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.
Measuring magnetic viscosity significantly improves the information brought by the magnetic susceptibility about the history of soils. In the field its mapping can be achieved by TDEM measurement. Here we study the applicability of multi-frequency FDEM viscosity measurement in the low frequency range using a commercial EMI instrument. The dependence of the in-phase and quadrature out-of-phase components of the ratio of secondary magnetic field to primary magnetic field, electrical conductivity, magnetic susceptibility and magnetic viscosity is first described. The procedure allowing the determination of the three apparent properties is then proposed. It delivers first the conductivity using the differences between the quadrature responses at two different frequencies. Then, after removing the conductivity effects both in the in-phase and quadrature components, it provides the values of the magnetic susceptibility and viscosity. This procedure is tested on 1D and 3D synthetic cases to assess any arising uncertainty. The application of the method is attested in two archaeological case histories in Thessaly in conductive and magnetic soil contexts. The apparent magnetic viscosity maps are significantly different from magnetic susceptibility and conductivity maps thus bringing new information into the game.
Time‐lapse applications of seismic methods have been recently suggested in the near‐surface scale to track hydrological properties variations due to climate, water level changes, or permafrost thaw, for instance. But when it comes to traveltime tomography or surface‐wave dispersion inversion, a careful estimation of the data variability associated with the picking process must be considered prior to any time‐lapse interpretation. In this study, we propose to estimate picking errors that are due to the inherent subjectivity of human operators, using statistical analysis based on picking repeatability. Two seismic datasets were collected along the same profile under distinct hydrological conditions across a granite–micaschist contact at the Ploemeur hydrological observatory (France). Both datasets were recorded using identical equipment and acquisition parameters. A thorough statistical analysis is conducted to estimate picking uncertainties, at the 99% confidence level, for both P‐wave first arrival time and surface‐wave phase velocity. With the suggested workflow, we are able to identify 33% of the P‐wave traveltimes and 16% of the surface‐wave dispersion data, which can be considered significant enough for time‐lapse interpretations. In this selected portion of the data, point‐by‐point differences highlight important variations linked to different hydrogeological properties of the subsurface. These variations show strong contrasts with a non‐monotonous behaviour along the line, offering new insights to better constrain the dynamics of this hydrosystem.
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