We model 3D seismic wave propagation in media with vertical fractures using the standard O(2,8) time- space staggered grid Finite Difference technique. This high order FD method has particular utility when modelling small fractures in 3D. Compared with the 2D case, full 3D simulation provides significantly more information with which to characterize fractures or fracture network properties. The known Coates- Schoenberg scheme is used to represent compliant discrete fractures with vanishing thickness in FD grids. We have built a group of 3D models among which all the model parameters remain the same except the fracture spacing. The calculated seismic response in the plane perpendicular to the vertical fracture plane shows characteristic features diagnosing the variation of fracture spacing. P-wave seismic anisotropy increases systematically as fracture spacing decreases. If the ratio of the fracture spacing to applied wavelength is more than 1/4, the reflection curves related to the fracture spacing can be clearly observed, and their properties used to infer fracture spacing. Scattering energy weakens and forms systematically different patterns in the different coordinate planes, as fracture spacing increases.
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