In general, the way to act consists of sending a current to the subsoil by means of an electrode device and measuring the potential difference, and finally, based on the resistivity values of the ground, making the geological attribution. Depending on the measuring device and the spacing between the electrodes it is possible to reach different depths of investigation and resolution.
VES (Vertical Electrical Soundings)
The purpose of the Vertical Electrical Sounding technique is to study the distribution of subsoil materials in the vertical of the measurement point, based on the existence of contrasts between the characteristic resistivity values of the various layers of the ground.
The way to act consists of sending a current to the subsoil through two electrodes, AB, and measuring with other electrodes, MN, the voltage difference, which is attributed to the center of the device (measurement point). By changing the spacing between the electrodes it is possible to measure the resistivity of the ground in the vertical of the same point at different depths. Thus, after each measurement, electrodes A and B are separated, which implies an increase in the depth of investigation with successive measurements.
ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT)
By means of electrical resistivity tomography it is possible to obtain a geolectric sectiont in which the distribution of resistivities under the topographic surface is observed, up to a certain depth that depends on the measurement device used. Taking into account the resistivity values obtained after processing of the section, a geological attribution is made, differentiating geological formations, their state of weathering, fracture zones or faults, dikes, cavities, anthropic elements (walls, footings, ...) , etc. The greater the resistivity contrast between the materials or elements studied, the easier it will be to differentiate them and therefore make a more accurate and truthful interpretation.
INDUCED POLARIZATION (IP)
The IP method involves inducing a current through the ground by two current electrodes and measuring a voltage by two potential electrodes. Same electrode configurations can be used than in resistivity tomography, but the most common devices are dipole-dipole and pole-dipole. The depth of investigation for any electrode gap is increased through the use of higher electrode spacing values.
By injecting a current into the ground, it is charged or polarized, like a capacitor. When the current ceases, it takes a finite time for the induced charge to dissipate. The amount of charge and the time it takes to inject and dissipate are the parameters used in the analysis. In IP tomography, nonpolarizable electrodes are used.