RADIATION OF BODY WAVES FROM NEAR‐SURFACE EXPLOSIVE SOURCES

Amplitude distributions obtained from field observations of the azimuthal distribution of motion from cratering shots near a vertical face in a limestone section yielded data on radiation into a half‐space. These effects have been approximately reproduced in the laboratory by means of two‐dimensional seismic models. Small chemical explosions were fired on or near the edge of a large plexiglas sheet and the radiation of both P and S waves observed. Shots on the edge of the model sheet produce P and S radiation patterns expected from a normal downward impulse on the free surface. The radiation patterns from cratering shots may be qualitatively explained by the combined action on the free surface of a normal downward stress and a pair of horizontal stresses (dipole without moment) at the source point. The observed data are not sufficient for verifying theoretical S wave distributions. Observations of SV amplitudes from nuclear explosions could yield useful information concerning the relation between the angle at which the waves leave the source and the distance at which the wave emerges.

ATTENUATION AND DISPERSION OF SHEAR WAVES IN PLEXIGLAS

Wuenschel (1965) measured the attenuation of plate waves in a Plexiglas sheet, finding a value of [Formula: see text] for the attenuation coefficient at frequency f(cps). Using only the first term in the attenuation, he then employed the results of Futterman (1962) to predict the intrinsic dispersion which should accompany this attenuation. Good agreement was obtained between the predicted and observed dispersion. Changes in pulse shape with distance traveled proved to be a sensitive indicator of the presence and nature of the dispersion; when pulses at a location remote from the source were predicted on the basis of a pulse observed near the source plus the transmission characteristics of the material, inclusion of the Futterman dispersion substantially improved the match between predicted and observed waveforms.