Calibration of frequency-domain electromagnetic devices used in near-surface surveying
Résumé
For the past forty years electromagnetic prospecting instruments have played a growing role in the mapping of soil EM properties in the very low-frequency (VLF) range for a large variety of applications and they are now beginning to be applied in the medium-frequency range. Measurement interpretations, however, necessitate expressing the results in terms of physical properties. This step allows not only comparisons and joint interpretation with data generated by different electromagnetic induction (EMI) instruments but also with other types of field measurements e. g., vertical electrical sounding (VES) or electrical resistivity tomography (ERT) or laboratory tests on samples. The calibration process here proposed is based on comparisons between the instrument responses and: (1) an exact 1D multi-layer analytical modelling that takes the three EM properties into account, i. e., the electrical conductivity, the complex magnetic susceptibility and the complex dielectric permittivity when the instrument is elevated above a layered ground; (2) the response to purely conductive metallic spheres, which only depends on the diameter of the spheres. It is applied to two instrument prototypes: one in the VLF frequency range and the other in the medium-frequency (MF) range.