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.