Negative apparent resistivity values are often obtained in field exploration using the dipole-dipole configuration. It has been believed that negative apparent resistivity cannot exist theoretically, and most of negative readings in field data have been regarded as measurement errors or noise. In recent years, some studies and experiments presented that negative apparent resistivity can be induced by geological structures. In this study, we show that negative readings can be generated even in very simple structures, if the conductive body is located around the surface and current flows into the conductive body. By analyzing the electric potential distribution and the current flow maps, we investigate the main mechanism of generating negative apparent resistivity. Our experiments show that negative apparent resistivity is a kind of 3D effect. When we apply the line source to simulate 2D modeling, negative readings are not recorded even for the same structures, because the line source blocks the current flow. The inversion results are distorted severely by these negative readings, and deviated from the original model. To properly interpret subsurface media from field data including negative readings, we need a new interpretation algorithm that can deal with negative readings.


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