Modeling the response of a field probe for nondestructive measurements of the magnetic susceptibility of soils

Abstract

Nondestructive volume magnetic susceptibility measurements (MS) from the surface do not provide information about the depth distribution of a magnetic material or about the amount of magnetic material. We have developed a model that can be used to predict the volume magnetic susceptibility from the surface for a given (known or hypothesized) stratification of the magnetic layers in the soil profile. The measurements were performed with the MS2D Bartington sensor. The antenna signal from the MS2D probe decreases rapidly with depth. We show that the relative decrease in MS depends not only on the concentration of the magnetic tracer, but also on the distribution of the magnetic tracer in the soil profile. The decrease in sensitivity was fitted with a double exponential function. The function was implemented in a newly developed MagHut model. The MagHut model is a tool that can be used for forward modeling of the volume magnetic susceptibility when the tracer distribution in the soil profile is known. The model was successfully calibrated and validated with the measured data and with data from the literature. The Nash–Sutcliffe coefficients for goodness of fit were above 0.99 in all cases. MagHut can help interpret MS mapping results or it can be used to optimize amount and placement of the magnetic tracer for soil erosion experiments. However, the MagHut tool is only limited to the top 10 cm of the soil profile and cannot replace, but only complement, the standard procedure of occasional soil profile sampling and laboratory mass MS measurements.