The field data processing to reduce NSI influence we fulfill with the IPI-2D
package. Two algorithms: the principal component's method (MPC) and
median polishing (Median) are applied for that [2-3].
The algorithm of median polishing was offered by John W. Tukey (1981),
and was applied after modification by E.V.Pervago for processing of Resistivity
Imaging or Total Electrical Sounding (TES) data. The algorithm’s operation
is based on regularity of distortion effects and allows to cancel effectively
P-, and C-effects on apparent resistivity pseudo-cross-section. Tukey described
a processing algorithm for data, given in the table as following: at the
first step - counting and subtracting median value for each column; at
the second step - counting and subtracting median value for every row.
Then the 1st and 2nd steps repeat several times.

We present soundings data as the table of Rho_{a} logarithms,
where each spacing corresponds to a row, and each sounding site - to a
column. The effect of horizontally-layered medium in such table will be
displayed simultaneously for all sites, P-effect
will be displayed simultaneously on all spacings, and C-effect
- as lines, inclined under angele of 45 degrees; to the left or to the
right, depending on type of array and sounding technology. To this table
we apply algorithm of median polishing, but in addition to rows and columns
we shall also include in processing inclined lines, corresponding to C-effect.
The result of algorithm operation is the decomposition of an initial field
into several components, connected with: a) position of moveable (C)and
unmoveable electrodes(P); b) horizontally-layered structure (HLS)
and c) some rests (R).

After
decomposition each component is filtered from high-frequency noise, connected
basically with near-surface inhomogeneities and errors of measurements.
Parameters of filtering (width of smoothing window) are selected by user.
At the last stage the reconstructing process is applied - the smoothed
components are united back into Rho_{a} field (see fig.). The values
of P- and C-effects can be estimated separately for each site of a traverse
and can be drawn as separate diagrams.

Thus, the technology of processing with the program Median in IPI-2D
package allows to reveal effects of distortions caused by near-surface
inhomogeneities and to remove them, to see effects from deep inhomogeneous
objects and to separate the influence of horizontally-layered medium. Layered
medium can be subjected to quantitative 1D interpretation. Deep objects
can be interpreted with the help of 2D forward problem account, including
both deep inhomogeneities and layered medium.

119899, Russia, Moscow, Moscow State University, Geological Faculty, Department
of Geophysics
V.A. Shevnin,I.N. Modin
Tel. & fax: (7095) 939 49 63
E-mail: sh@geophys.geol.msu.ru