27.07.2021 •

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

Two sites located in the Northern Piedmont of Pennsylvania suspected to have different levels of magnetic susceptibility (k) were examined using electromagnetic induction (EMI) and portable X-ray fluorescence (pXRF). One site is underlain by micaceous schist and serpentinite; the other site by micaceous schist only. The responses of an EM38-MK2-1 meter and the estimated k were greater and more variable at the site underlain by serpentinite and micaceous schist. Also, the average concentrations of Fe, Cr, Ni, and Ti were significantly higher at this site, and significant correlations were derived between the concentrations of several metals and the in-phase response and k of the upper 30 cm of the soil. These correlations were generally lower and less significant at the site underlain by micaceous schist alone. As k is associated with greater amounts of ferromagnetic constituents in soils, the greater concentration of Fe measured with pXRF at the site underlain by micaceous schist and serpentinite helps explain the greater averaged and more variable EMI responses measured with the EM38-MK2-1 meter at this site. The contrast in the EMI and pXRF data between these two sites was associated with differences in the mineralogy and lithologies of serpentinite- and nonserpentinite-derived soils.

Introduction

Little is known about the magnetic properties of soils and their spatial variability, but they are largely determined by the presence of iron oxides in different forms and concentrations [1]. The magnetic susceptibility (k) indicates the presence of iron-bearing minerals in soils and rocks. As k describes a material's ability to become magnetized, it is roughly proportional to the concentration of ferromagnetic minerals. However, the in-phase (IP) response of electromagnetic induction (EMI) sensors is also used to measure the magnetic properties of soils [2].  

EMI surveys traditionally focus on the electrical properties of soils, neglecting the magnetic ones. In general, k of most soils is low and has negligible effects on electromagnetic field strengths; however, in magnetic soils, the presence of ferromagnetic minerals interferes with the efficiency of magnetic and electromagnetic sensors in detecting buried metallic objects [1].

The interpretation of k and IP data obtained from EMI sensors remains challenging due to various technical and environmental limitations, especially EMI sensor drift [3], arbitrary "zero level" [4], limited exploration depths [5], and changes in the sign (±) of the response at certain depths and in relation to the target position [6]. Regarding the latter, for EMI sensors operating in the vertical dipole orientation (VDO), the IP response experiences a sign change with depth [2]. For example, for the EM38 meter operating in the VDO, the response is positive for the upper 60 cm of the soil profile and weakly negative below 60 cm [2].


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