Reflection Response of a Fractured Medium: Physical Modelling Versus Numerical Simulations

Azimuthal variation of the AVO response (AVOaz response) of fractured reservoirs is usually modeled using equations for reflection coefficients obtained for plane waves. However, plane wave approximation can break down at long offsets where incidence angle approaches critical angle. Since AVOaz response is often more noticeable at large offsets, spherical wave effects must be carefully analysed and taken into account. In order to analyse these effects quantitatively we performed an AVOaz laboratory experiment and then numerically simulated this experiment. The AVOaz response of a physical model is studied in laboratory with finely layered Plexiglas simulating vertical fractures. Transmission measurements are performed to construct the elasticity tensor for the HTI model. This elasticity tensor is used as an input into numerical simulations which are performed using an anisotropic full-wave reflectivity algorithm. The comparison of the experimental data with simulations shows a very good match. The agreement is especially good for critical angles extracted by picking inflection points on AVO curves for each azimuth. This shows that (1) reflection measurements are consistent with the transmission measurements; (2) the methodology of picking critical angles on seismograms using the inflection point is robust, even in the presence of random and/or systematic noise.