Many models in exploration seismology naively presume that the earth is isotropic, that is, seismic velocities do not vary with direction. Yet individual crystals and most common earth materials are observed to be anisotropic with elastic parameters that vary with orientation (Shearer, 1999). Thus, it would be surprising if the earth was entirely isotropic. Further, it is now commonly accepted that most upper crustal rocks are anisotropic to some extent (Crampin, 1981) and more recently it has become apparent that anisotropy is evident in many other parts of the earth (Shearer, 1999). In addition, alternating layering of high and low velocities where the thickness of the layer is less than the wavelength of the seismic signal will also appear anisotropic (Backus, 1991).
Although seismic processors have been aware of anisotropy, it was ignored because its effect was successfully absorbed into the stacking velocity when processing horizontally layered media. However, erroneous assumptions of an isotropic velocity lead to flawed images and thus incorrect interpretations where targets can appear shifted both laterally and vertically (Isaac et al., 2004).
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