Vertical Spatial Sensitivity and Exploration Depth of Low‐Induction‐Number Electromagnetic‐Induction Instruments

TLDR

Earlier work suggested that, for low‑induction‑number instruments, vertical spatial sensitivity depends only on depth below the surface, but outside LIN conditions it also varies with apparent soil conductivity and induction number. This study sought to determine the conductivity and induction number ranges where LIN conditions hold and to quantify how effective depth changes when they do not. The authors employed a complete numerical solution of Maxwell’s equations to simulate two‑layer soil models with horizontal and vertical coplanar coil orientations, evaluating vertical spatial sensitivity and effective depth. Effective depth decreased with increasing apparent conductivity, with the least conductive soils yielding LIN‑expected depths; for the VCP orientation depths ranged from 1.6 s to 0.8 s, and for HCP from 0.76 s to 0.51 s, differences attributed to skin‑depth and interface scattering, highlighting the need to account for conductivity dependence when using LIN instruments for subsurface mapping.

Abstract

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 1.6 s , decreasing to 0.8 s in the most electrically conductive soil. For the HCP orientation, d e decreased from 0.76 s to 0.51 s 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