Vertical spatial sensitivity and exploration depth of low-induction-number electromagnetic-induction instruments

Vertical spatial sensitivity and effective depth of exploration (d _e) of low-induction-number (LIN) instruments over a layered soil were evaluated using a complete numerical solution to Maxwell's equations. Previous studies using approximate mathematical solutions predicted a vertical spatial sensitivity for instruments operating under LIN conditions that, for a given transmitter-receiver coil separation (s), coil orientation, and transmitter frequency, should depend solely on depth below the land surface. When not operating under LIN conditions, vertical spatial sensitivity and d_e also depend on apparent soil electrical conductivity (σ_a) and therefore the induction number (β). In this new evaluation, we determined the range of σ_a and β values for which the LIN conditions hold and how d_e changes when they do not. Two-layer soil models were simulated with both horizontal (HCP) and vertical (VCP) coplanar coil orientations. Soil layers were given electrical conductivity values ranging from 0.1 to 200 mS m^-1. As expected, d_e decreased as σ_a increased. Only the least electrically conductive soil produced the d_e expected when operating under LIN conditions. For the VCP orientation, this was 1.6s, decreasing to 0.8s in the most electrically conductive soil. For the HCP orientation, d_e decreased from 0.76s to 0.51s. Differences between this and previous studies are attributed to inadequate representation of skin-depth effect and scattering at interfaces between layers. When using LIN instruments to identify depth to water tables, interfaces between soil layers, and variations in salt or moisture content, it is important to consider the dependence of d_e on σ_a.