approximation, shales are assumed to be made up of horizontal layering where the rock matrix is stiffer in the direction parallel to the layering. Te horizontal velocity can be up to some 10% faster than that in the vertical direction. Where the layering is flat, the anisotropy is constant with azimuth; hence the terms transverse isotropy with vertical symmetry axis (VTI) and polar anisotropy. Where the layering dips, or is fractured, the situation becomes more complicated. Many deep hydrocarbon reservoirs may have undergone tremendous overburden forces as well as local stresses. Such
forces exceed the strength of the rocks and open up fractures in them, resulting in stress-induced anisotropy. Depending on the number of fracture sets and fracture orientation, anisotropy varies from HTI (transverse isotropy with horizontal symmetry axis), orthorhombic, to the even more complicated monoclinic or triclinic type. From the rock physics point of view, seismic anisotropy
can result from preferred alignment of mineral grains and pore spaces, from preferred direction of fluid flow among pore spaces, and from intrinsic seismic anisotropy of minerals.
