An Analysis for Negative Apparent Resistivity Appearing in Dipole-dipole Electrical Survey

In dipole-dipole electrical resistivity survey, we have often encountered negative apparent resistivity in pseudosections of apparent resistivity. Negative apparent resistivity is defined as the apparent resistivity values with the opposite sign to the surrounding resistivity values. It has been proven that the negative apparent resistivity can be caused by some geological models. We analyze the main causes for negative apparent resistivity by plotting the electric potential and current flow distributions using 3-D electrical resistivity modeling. We could observe that the current flows and the electric potentials are severely disturbed when transmitters are located around a branch of the U-shaped conductive model. Especially, the current flows in the opposite directions to those of the homogeneous body in some regions. This is because the current flows along the U-shaped conductive body and the other branch of the U-shaped conductive body acts as a secondary source. The resulting electric potentials and current flows are expressed as the combination of electric potentials and current flows generated by both the primary and secondary sources. From these results, we can confirm that negative apparent resistivity occurs due to some geological models.