Apparent magnetic susceptibility (MSa) as recorded by electromagnetic induction (EMI) instruments could offer relevant information about non-soil subsurface features. It is less affected by natural soil properties than its prominent counterpart, i.e., apparent electrical conductivity (ECa). Hence, MSa is generally a promising approach to investigate artificial inclusions and structures in soil. However, while the origin depth of EMI based ECa is widely accepted, the depth sensitivity (DS) of MSa measurements remains poorly understood. The depth interpretation of MSa is particularly challenging due to negative values especially for objects that are randomly distributed over different depths.
Here we assessed the performance of both multi-coil (MC) and multi-frequency (MF) EMI sensors for identifying and determining the DS of MSa measurements in shallow soils through detection of buried small targets of known conductivity. Two experiments were conducted in a sandy loam podzolic soil in western Newfoundland, Canada. Materials of different conductivities, including metal and plastic targets, were buried at depths between 20 and 80 cm. Three inter-coil separations (32, 71 and 118 cm) of the MC sensor and four factory-calibrated frequencies (18, 38, 49 and 80 kHz) of the MF sensor were tested in both horizontal and vertical coil orientations.
The MC sensor clearly detected all four metal targets from three coil separations in both coil orientations while the MF sensor identified more anomalies than targets limiting its information value. Based on the measurements from MC and the theoretical DS function, a criterion was developed and validated to assess the potential depth origin of MSa. We found that negative or less than the background values occur, if the depth of the target is shallower than 0.36 times the coil distance of the employed EMI sensor. According to this criterion, the depth origins of metallic targets were correctly identified under the assumption of low induction numbers, even if values were negative.