P‐to‐S conversion for a thin anisotropic zone produced by vertical fracturing

Abstract

Point‐source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out for a compressional source and multicomponent surface receivers. The anisotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil‐ and gas‐filled, aligned vertical fractures for a broad range of fracture densities and aspect ratios. P‐to‐S reflected conversions recorded on the horizontal geophones show both kinematic and dynamic anomalies that increase with increasing fracture density and are only weakly dependent on aspect ratio. In contrast, the vertical component P‐wave reflections provide a much poorer diagnostic of fracturing. Analytic expressions for the eigenvalues and eigenvectors of a vertically fractured system are presented, that have the same simplicity as those for transverse isotropy. New linearized expressions for mode‐converted amplitudes are developed for small angles of incidence and are used to interpret the synthetic response.