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Median algorithm

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 algorithmís 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 Rhoa 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 Rhoa 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

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