Acoustic Reflectivity From Variously Oriented Orthorhombic Media: Analogies to Seismic Responses From a Fractured Anisotropic Crust

Abstract

AbstractThe variations in the strength of seismic reflections with both angles of incidence and azimuth are being used to constrain the orientations and degrees of mechanical anisotropy of geological formations. This information is often used in turn to infer the directions of fracturing and stress usually assuming simplified geological structures that in many cases are not realistic. In order to further understand this problem, the reflectivities from a set of variously titled samples cut from a single block of a common anisotropic composite material are measured with both angles of incidence and azimuth in the laboratory. Each sample is analogous to a formation in which the fracture sets increasingly dip. The observed responses differ from plane wave theory due to the finite nature of the experiment. However, the deviations are well accounted for by modeling the acoustic reflectivity using its bounded pulse. As expected, the reflectivities vary with the sample tilt, the angle of incidence, and the azimuth at which the measurements are made. However, some of these responses, such as the precritical reflectivities, do not appear to strongly vary with azimuth for a given sample. Critical angle phenomena, too, must be used cautiously as the observed reflectivity peaks do not lie at the exact critical angle. These results suggest that reflectivities could be an important source of information with regard to the spatial orientation of anisotropic layer.