Bracketed Intercept and Gradient Analysis for the Prediction of VTI AVO Anisotropy - Theory and Application

Abstract

The measurement of anisotropic AVO (Amplitude Versus Offset) characteristics with azimuth is a long established method for determining open fracture orientation. This exploits the change in seismic velocity due to vertical or sub-vertical features in the subsurface that result in HTI (Horizontally Transverse Isotropy), the seismic velocity varies with respect to seismic recording azimuth which has an impact therefore on the recorded AVO response. However, in many hydrocarbon exploration provinces fracture porosity is not of primary interest and AVO anisotropy is ignored, although high fidelity seismic migration includes anisotropy terms. With improved imaging at higher reflection angles the effect of VTI (Vertical Transverse Isotropy) due to horizontal layering can be important. This paper demonstrates through a simple model of a sand embedded in shale how the AVO response can be affected and presents a method for discriminating isotropic and non-isotropic AVO character using angle bracketed 3-term intercept and gradient analysis. This method is then applied to field seismic data highlighing areas of non-standard, or non-isotropic, AVO responses. Importantly this approach is applicable to any pre-stack seismic data as only a single azimuth (e.g. inline direction) is required and the approach can be used to de-risk AVO signatures related to prospectivity.