Magnetic viscosity of tropical soils: classification and prediction as an aid for landmine detection

SUMMARY Electromagnetic induction (EMI)‐based metal detectors are the most widely used sensing techniques in landmine clearance operations; however, they are negatively influenced by frequency dependence of magnetic susceptibility. A total of 466 rock and soil samples collected from across the tropics are investigated in this study. The data show that frequency‐dependent susceptibility depends on the parent material as well as on the degree of weathering. Ultramafic and mafic rocks and their derivatives have higher susceptibility and absolute frequency dependence than material originating from intermediate, felsic and sedimentary rocks. Within each parent material group, absolute frequency dependence increases steadily with increasing alteration from unweathered rock to topsoil. This effect is likely due to either the residual enrichment of weathering resistant ferrimagnetic minerals including superparamagnetic (SP) grains, the comminution of larger ferrimagnetic minerals or the neoformation of SP minerals during soil formation. Relative frequency dependence is generally lower than 15 per cent for the investigated samples with a few exceptions. It increases with alteration for igneous rocks but remains at the initially high level for sediments. This finding indicates that the relative concentration of SP minerals changes with respect to the total magnetic fraction for igneous rocks but remains constant for sediments. Soils derived from ultramafic, mafic and intermediate rocks show low relative frequency dependence, and their magnetic susceptibility is mainly the result of multidomain lithogenic minerals. In contrast, soils derived from felsic rocks and sediments show the highest values, and their susceptibility is due to SP minerals that are either formed during pedogenesis or residually enriched. The average and extreme values of the absolute frequency dependence within each subgroup, based on parent material and alteration grade, are used to design a prognosis system for assessing the impact of the subsurface on EMI sensors for landmine detection. In general, intermediate, felsic and sedimentary rocks have no influence on the detectors and only a weak influence in the most extreme cases. Soils derived from these rocks typically have no influence; however, they can have a very severe influence in a few cases. In contrast, ultramafic and mafic rocks typically have a moderate influence and a very severe influence in extreme cases, with the associated soils resulting in a severe influence in general. The deduced prognosis system can be used by demining organizations to help them predict metal detector performance in tropical regions on the basis of geologic and/or soil maps, which do not supply information on electromagnetic properties. In this way, such a system may eventually help with the planning of demining campaigns and selection of appropriate sensors.