Calibration of frequency‐domain electromagnetic devices used in near‐surface surveying

Abstract

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 applica tions 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 electromag netic 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 respons es 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 die lectric 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.

Mapping of quadrature magnetic susceptibility/magnetic viscosity of soils by using multi-frequency EMI

“…Multi-frequency measurements allow removing the effect of the magnetic viscosity on the quadrature part of the signal. Considering the difference between two different frequencies this difference is then transformed in apparent electrical conductivity by correspondence with a reference curve calculated using the complete formulas (Thiesson et al 2014). …”

“…Thus, one obtains the calibration coefficient between the digits measured by the instrument and the theoretical response for the quadrature part of the EM signal. In the second calibration step the experimental in-phase response acquired with an aluminium sphere is compared with the theoretical one (Thiesson et al 2014). For our instrument, in-phase coefficient is − 0.70 digit/ppm and the quadrature coefficient − 1.0 digit/ppm.…”

“…transformation of instrumental digits into experimental (Hs/Hp) ratio in ppm, established on a methodology described by Thiesson et al 2014). For the quadrature channel it is based on a comparison between an electrical sounding and measurements at (at least) two heights above the ground surface.…”

“…The calculation of the response of a homogeneous ground, or of layered one, has been established by the work of Wait (1959), and is based on Hankel's transforms that can be rapidly calculated using convolution products (Guptasarma and Singh, 1997). Complete expressions and a description of the different subsequent approximations can be found in (Thiesson et al 2014). They allow expressing the measurement in terms of apparent conductivity and apparent susceptibility, based on a simplified conversion since the variations are monotonous although not simply linear.…”

Multiconfiguration electromagnetic induction survey for paleochannel internal structure imaging: a case study in the alluvial plain of the River Seine, France

“…It is worth noting that other in situ ways of calibration could be performed (e.g. Delefortrie et al, 2014) -particularly, using the theoretical response of a metallic and non-magnetic sphere (Thiesson et al, 2014).…”