Here we include methods and techniques capable to measure electrical natural potentials or induced by current injection between two grounded electrodes. This current can be simple DC, stepped DC or low frequency alternative and the coupling with the  terrain is galvanic. The potentials produced between another pair of grounded electrodes depends on underground distribution of current lines. It is related to lithology, moisture, groundwater and dissolved ions, temperature etc. The relative position (geometry) of the four grounded electrodes defines the, so called, measurement array. The investigation depth and resolution are different from one array to another, the proper one being chosen on performance, sensitivity and cost effective criteria.
          The key physical parameter of electrical measurements is electrical resistivity (or its inverse, conductivity) generally related to geological characteristics of terrain.
          We use electrical measurement to reveal targets located at different depths, using classical VES, for depths up to 400 m (using power transmitter) and  ERT to depths up to 70 m. In special situations we can also perform 3D measurements, at a requested level of detail.
          For deeper structures we use EM techniques, more efficient and cost effective.
          IP (induced polarization) is a phenomenon present both in time and frequency domain. In the first case it appears due to capacitive effects produced after DC cut off (discharge ); in frequency domain out of phase potentials can be measured. IP effects are recorded around metallic ores specially if they are disseminated. They are also observed in presence of clay films or even simple clays,  graphite and few other materials, being a modality  to highlight them.
            The SP (Self Potential) method consist of adequate measurement on natural potentials occurred in presence of ionic solutions due to some physio-chemical reactions such as redox and electrofiltration. They involve the water table  level and fluid circulation beneath and over it.


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ERT layout in urban area