Negative apparent resistivities in surface resistivity measurements

Abstract

Apparent resistivity is traditionally defined as the resistivity of a hypothetical homogeneous half-space that would yield the same response as that measured over the real earth with the same acquisition parameters. Negative apparent resistivity contradicts this definition of apparent resistivity as we cannot physically attain a homogeneous medium with negative resistivity. However, we often encounter negative apparent resistivity in field data acquisition; it is regarded as the effect of measurement error or noise. In this paper, we re-investigated the idea that negative apparent resistivity, especially in the dipole-dipole measurement, can be caused by a natural geological structure even when the earth is isotropic and flat, and the survey line is straight. The results from 3D numerical modelling revealed that negative apparent resistivity in the dipole-dipole measurement can be encountered particularly when the resistivity contrast between the conductive body and background medium is extraordinarily high. Moreover, negative apparent resistivity can occur only when the current source is very close to one side of the conductive body, like in the mise-ả-la-masse case, and the potential dipole is placed at the opposite side of the body.