On the low-frequency EMI response of coincident loops over a conductive and permeable soil and corresponding background reduction schemes

The performance of metal detectors (low-frequency electromagnetic induction (EMI) devices) employed for landmine and ordnance detection is well known to be adversely affected by the soil response, a fact which is, however, not very often considered in the scientific literature. We have, therefore, started from the analytical model of a frequency domain coincident loop system over a homogeneous half-space to calculate directly the voltage induced in the system's receive coil, for a number of scenarios and soil parameters of interest, and with emphasis on the operating conditions prevailing in humanitarian demining applications. The role of the soil's permeability , which heavily affects the real part of the system's response function, has been clearly shown (plateau effect at low-frequencies), as well as the effects of changes in the detector's height . Some of the background rejection techniques used in practice, in particular frequency differencing methods to suppress the effect of magnetic and/or conductive soil, have been described as well, and two of them studied in more detail, including the unavoidable target response reduction. Finally, we have briefly dealt with the effect of a conductive soil on the primary and scattered fields themselves, which could affect in a nonadditive way the target signature. Practical background information and an extensive list of references complement the paper.