Spatial information on soils generally results from local observations that are destructive and time consuming especially
in the case of heterogeneous soils. Geophysical technics can be a great help for soil mapping since they
are non-destructive and fast. Large areas can be surveyed with a high density of measurements. Electrical resistivity
is particularly interesting for soil study because it covers a wide range of values (several decades) and depends
on many characteristics of the soil.
The main objective of this paper was to study soil spatial variability using an original approach to electrical data
processing. Electrical data from a 3-depth survey, usually treated as three apparent resistivity maps, were considered
as many electrical soundings each with three apparent resistivity values. The study of the vertical succession
of these values led to define nine reference geophysical taxa. This taxonomy relies on a parameter (α) which discriminates
soil layers and is defined as the interval (in Ω m) beyond which two successive apparent resistivity
values on a sounding are considered different. Geophysical taxa mapping highlighted their spatial coherence,
which was related to pedological characteristics such as the presence of a clay layer or the depth of the soil profile.
The comparison between the spatial distribution of geophysical taxa and a pre-existing soil map showed that the
delineations of taxa clusters closely matched soil unit boundaries but were much less smooth. This method
allowed the assignment to each soil type of a specific apparent electrical resistivity profile which was consistent
with soil profile description. The method is straightly applicable to data from other surveys, and opens the way to
the development of semi-automatic soil mapping from electrical resistivity data.