Geoelectrics
Geoelectrics is one of the classic methods of exploration geophysics. Geoelectric measurements play an important role, particularly in groundwater and soil exploration, but also in cavity and fracture detection, archaeology, and borehole logging.
In electrical resistivity tomography (ERT) or geoelectrics, a direct current [I] is fed into the earth's surface at two points A and B, and the resulting electrical potential [U] is measured at two further points M and N. From both measured values I and U, the so-called apparent resistivity [rho a], the inverse of the electrical conductivity, is calculated, taking the geometry of the electrode configuration into account.
During a measurement, the distance between the current electrodes is systematically increased, thus determining the apparent electrical resistance as a function of the electrode spacing (resistivity). The apparent resistivity value measured by a particular electrode arrangement represents the volume of material along the pathway auf the currents and it is only identical to the specific resistance if the surface is a homogeneous halfspace. However, such ideal conditions are rarely found in nature. The model most commonly used in geophysics is therefore that of a layered subsurface and the apparent resistivity is a combination of the individual resistivities of the layers. The apparent resitivity is usually plotted as a function of the electrode spacing, which is a rough proxy for the measurement depth . These sounding curves, which were lines up along a 2D profile of measurement positions is referred to as a pseudosection.
Depending on the application, electrode arrays are often used along a profile or across an area. Each electrode can be used for both current injection and voltage measurement. The use of multi-electrode arrays and multi-channel data loggers makes it possible to measure different configurations without changing the position of the electrodes and minimises the duration of the measurements.
Thanks to technical advances, it is now possible to measure several hundred electrodes and several thousand measurement configurations in less than an hour, which also allows for the observation of flow processes, for example.