Electrical conductivity mapping is a prerequisite tool for hydrogeological or environmental studies. Its interpretation still remains qualitative but advantages can be expected from a quantitative approach. However a full 3D interpretation is too laborious a task in comparison with the limited cost and time which are involved in the majority of such field studies. It is then of value to define the situations where lateral variations are sufficiently smooth for a lD model to describe correctly the underlying features.For slingram conductivity measurements, criteria allowing an approximate 1D inversion are defined: these mainly consist of a limited rate of variation over three times the intercoil spacing.In geological contexts where the weathering has generated a conductive intermediate layer between the underlying sound rock and the soil, this processing can be applied to determine the thickness of the conductive layer from the apparent resistivity map when the other geoelectrical parameters are known. The examples presented illustrate this application. lntroduction Conductivity or resistivity mapping plays a major part in near-surface geophysics. These physical properties exhibit the widest range of variations and are sensitive to the presence of water in a soil or in a rock, to the salinity of this water or to the content in clay particles. These parameters are dominant in environmental or geotechnical studies. As the cost of such surveys is necessarily limited, electrical or electromagnetic methods used for resistivity mapping must involve fast measurements with light and robust instruments and must be qualitatively interpretable without difficulty. The speed of measurement determines both the extent of the surveyed area and the size of the sampling mesh, which are the main influences on interpretation. I

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