Theory
Electrical polarization measurements are widely used to infer
electrical material properties of laboratory samples. Here the
electrical impedance (alternating-current resistance) is measured
as a complex number. By taking into account the measurement
geometry of the sample and the position of the measurement
electrodes, the impedance can be transferred to a material
property, the resistivity (with the inverse, the conductivity).
Resistivities and conductivities are still complex numbers, which
can be represented as a real and imaginary parts, or as magnitude
and phase values.
The real part of the conductivity denotes the
ohmic conduction properties of the sample, dominated by ionic
conductivity of the poor fluid or interface conduction properties
of geophysical samples. Metallic conduction can play an important
role in other fields of applications.
The imaginary part of the conductivity represents non-ohmic
polarization signals which, in geophysical applications, as
mostly of a capacitive nature. Possible source of polarization
are manifold, but low-frequency polarization in geophysical
contexts is believed to be dominated by ionic dislocation at the
electrical double layer (EDL).
Polarization processes are usually frequency dependent and
correspondingly are measured a various frequencies ranging from
the low mHz range to the kHz range (or even further). This
results in a spectrum of the complex impedance, which is why the
method is also called electrical impedance spectroscopy (EIS).
