In DC resistivity field measurements negative readings can occur, but it has been explained until now that these are not caused by subsurface formations if standard electrode arrays are used, therefore these are due to measurement errors. But un-noisy negative data exist even in shallow cases of dipole-dipole spread n = 1 or 2. There was not any numerical study showing the possibility of actual negative apparent resistivity different from negative IP effect. In this study negative apparent resistivity effect due to a steel casing borehole is examined by verified 3-D numerical modeling program. The apparent resisitivity pseudosection shows pants-leg effect, and abnormal high apparent resistivity trends occur closely to negative data. From various numerical modeling results, -ρa by one steel casing borehole near resistivity survey line can be acquired with dipole-dipole array or pole-dipole array. Obviously, negative apparent resistivity anomalies even in the flat area are subsurface information. Because the misinterpretations lead to seriously wrong pitfall, inversion techniques for those effects need to be developed in the near future.


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