Soil magnetism and proximal soil sensing: a perspective
Philippe De Smedt1*, Ynse Declercq1, Jeremy Devos1,2, Lander Baeten2, Gaston M. Veirana1, Ellen Van De Vijver1
1 Department of Environment, Research Group Soil Spatial Inventory Techniques, Faculty of Bioscience Engineering, Ghent University, Belgium
2 Department of Environment,Forest & Nature Lab, Faculty of Bioscience Engineering, Ghent University, Belgium
* Philippe.desmedt@ugent.be Abstract
The rate at which applications dealing with the shallow subsurface are using sensor data to support the evaluation of soil-based ecosystem services is continually increasing. This expansion relates to advances being made at different levels: from the development of small and affordable sensors to compose efficient soil monitoring networks, to the increasing accessibility of remote sensing information through openness of data and cloud computing. When considering geophysical approaches to soil mapping, often primarily targeting electromagnetic variations, such technical advancements are in stark contrast to the lack of fundamental insight into the magnetic soil variability.
The emphasis on electrical conductivity and dielectric permittivity in many cases stems of necessity, as fluid dynamics and associated properties of soils are often crucial to soil studies and resource management (consider, for instance, the definition of ‘agrogeophysics’ as a subclass of hydrogeophysics (Garré et al., 2021)). Although soil magnetism does not inform on short-term changes in moisture dynamics, its direct relationship to the content and form of iron oxides makes it a valuable proxy for, e.g., mineralogical and geochemical variations, or the influence of weathering and erosive processes (Jordanova, 2017).
At current, magnetic soil exploration is directed mainly at finding traces of anthropogenic activity, with its most common use in UXO detection and archaeological prospecting, the latter application being one that demands high instrument sensitivity and sampling densities (Fassbinder, 2015). While having potential for targeting the often low concentrations of iron oxides in soils, the frequent use of gradiometer instruments, combining two coaxial magnetometers separated by a fixed distance, limits soil mapping beyond discrete subsurface features. However, alternative configurations of magnetometry instrumentation (Mathé and Lévêque, 2003) and magnetic susceptibility meters (Grimley et al., 2008), as well as detailed studies into the magnetic properties of soil profiles (Jordanova, 2017) underline the potential of such approaches for soil studies.
Here, we discuss how magnetic soil mapping can provide insight into physical and chemical soil variations, and the influence of anthropogenic land-use. We elaborate on this through case studies using, particularly for large-scale applications, frequency-domain electromagnetic induction (FDEM) instruments. While used primarily as ‘conductivity meters’, the in-phase response of these instruments can enable detailed mapping of the induced magnetisation of soils, revealing, for instance, the presence of iron pans in podzols as shown in Fig. 1. In a similar way, we demonstrate how magnetic susceptibility mapping can support soil nutrient as well as land-use studies. Based on the current state of the art, we consider magnetic soil mapping as key component of proximal soil sensing, and elaborate on the potential and technical limitations of current approaches to investigate soil magnetism.
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Fig. 1: In-phase (IP) FDEM map of soil magnetic variation in a wetland environment (blanket bog covering freely draining acid loamy soils over mudstone bedrock) whereby high IP magnetic susceptibilities indicate the presence of podsols with strong iron panning (right borehole log with downhole magnetic susceptibility (MS) profile), contrasting with podsols lacking iron panning (left borehole log)
References
Fassbinder, J.W.E., 2015. Seeing beneath the farmland, steppe and desert soil: magnetic prospecting and soil magnetism. Journal of Archaeological Science 56, 85–95.
Garré, S., et al., 2021. Geophysics conquering new territories: The rise of “agrogeophysics.” Vadose Zone Journal
Grimley, D.A., et al., 2008. Soil Magnetic Susceptibility: A Quantitative Proxy of Soil Drainage for Use in Ecological Restoration. Restoration Ecology 16, 657–667.
Jordanova, N., 2017. Soil Magnetism. Applications in Pedology, Environmental Science and Agriculture. Academic Press, London, UK.
Mathé, V., Lévêque, F., 2003. High resolution magnetic survey for soil monitoring: detection of drainage and soil tillage effects. Earth and Planetary Science Letters 212, 241–251.
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