Seismic anisotropy in reservoir rocks can result from the alignment of fractures of any size from grain scale to lengths of meters or tens of meters, provided the seismic wavelength is long relative to the size and spatial distribution of the fractures. It can also be caused by correlated pore space, alignment of minerals or grains, or any combination of the above. It is of huge practical importance to determine the primary cause of observed seismic anisotropy since the presence or otherwise of a limited number of open macrofractures is likely to greatly influence the calculations of fluid flow within a reservoir. For the purposes of this article, macrofractures are defined as having lengths of approximately a meter to tens of meters but to be of subseismic scale, meaning that they are not individually imaged within a seismic reflection section.

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