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C-effect - was found and described in 1991, firstly in modeling results and only after that in field data. The main cause of that is in the difficulty of finding C-effect on Rhoa pseudo-cross-section when all VES's were measured with logarithmic distance step. On fig.2 and 5 are shown results of modeling VES over two-layered structure with one NSI. (2a - the model; 5a - Rhoa pseudo-cross-section for AMN array, 5b - V-transformation; 2c - different VES curves for several variants of meeting elements of array and NSI (fig.2b), 0 - non-distorted and 1-4 - distorted by P or C-effect). Some distortions of sounding curves are conformable (fig.2c,1-2), whereas others are nonconformable (fig.2c,3-4). On apparent resistivity crosssection distortions display as vertical and inclined strip zones (fig.5). When moving current electrode hits the NSI, VES curve noticeably changes on one or two distances due to abrupt change in current density in the cross-section.

C-effect has several particularities, which do it more dangerous, than P-effect: a) The form of VES curve and consequently visible layers number changes; b) On profile VES data C-effect shows itself on Rhoa cross-section as dipping layer (fig.5b), and in the case of logarithmic scale along AO axis it seems to be bent layer. c) when standard sounding technology is used (four electrodes Schlumberger array and logarithmic step in distance growth), C-effect results from A and from B electrodes occasionally and irregularly only at the moments when current electrodes exactly hit NSI. In this case the main sign of C-effect - its form - disappears. Case C on fig.4 shows V-transformation for AMN array with logarithmic step in distance growth. C-effect in this case loose its regularity. d) On Rhoa cross-section C-effect is not distinctly visible due to great changes in Rhoa level with distances (fig.5a). To find and recognize C-effect the special technology of field measurements and results visualization is needed (V-transformation).

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

V.A. Shevnin,I.N. Modin
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