Untitled

› Geo-Electric Explorations
48
› INDUCED POLARISATION
The Induced Polarisation (PI) is an
electric phenomenon that occurs inside
of material media in the time domain,
with the release of stresses upon
the interruption of a flow of electric
current of step type (in this case, it is
measured as chargeability) and also in
the frequency domain, with a precise
dispersion law of the electric resistivity
upon the variation of the frequency
of an alternated current flow. A PI
source is connected to the oxido-reductive
processes along the interface
between the metal grains and interstitial
fluids (electrode polarisation). Another
important PI source consists instead
of ionic accumulations in moving
electrolytes due to mobility variations
along the path (electro-kinetic polarisation).
The tomographic inversion of
the data obtained on the surface returns
the ground in “output” based on
chargeability images, thanks to which
it is possible to identify possible accumulation
areas or significant concentrations
of hydrocarbons. The chargeability
is proportional to the charge
stored by the lithotype and it represents
the concentration of a conductor
in the multi-electrode area.
› SPONTANEOUS-POTENTIAL METHOD
The Spontaneous Potential method
(PS) consists in determining on
the surface, the differences of potential
related to a natural electric
field, linked to the underground circulation
of aqueous electrolytic solutions
in porous media. From the analysis
of PS anomalies in the surface, it
is possible to determine the intensity
and position of the concentrations
of ionic charges with both polarities.
The test is carried out by positioning
two electrodes: the first one close to
the measurement station and the other
one is moved on the following stations
of the line; of both electrodes
are moved, by keeping stable the interval
between them, mapping the
ground based on the spontaneous potential.
The use of this method is useful
in the mining field for the search
of sulphides and graphite, but also in
the archaeological field, indeed water
circulations can be affected by underground
archaeological structures,
that could act as drain or as obstacle,
and therefore, with the identification
of PS anomalies, it is possible in general
to find underground archaeological
structures indirectly.