Anis. Page2

4. Data processing and interpretation.

Now we apply several operations of data processing and interpretation in AnisPack software. First operation is spectra calculation. The spectral approach helps to fulfill deep and full ARS data analysis. The ARS diagram is a periodic function, and has a discrete spectrum. Fig.1 shows the results of spectra calculations for theoretical ARS diagrams of pole - dipole (1) and dipole equatorial (2) arrays above anisotropic half-space with longitudinal and transversal resistivities equal to 1 and 3. Lines show bending around discrete spectra of

only even harmonics (odd ones are equal to zero). The spectrum for AMN array (1) has maximum at zero harmonic (which reflect mean resistivity of the medium), while the second and the next even harmonics are much smaller. The spectrum for "D" array (2) has maximum at the second harmonic, which is higher then zero one. The second harmonic is the most important for sensitivity to anisotropy. That is why D array is much more sensitive to anisotropy in comparison with any linear array. All linear arrays measure only radial component of electric field intensity (except pole-pole one, which measures potential), while non-linear arrays measure both radial and azimuthal components. Azimuthal component is much more sensitive to anisotropy (fig.2) and give this high sensitivity to non-linear arrays.
Spectral approach give us possibility to calculate anisotropy coefficient with more accuracy then axis ratio and estimate asymmetry coefficient, O/E ratio (sum of odd divided to sum ov even spectral components, which haracterizes relative influence of inhomogeneity in comparison with that of anisotropy), etc. Spectral approach has allowed to simplify the process of interpretation. The iterative process is not required, the parameters can be determined at one step. Together with the azimuth of an anisotropic medium's strike we estimate its error. Instead of interpretation of longitudinal and transversal VES curves, what gives distorted value of the upper layer thickness above the anisotropic basement, we use now dependence of zero harmonic from spacing, allowing to estimate this thickness precisely. Spectra allow to transform sounding curves for non-linear array to linear one, and estimate for it the "true" values of longitudinal and transversal resistivity. In comparison with direct measurements with linear array, that way gives much greater accuracy (Bolshakov et al., 1997).

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

V.A. Shevnin, I.N. Modin
Tel. & fax: (7-095) 939 49 63
E-mail: sh@geophys.geol.msu.ru

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