ABSTRACT

The 3D reflection seismic response is associated with a zone (the Fresnel zone), rather than with a single point used in the idealized 1D convolution model. Unlike a point of incidence, the Fresnel zone is complicated by its textural characters that are defined by the dip and azimuth of microreflectors in the zone. The Fresnel-zone texture makes seismic amplitude interpretation more complicated than previously documented. A conceptual model suggests that seismic amplitude variations with offset (AVO), azimuth (AVAz), and frequency (spectral decomposition) were physically related to textural roughness, textural anisotropy, and textural scale of the Fresnel zone, respectively. Textural roughness is defined by the dip deviation of microreflectors and contributes to the AVO intercept and gradient. Textural anisotropy is defined by the degree of the preferred orientation of the microreflectors and directly affects the AVAz signature. Textural scale is defined by the spacing of the microreflectors and controls the selective frequency tuning in spectral decomposition data. The Fresnel-zone texture gives rise to amplitude variations that can not be accurately modeled by using a 1D reflectivity-wavelet convolution algorithm, and thus poses challenges to the reliability of many previous predictions of rock properties and thickness from amplitude. The AVO, AVAz, and spectral decomposition data should be used to characterize Fresnel-zone texture for predicting depositional facies, deformational fabrics, and hydraulic properties in the subsurface.

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