Electrical resistivity tomography
The geoelectric belongs to Applied Geophysics and includes methodologies for studying the Earth's crust by measuring electrical voltage and current at the surface. These include:
- Self-potentials (natural, galvanic elements in ore );
- DC method and
- AC method in which the ground electrode out artificial streams.
A method of artificial power often use four dot arrays (two electrodes A, B to the power supply, two probes M, N to the potential measurement ), there can only be eliminated in this way, which occurs at the electrode contact resistance. In the arrangement of the electrodes in a line ( for example, power supply by the external electrodes, the measurement of the internal electrodes = probe ), there are various possibilities, for example:
- By Wenner method: All electrodes have the same distance to each other,
- By Schlumberger method: The current electrodes have a larger spacing than the potential probes
- Dipolar: Voltage and current electrodes each form a dipole at a greater distance to one another,
- Pole-dipole forward and reverse: The voltage electrodes form a dipole, a current electrode is located at a greater distance to the voltage electrodes, the second current electrode is located at infinity.
The Wenner method is well suited for mapping changes in conductivity over a larger area, the Schlumberger method is used primarily for exploratory - used for depth sensing. The dipole versions offer better contrast resolution of conductivity contrasts, especially for smaller structures. The pole -dipole method can be used in combination with a forward and reverse model particularly suitable for the mapping of borders - eg be used, at which changed owing to the water conductivity - to fault zones. The problem behave in practice transition resistors so that the voltage to be measured differences can be very small part. The selection of the suitable for the measurement task configuration decisive impact on the future possibilities of expression of results.
The measured value in the geoelectric is the apparent resistivity in.
Common applications for the geoelectric especially in groundwater exploration, the search for contaminated sites, but also in archeology for the detection and mapping of former settlements and other historic or prehistoric structures. Here also the geomagnetic exploration is often used as an alternative or supplement to the geoelectrical exploration.
Basically, two main objectives: the exploratory, which provides the structure under the probing point with increasing depth, and mapping that represents the planar structure in the selected focus depth. Newer multi-electrode methods allow the simultaneous scanning and mapping of a limited range, and are also referred to as geo-electrical imaging.
By using inversion method, the real resistivity from the recorded measurement data can be calculated, which, inter alia, allows a depth specification of the structures found. The computational algorithms frequently use the finite difference or finite element methods.