SCATTERING OF ELASTIC-WAVES IN 2-D COMPOSITE MEDIA .1. THEORY AND TEST

dLocalised regions in the earth's crust exhibiting complex variations of density and seismic wave velocities can be represented by random di stribution of cavities, in a manner described by Matsunami (1983) for two-dimensional (2-D) media. In order to study the multiple scattering of seismic waves propagating in such media, we develop an indirect bo undary integral scheme with discretisation based on wave source distri bution around The cavities. Numerical experiments using seven generic 2-D models and incident P, SV and SH plane waves, as well as el:plosiv e line sources, are carried out. These experiments are intended to bot h assess the accuracy of the method, and to examine the character of a ttenuation of the direct wave, coda waveforms, and travel time feature s that emerge from pure scattering (no intrinsic attenuation), compute d in all cases for wavelengths comparable to the size of the heterogen eities. The wavefield computed for one cavity shows a remarkable diffr acted wave that creeps around it, for all the incident waves, regardle ss of the shape of its cross-section. This wave contributes significan tly to the multiple scattering caused by the direct and all reflected/ converted waves in the presence of many cavities. For complex regions defined by random distribution of cavities, an explosive line source l ocated below the region produces slight amplification of the horizonta l component of the wavefield, apparently due to constructive interfere nce, at observation points above the region, while the vertical compon ent is strongly attenuated. The durations of the seismograms are about the same for observation points located towards both ends of tine reg ion. These results appear to be reversed when the source is above the region. Ln this case, the horizontal component is strongly attenuated, and the duration of the seismograms is significantly larger at observ ation points on the side of the incidence than at the opposite side, s uggesting the dominant effect of backscattering. The amplitudes of the multiple scattered phases, the attenuation of the direct wave and the duration of the seismograms, appear to be larger when the line source is very near or within the heterogeneous region, than when it is outs ide. For the same geometry of the scattering region, the seismograms a ppear to be more complex and amplitudes of multiple scattered phases l arger for a plane SH wave in a half-space. (C) 1997 Elsevier Science B .V.