Open AccessVertical Spatial Sensitivity and Exploration Depth of Low‐Induction‐Number Electromagnetic‐Induction Instruments
Author(s) -
Callegary James B.,
Ferré Ty P. A.,
Groom R. W.
Publication year - 2007
Publication title -
vadose zone journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.036
H-Index - 81
ISSN - 1539-1663
DOI - 10.2136/vzj2006.0120
Subject(s) - electromagnetic induction , sensitivity (control systems) , electrical conductor , electrical resistivity and conductivity , orientation (vector space) , soil science , water content , electromagnetic coil , induction coil , geology , materials science , geometry , optics , geotechnical engineering , physics , mathematics , electrical engineering , composite material , engineering , electronic engineering
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