Seismic Signatures of Anisotropic Dispersion Anomalies

Observation from VSP data indicates that fractured hydrocarbon reservoirs often exhibit significant anisotropic attenuation and dispersion. It is known that, in the isotropic case, reflections from the interface between an elastic layer and a strongly dispersive are affected by a strongly frequency-dependent reflection coefficient, with the nature of the response being dependent on the AVO class of the interface. In this paper, we extend this analysis to the case in which the dispersion is anisotropic. We show that the frequency-dependence of two parameters, the vertical incidence reflection coefficient and the normal fracture compliance, control the behaviour. Once again, the modeled response depends on the AVO class of the interface. The strongest azimuthal frequency variations are expected for class I interfaces, where we find that the dispersion tends to boost the high frequencies of the reflections perpendicular to the fracture strike. It is hoped that azimuthal frequency analyses could improve our ability to characterize fractured reservoirs.