Spatial variability of magnetic soil properties

Abstract

The presence of magnetic iron oxides in the soil can seriously hamper the performance of electromagnetic sensors for the detection of buried land mines and unexploded ordnance (UXO). Previous work has shown that spatial variability in soil water content and texture affects the performance of ground penetrating radar and thermal sensors for land mine detection. In this paper we aim to study the spatial variability of iron oxides in tropical soils and the possible effect on electromagnetic induction sensors for buried low-metal land mine and UXO detection. We selected field sites in Panama, Hawaii, and Ghana. Along several horizontal transects in Panama and Hawaii we took closely spaced magnetic susceptibility readings using Bartington MS2D and MS2F sensors. In addition to the field measurements, we took soil samples from the selected sites for laboratory measurements of dual frequency magnetic susceptibility and textural characteristics of the material. The magnetic susceptibility values show a significant spatial variation in susceptibility and are comparable to values reported to hamper the operation of metal detectors in parts of Africa and Asia. The absolute values of susceptibility do not correlate with both frequency dependence and total iron content, which is an indication of the presence of different types of iron oxides in the studied material.

Multi-frequency metal detector in high mineralization

“…In the following we will model this effect, considering the extreme mineralization case where the magnetic susceptibility is χ soil = 0.03 or 3,000×10 -5 SI units [2]. This is a rather large value, but encountered in some places like Australia, Mozambique [3] and Hawaii [4] for example.…”

Using EMI and P-XRF to Characterize the Magnetic Properties and the Concentration of Metals in Soils Formed over Different Lithologies

“…Little is known about the magnetic properties of soils and their spatial variability (Van Dam et al, 2004). The magnetic properties of soils are largely determined by the presence of iron oxides, in different forms and concentrations (Van Dam et al, 2004). Strongly magnetic soils have large concentrations of ferromagnetic minerals, such as magnetite, maghemite, and titanomagnetite, and to a lesser degree pyrrhotite and chromite (Takahashi et al, 2011; Van Dam et al, 2004; Mullins, 1977).…”

“…The magnetic properties of soils are largely determined by the presence of iron oxides, in different forms and concentrations (Van Dam et al, 2004). Strongly magnetic soils have large concentrations of ferromagnetic minerals, such as magnetite, maghemite, and titanomagnetite, and to a lesser degree pyrrhotite and chromite (Takahashi et al, 2011; Van Dam et al, 2004; Mullins, 1977). While hematite, pyrite, ilmenite, goethite, ferrihydrate, and lepidocrocite occur in many soils, they appear to play a minor role in determining the magnetic properties of soils (Shamatava et al, 2007; Van Dam et al, 2004).…”

“…In general, the magnetic susceptibility of most soils is low, has negligible affects on electromagnetic field strengths, and is ignored. However, in magnetic soils, the presence of ferromagnetic minerals interferes with and reduces the efficiency of magnetic and electromagnetic sensors in detecting buried metallic objects (i.e., unexploded ordinance, mines, pipes, drums, and other artifacts) (Van Dam et al, 2004).…”

“…L ittle is known about the magnetic properties of soils and their spatial variability (Van Dam et al, 2004). The magnetic properties of soils are largely determined by the presence of iron oxides, in different forms and concentrations (Van Dam et al, 2004).…”