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 6. Anisotropic and pseudo-anisotropic models

AnisPack software, developed in our laboratory, includes several programs, which allow us to fulfill modeling next anisotropy situations, shown on fig.1: 1. Halfspace; 2. Vertical contact; 3. Anisotropic halfspace covered with isotropic overburden; 4. Two-layered model with upper anisotropic layer; 5. Two-layered model with arbitrary anisotropy in each layer. The sixth multi-layered model is not realized yet in software. According to K.Watson and R.Barker, 1998 the most cases in azimuthal resistivity survey (ARS) are not really anisotropic, but give us examples of pseudo-anisotropy. The important problem of ARS consists in distinction of these cases: 1.- a homogeneous isotropic earth; 2.- a homogeneous anisotropic earth; 3.- a dipping interface separating two homogeneous isotropic layers; 4.- a laterally changing resistivity model; 5.- that with isotropic overburden; 6.- two-layered model with a laterally changing resistivity in upper layer; 7.- a dipping interface with anisotropy in lower layer with its orientation in dip direction; 8.- that with anisotropy orientation in strike direction. We classify all cases as anisotropy, inhomogeneity and inhomogeneity with anisotropy. Anisotropy gives rather weak influence on resistivity data in comparison with inhomogeneity. That is why we recommend to apply more sensitive to anisotropy non-linear arrays instead of collinear arrays.

119899, Russia, Moscow, Moscow State University, Geological Faculty, Department of Geophysics

V.A. Shevnin, I.N. Modin
Tel. & fax: (7-095) 939 49 63

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