlying the conversion were often developed in the exploration companies and — originally — were carefully guarded. But at least the underlying principles were exchanged throughout the industry through books, journal articles, and presentations at meetings, such as noted in nearly 300 references in C. H. Dix's Seismic Prospecting for Oil (1952) . The techniques of data acquisition and data interpretation have changed considerably, but the underlying principles of ray geometry are the same. Therefore, many new methods are based on ideas formulated in the early times of the industry.