Measuring instruments, geophysical instruments

Electrical imaging

Despite starting from the basic principles of geoelectrics, electrical imaging or electrical tomography (ERT) has opened a new chapter in the history of geophysical surveys. Instead of using only 4 electrodes, as in "traditional" vertical electrical surveys, the investigation on the ground is carried out using a set of electrodes (from a minimum of 16 up to several hundred), distributed along a profile with an interdistance of few metres. Using a special system - based on one or more multi-electrode cables and related "switching boxes" - these electrodes are connected to the data acquisition unit/energiser so as to be able to operate alternately as current electrodes or measuring electrodes. In this way, the measurements along a profile can proceed in an automatic manner according to the desired sequence (the most commonly used spreadings include Wenner, Wenner-Schlumberger, Dipole-Dipole), returning apparent resistivity values at different depths and locations along the profile itself. The result of processing this data is a real two-dimensional cross-section which represents the distribution of the resistivity values ??of the ground. By combining several parallel ground resistivity profiles, it is possible to obtain a 2.5D high definition display. With a greater number of electrodes, it is even possible to perform real 3D tomography investigations, suitably interpreted by means of sophisticated processing software.

The extreme versatility and measuring accuracy makes this method of investigation suitable for the widest areas of application. Here are just some examples of the most common applications: - Environmental monitoring: in dumping areas, using electrical tomography it is possible to display the presence or absence of leaks in the waterproof insulating substrate quickly and clearly. The method can be also applied in also lends itself greatly to medium- to long-term monitoring of the dispersion of pollutants in the ground in cases of environmental accidents. - Hydrogeology: the creation of electrical tomographic cross-sections allows the highly precise identification of lithological discontinuities, water accumulation zones, rock fracturing zones and circulation of fluids inside the discontinuity. - Archaeology: identification of artefacts and remains by measuring the difference in resistivity between the materials - Geotechnics, surface geology, geomorphology: identification of possible causes of subsidence, locations and structures of underground cavities and tunnels, detailed investigations of mineral deposits and quarry areas, identifying breakaway surfaces in landslide areas.