Generally seismic amplitude varies with azimuth in a horizontal transverse isotropy medium (HTI). Uniform direction vertically fractured rock composes a HTI medium. The magnitude of seismic amplitude variation with azimuth (AVAZ) is determined by anisotropy parameters and mechanical property of the host rock. However the distribution of AVAZ result is usually interpreted as the representation of anisotropy variation caused by vertical fracturing change in the host rock. The change of mechanical property, such as Poisson ratio, is ignored. In this paper we demonstrate that host rock mechanical property change can complicate the correlation between fracture density and AVAZ amplitude. From a shally sand model we reveal that the AVAZ amplitude is not uniquely correlated to the fracture density change when the fracture density is small. Our results also indicate that host rock Poisson ratio change can manifest seismic data amplitude variation along azimuth direction. The complexity of AVAZ in response to the fracture density is illustrated from a fractured sand-shale mixing model. From the model study we find that anisotropy AVO intercept and gradient cross-plot technique is a more robust method for identification of fracture density change.


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