THE BLIND ZONE PROBLEM IN ENGINEERING GEOPHYSICS

Abstract

General discussions of the so‐called “blind zone” in refraction seismograph studies have been regarded by most practical geophysicists as an academic treatment of a subject having little, if any, field application. The purpose of this paper is to illustrate that the consideration of the blind zone in specific cases definitely improves the correlations between the interpretations of refraction work and the engineering data obtained by core drilling. In this discussion the definition of the blind zone has been narrowed to refer only to that portion of the underground which lies between the surface layer and a high speed marker in a three‐layer problem and is not represented in the graph of the “first breaks.” This volume of material is not suggested by the first breaks because of the relative thicknesses of the layers and the effect of the extent to which the velocity of the third layer exceeds that of the second. This definition excludes any of those anomalous zones which may not be represented by the first breaks because of marked decreases of the seismic velocity. Portrayal of the blind zone on the graph of the first breaks cannot be obtained by merely rearranging recording distances from the shotpoint to the seismometers. The speed of the seismic wave in the blind zone is generally intermediate in value between that of the surface layer and the high speed marker zone. In ordinary practice it is often represented by the zone of the water table above a high speed bedrock surface.