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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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your comments.