THE LAW OF PARALLELISM IN REFRACTION SHOOTING*

Abstract

AbstractThe half‐intercept‐time for in‐line reversed profiles is highly adaptable to multi‐layer cases, but the deep evidence of the intercept time on the depths and the velocities at the shotpoint and detector position poses a problem. The law of parallelism provides a reliable and simple means to solve this problem. The interpretation procedure implies that from a travel‐time curve the refractor segment is projected back to the shotpoint parallel to a refractor curve from an offset shot. The intercept time thus obtained refers only to the geological conditions in the immediate vicinity of the shotpoint, and varying conditions outside this region are automatically climinated. The assumptions involved in the interpretation technique are confined to the region where, in relation to the shotpoint, the rays leave and strike the refractor surface.The use of the “ABC method” to establish true intercept times at the shotpoint is also discussed briefly as well as the applicability of the method to eliminate or minimize arrival time disturbances in order to improve velocity determinations.