Measurement and inversion strategies for 3-D resistivity surveys with vector arrays

Abstract

Three-dimensional geoelectrical surveys are widely used to map the subsurface in areas with complex geology. Field survey methods using the offset pole–dipole and dipole–dipole arrays have been proposed to map large areas efficiently. However, it has been found that negative apparent resistivity values are sometimes encountered in areas with large resistivity contrasts, particularly when large offsets between the current electrodes and potential dipoles are used. The vector array configuration that makes two measurements of the electric field at each station with two potential dipoles at right angles avoids this problem. The combined potential measured at the two dipoles is independent of the orientation the current electrodes to the receiver station. The amplitude of the vector apparent resistivity value is always positive. A commonly used configuration is to lay out the current and potential electrodes along alternating parallel lines. We examine the use of different offsets between the current electrodes and potential dipoles to improve the survey spatial resolution. A new inversion method using both the amplitude and direction of the vector array potentials is described to improve the resolution. Results from the inversion of a synthetic model data set and a field survey over a deep-seated landslide are shown.