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› Mae 47
› S.E.O. (Horizontal electric explorations)
This type of survey is carried out with
the purpose to obtain a resistivity profile
of the ground along a direction and
at a certain depth. Each unhomogeneity
in the medium tested, like bodies with
different electric conduction capacity,
is identified because it deflects the current
lines and distorts the regular distribution
of the electric potential. Furthermore,
by measuring the potential
drop on two random points, it is possible
to determine the electric resistivity
of the medium by multiplying the ratio
between the potential drop and the
current sent by a geometric coefficient
which depends on the position of the
electrodes on the ground. By changing
the position of the electrode device on
the area to test, it is possible to determine
the distribution of the resistivity
in the volume subject to electric current.
Since rocks are resistive materials
by nature, the resistivity variations are
due almost exclusively to the presence
of water in different quantities. The SEO
is carried out by fixing 4 electrodes in
the ground, two of which (the external
ones) are for anchoring purposes
and the other 2 (internal ones) are for
measuring purposes, at a certain distance
from each other. It is possible to
obtain a horizontal profile of the resistivities
of the grounds, which is useful
to identify a vertical passage between
two bodies with different resistivity.
› RESISTIVITY TOMOGRAPHIC GEO-ELECTRIC PROFILE
(Electric tomography)
This type of survey is based on the
calculation of the resistivity from the
d.d.p measurements in the ground. The
method is based on the introduction of
an electric field in the ground through
electrodes (input or current) and the
measurement of the d.d.p. in other
electrodes (of measurement type).
From the d.d.p. value, it will be possible
to find the resistivity value through the
second law of Ohm, which is a typical
characteristics of all materials. Since
rocks are resistive materials by nature,
the resistivity variations are due almost
exclusively to the presence of water in
different quantities. Electric tomography
2D-3D is carried out by fixing electrode
grids in the ground (16, 32, 64,
128….) at constant pitch. All of them
are connected to boxes which allow
commutation among them, defining
which are the measuring and current
electrodes from time to time and automatically.
The system will automatically
perform all possible combinations.
For this purpose, a series of measurements
will be obtained (as many as the
combinations available), based on: the
number of electrodes and type of geometric
configuration used. The tomographic
inversion of the data obtained
on the surface generates a bidimensional
or tridimensional reconstruction
in “output” of the ground, from which it
is possible to identify possible anomalies
due to cavities, water bodies, etc,
and identify shapes, sizes and spatial
distribution.