Electrical resistivity tomography (ERT) is a geophysical tool used to obtain an image of the subsurface using electrical measurements made at the surface. The ERT technique provides an image of the electrical properties of the sub-surface in a cross-sectional format. ERT data are collected using a linear array of electrodes connected to a resistivity transmitter/receiver and an electrode control box. Various electrode spacings and arrangements can be used, depending upon the objective of the survey and local geology.

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In geoelectric prospecting, the physical parameter that is determined is the electrical resistivity of the formations that constitute the subsoil and that can vary according to the lithotype, the degree of homogeneity, the degree of alteration and fracturing, the content in water and salts, the granulometry, etc. Other electrical methods, usually dedicated to mining research, geothermal energy, or the dispersion of liquids in the ground, are instead based on the measurement of Self Potential or Induced Polarization.

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Although it started from the basic principles of geoelectric, electrical tomography (ERT) has opened a new chapter in the history of geophysical surveys: in this case, the ground survey is carried out with a set of electrodes distributed along a profile a few metres away. The measurements proceed automatically according to the desired sequence, returning apparent resistivity values at different depths and locations along the profile itself. The result of the processing of this data is a two-dimensional profile of the distribution of soil resistivity values. A high resolution 2.5D image can be obtained by combining multiple soil resistivity profiles parallel to each other. With a greater number of electrodes, it is even possible to perform true 3D tomography investigations, suitably interpreted using sophisticated processing software.