Spatial sensitivity of low-induction-number frequency-domain electromagnetic-induction instruments

Abstract

Numerical simulations were used to study spatial averaging in low-induction-number frequency-domain electromagnetic induction (LIN FEM) instruments. Local ( LS) and cumulative (CS) sensitivity were used to analyze three different aspects of LIN FEM spatial sensitivity. LS is the variation in a measured property given a small change at a given location of the property of interest. CS contours are derived from LS and reveal the shape and the fraction of total instrument sensitivity enclosed within the contours. The first study re-evaluated the asymptotic approach to LIN FEM spatial sensitivity. Using this approach, LIN FEM measurements have often been assumed to represent electrical conductivity (sigma) at discreet depths that do not vary with the sigma of the ground. This assumption was tested using simulations of electromagnetic fields in environments with homogeneous and layered sigma distributions. When the induction number was greater than 0.01, the 1-D vertical CS distribution and the depth of investigation varied up to 20% over the range of sigma simulated. As sigma increased, CS contours and depth of investigation decreased in depth. In the second study a small perturbation approach was used to calculate CS distributions so that each distribution is unique to a given LS distribution. CS was summed from regions of high to low LS, and retained information on the magnitude and location of LS. As sigma increased, CS became focused around the highest LS values. The maximum reduction in depth of investigation was about 40% at the highest sigma investigated. In the final study, a series of small, electrically conductive perturbations was simulated in a three-dimensional, homogeneous environment. Three-dimensional LS varied markedly with a large difference between horizontal (HMD) and vertical (VMD) orientations of the transmitter and receiver dipoles. In some regions, the calculated magnetic field intensity with the perturbation was less than that calculated for the host without the perturbation. This occurred for both VMD and HMD orientations of the transmitter. CS contours were highly complex. One dimensional, vertical LS curves extracted from the three-dimensional data were very different from curves from infinite layer simulations.