P-TO-S CONVERSION FOR A THIN ANISOTROPIC ZONE PRODUCED BY VERTICAL FRACTURING

Point-source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out fo r a compressional source and multicomponent surface receivers. The ani sotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil- and gas-filled, aligned vertical fractures f or a broad range of fracture densities and aspect ratios. P-to-S refle cted conversions recorded on the horizontal geophones show both kinema tic and dynamic anomalies that increase with increasing fracture densi ty and are only weakly dependent on aspect ratio. In contrast, the ver tical 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 si mplicity as those for transverse isotropy. New linearized expressions for mode-converted amplitudes are developed for small angles of incide nce and are used to interpret the synthetic response.