Ultrasonic Model Investigation of the Effects of Lateral Heterogeneties on Seismic Surface Wave Propagation and Scattering

The effects of topographic features on Rayleigh wave propagation and scattering are investigated using two and three-dimensionald ultrasonic models. Starting from simple steps, different topographic features are modeled. The effects of these features on Rayleigh wave transmission and scattering are examined as a function of wavelength. In general, backscattered or reflected of the Rayleigh wave energy is scattered into body waves. To complement the laboratory studies, two-dimensional finite difference calculations are carried out. These show the details of the scattering of incident Rayleigh waves at a step discontinuity intoP, S and Rayleigh waves. More energy scatters into S waves than into P waves. Using three-dimensional laboratory models of steps, the dependence of wave scattering on angle of incidence is studied. Transmission and reflection coefficients (transmitted or reflected energy/incident energy) computed from spectral ratios vary strongly with incident energy) computed from spectral ratios vary strongly with incidence angle. At wavelengths equal to twice the step height, the fraction of incident energy scattered into body waves ranges from more than 90% at normal incidence to about zero at near-grazing incidence. At each angle, transmission coefficients vary strongly with frequency. Because of frequency-dependent phase shifts, the transmitted and reflected waves are distorted. Original contains color plates: All DTIC and NTIS reproductions will be in black and white.