Geoscience Reference
In-Depth Information
Current I
V
Prongs that
penetrate the soil
and scratch it if
there is movement
perpendicular to
the plane of the
diagram
Battery
Potentiometer
Soil
Fig. 13.1
Principle of measurement of resistivity
in situ
.
Ohm's law
V
=
RI
is applied. By taking into account the geometry
of the system (gap between electrodes) the average electrical resistivity
of the soil in ohm cm is deduced and, from it, the conductivity. If the
electrodes are spread out further, the size of the section of soil examined
increases and may go up to 10 m. If the electrodes are fixed on a mobile
frame drawn by a tractor, a resistimetric plot is obtained. By juxtaposing
several parallel linear plots we obtain a picture of the resistivity of an
area, or a resistivity map.
The soil is subjected to a high-frequency magnetic field generated by
a primary coil. The higher the conductivity of the soil, hence greater
salinity, the larger are the currents induced in the various horizons.
These currents in turn set up a secondary field that is measured in a
secondary coil. The depth surveyed depends on the gap between the
coils and their orientation, but it will not be more than 2 m of profile
thickness. The procedure is not destructive. It allows us to map the
salinity easily (Boivin
et al
. 1988; Job 1992; Corwin
et al
. 2006). Similar
to the preceding method, it has the usual problems of any 'all-inclusive'
determination: on the one hand we get a single result, on the other
hand, the determining factors are many (salinity and also water content,
structure and organization of the soil). Not easy to sort out…
Electromagnetic induction
The soil solution is collected in the field through a porous candle. But
the sampling is not possible unless the soil is wet. The hydric potential
must be lower than 2 bars or else a suction must be applied to extract
the water. The quantity of each of the ions present and the total dry
Chemical analyses
