Reflection moveout in azimuthally anisotropic media

Abstract

Natural fractures in reservoirs, and in the caprock overlying the reservoir, play an important role in determining fluid flow during production, and hence the density and orientation of sets of fractures is of great interest. Chang and Gardner have suggested that the fracture orientation of a subsurface fracture zone may be determined using P-wave interval velocities. In the general case of a rock possessing both an anisotropic fabric and a preferred orientation of fractures, the rock will display both polar and azimuthal anisotropy. A non-hyperbolic moveout equation, based on an expansion of the inverse-squared group velocity in spherical harmonics, is presented which can be used in the presence of azimuthally anisotropic layered media. The theory is applied to the case of horizontal layers having monoclinic symmetry with symmetry plane parallel to the layers. A sedimentary rock containing several sets of fractures with normals lying in the bedding plane is an example of such a medium if, in the absence of fractures, the rock is transversely isotropic with symmetry axis perpendicular to the bedding plane.