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The signs of distortions on VES data and their role for data interpretation

There are several groups of VES data distortion's signs:

Signs for a single non-segmented VES curve: a) when part of VES curve goes up at an angle more than 45°; b) when part of VES curve goes down more steep, than VES with mue=0; c) there are abrupt extremums or break points on VES curve, that are not match with theoretical horizontally-layered curve.
For segmented VES curve: great gaps on vertical (Rhoa) direction between segments, or crossing segments, or any other their arrangement differs from theoretical one.
For the pair of pole-dipole VES curves, with a reference point in the center of MN (AMN+MNB): differences in form of these two curves.

For VES' profile, when distances between VES points are smaller than maximum depth of investigation: abrupt changes in form or level of VES curves from point to point. (The distance between VES point is one meter in this field example)

Field VES data analysis for different regions has shown, that distortions' signs could be found at more than 70% of VES curves, even in simple geological situations. Thus, VES distortions exist nearly on all VES data. The influence of near-surface inhomogeneities is analogous to "broken glass" or wavered see surface, preventing from clear seeing deeper objects through them. Our experience of VES data interpretation shows the next facts. The instrumental measuring accuracy for resistivity method is usually better than 3%. Wrong electrode positions can give errors less than 1-2% of Rhoa value. The accuracy of automatic VES data fitting frequently consists 8-12%. This fact means, that the main part of general VES data dispersion is a result of geological noise's influence.

119899, Russia, Moscow, Moscow State University, Geological Faculty, Department of Geophysics
V.A. Shevnin,I.N. Modin
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