The results are presented in the form of profiles or maps of apparent resistivity on which appear zones of low values, showing clay or water-filled layers, and higher resistivity layers that may correspond to more resistant soils or coarser materials, depending on the geological context.

EM 31

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  Step 1: Resistivity Map of the altered zones of a bedrock before civil works in the area.
  Mapping is done prior to the opening of a Technical Landfill Center. The objective was to identify the most conductive areas for the placement of traps (blue and green). Values correspond to highly shaly silts. The type of map is also used in the construction of building zones. On resistant areas (red), the buildings and the conductive areas, the retention ponds to store runoff water, are preferentially installed.


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  Step 2: Resistivity map used as a quality indicator for a residual clay deposit.
  Blue and light green indicate the potentially exploitable clay areas. Orange indicates the zones where the bedrock is rising up.


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  Steps 3 and 4: Detection of an old metallic pipe network and an old well.
  The acquisition is done in a schoolyard located on former homes built after the World War II (bombing of Saint-Nazaire).
  The longitudinal anomaly in beige-orange on the phase map (step 4) or in yellow-green on the conductivity map (step 3) has to be connected with an old network, it bordered an old road. In the middle, we see a circular structure that could correspond to an old well filled with more resistant materials (blue on both maps). The other anomalies are due to more or less coarse embankments or to old constructions (less conductive anomalies, blue on both maps). The extreme values at the periphery are erroneous values due to a metallic fence. The interpretation is confirmed by aerial photographs.


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  Step 3: Conductivity map (mS/m)


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  Step 4: pseudo-phase map (unitless)

EM 34

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  Step 5: Example of an EM 34 profile recorded in the Marais Poitevin (France).
  • Top: the raw profiles in apparent resistivity for horizontal and vertical dipoles. The vertical dipole enables a greater penetration in depth while the horizontal dipole is more sensitive to lateral resistivity variations and appears noisier.
  • Bottom: Result of apparent resistivities inversion. The curve indicates the limit between the Flandrian clays and the Jurassic limestone.


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  Step 6: Map of resistivities used as a proxy for paleo-channels detection.
  The device is an EM 34-3 horizontal dipole. The coil spacing is 10 m. Resistivity values greater than 90 ohms correspond to coarse materials. The limits of the paleo-channels are indicated by red solid lines on the map.