Rayleigh waves from small explosions

Abstract

A s part of a program to study ground disturbances that interfere with seismic prospecting, an experimental seismic crew of the Magnolia Petroleum Co. has been investigating Rayleigh waves from shot‐hole explosions at distances up to about 3000 ft. Waves have been recorded by vertical and horizontal geophones through a system giving flat response between 5 and 200 cycles/sec. The geophones have been disposed along surface profiles with separations that are short compared to Rayleigh wavelengths and also at various depths up to 100 ft in boreholes. Rayleigh waves from air explosions have also been recorded. Sample arrays of records are presented on which individual waves from hole and air shots can be followed for horizontal distances up to about ten wavelengths. Dispersion characteristics observed on the records are plotted in a form permitting comparison with theoretical dispersion curves for various kinds of surface layering. Effects of varying the depth of the explosion are observed and compared with theoretical predictions. Particle‐motion trajectories are plotted both at the surface and at various depths. Results all show as good agreement with classical theory as can be expected in view of the simplifying assumptions which must be made in deriving this theory. Constant‐frequency wave trains were observed immediately after the air‐wave arrival on the records made from single air explosions eight feet above the ground. These are shown to be waves of the type predicted by the Press‐Ewing theory of surface‐wave coupling to the atmosphere.