VARIATION OF REFLECTION AND TRANSMISSION COEFFICIENTS WITH CRACK STRIKE AND CRACK DENSITY IN ANISOTROPIC MEDIA

Recent observations show that the differential amplitudes between the faster and slower split shear-waves in reflection surveys contain info rmation about lateral variations of crack density in cracked reservoir s. However, the variation of amplitude with crack geometry when the cr ack strike changes with depth has not been reported previously. In thi s paper, we derive expressions for reflection and transmission coeffic ients of plane split shear-waves at vertical incidence at an interface separating two cracked (anisotropic) media with different crack strik es. We examine the effects on these coefficients as crack strike and c rack density vary. For interfaces with large velocity-contrasts, the r eflection coefficients carry little information about crack geometry, and the effects of crack strike varying with depth are negligible. In such cases, the polarization and time-delay of the split shear-waves a re the only features which reliably diagnose anisotropy and contain in formation about the variation of crack strike and density. However, fo r interfaces with small velocity-contrasts, the effects of any variati on of crack strike with depth cannot be neglected. In such cases, in a ddition to the polarization and time-delays of split shear-waves, both the differential amplitude of faster and slower shear-waves and the a mplitude ratio of the two off-diagonal elements in the reflected data matrix after separation of split shear-waves, contain information abou t the variation of crack strike and crack density. In contrast, effect s of crack strike changing with depth on transmitted waves are more se nsitive regardless of the velocity-contrast and the degree of anisotro py.