AN AREAL PLAN OF MAPPING SUBSURFACE STRUCTURE BY REFRACTION SHOOTING

Abstract

Under suitable geologic conditions, arrival events of refraction wave trajectories following a relatively high speed “marker horizon” may be observed and identified and the associated travel times may be measured on refraction setups having detectors located at suitable distances from shot point locations. On each refraction setup, the sum of the two “delay times” at positions offset from the shot point and detector position may be determined. Depth of the marker horizon and the location of the “offset position” at which this depth applies are dependent on and determinable from “delay time” if the seismic wave speeds in the geologic section overlying the marker horizon are known. By arranging two or more refraction setups angularly disposed so that they have a common location for one offset position of each setup, differential delay times and corresponding differential depths at the free offset positions may be determined. By a geometrical arrangement of three setups in a triangular manner such that each offset position of each line is coincident with one offset position of another line, absolute delay times and depths at the common offset positions may be determined. By coordinating these two types of geometrical arrangements in shooting over an area, the depths and undulations of the marker horizon may be determined.