ENHANCED BACKSCATTERING OF SEISMIC-WAVES FROM A HIGHLY IRREGULAR, RANDOM INTERFACE - SH CASE

In this paper we study the scattering of SH waves from a highly irregu lar random interface between two elastic half-spaces to determine whet her the 'enhanced backscattering' observed in optics can also occur fo r seismic waves. We approach this problem by combining the elastic rep resentation theorem with the extinction theorem to develop a surface i ntegral representation of the total scattered displacement. The scatte red displacement is then expressed quantitatively in terms of a differ ential reflection coefficient (DRC). Statistically, the incoherent and coherent contributions to a mean DRC are calculated by averaging over many realizations of randomly irregular surfaces. We find that the in coherent field forms the major contribution to the total mean DRC, cle arly showing an 'enhanced backscattered' or 'retro-reflective' peak wh ich results directly from multiple scattering. The character of the pe ak varies strongly as a function of impedance contrast, incident angle , and rms slope of the interface. In each case, the peak height decrea ses dramatically with the decrease in impedance contrast and with the increase in incident angle. In addition, the longer the wavelength of the incident wave relative to the correlation length of the interface, the wider the retro-reflective peak. Overall, this study provides a m ore intuitive yet quantitative understanding of multiple scattering bo th as a function of frequency and interface properties.