Abstract

Propagation of direct waves and dispersive long-period waves along a layered system was investigated experimentally by means of two-dimensional ultrasonic models. Velocities of direct and head waves were measured within layers or in a medium adjacent to layers as functions of layer thickness to wave length or source-from-interface distance to wave length. Amplitudes of direct longitudinal, direct shear, and long-period waves were measured on three profiles, each perpendicular to the layers. Three models were used: the first consisted of a low-velocity layer between two thick sheets; the second of a high-velocity layer between two sheets; the third of six alternating high- and low-velocity layers between two sheets. The source was a wave train, simulating a wave from a seismic explosion. The frequency was varied so as to obtain different ratios of layer thickness to wave length. In the single low-velocity layer model the direct longitudinal wave contained a larger amplitude than the dispersive long-period wave in the layer at offset distance of 6 to 10 times the layer thickness. In the single high-velocity layer model the direct longitudinal wave was attenuated rapidly and the amplitudes of the long-period waves were negligigble. In the multilayered model, direct waves had negligible amplitudes at the corresponding distances; nearly all of the energy was in the dispersive long-period waves. In this model the low-velocity layer carried 1 1/2 to 3 times the amplitude observed in the high-velocity layers, whether the source was located in the high- or low-velocity layers. Dispersion of the long-period waves in the multilayered model was pronounced within the low-velocity layers and weak in the high-velocity layers, when the source was either in a high- or low-velocity layer. Dispersion was anomalous when the source was in a low-velocity layer and normal when in a high-velocity layer.

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