Geoelectrics
The Geoelectrics belongs to the applied geophysics , and includes procedures for the research of the Earth's crust by measuring the electric voltage and current at the surface. This includes:
- Own potentials (natural, galvanic elements in ore deposits);
- Direct current method and
- Alternating current methods in which electrodes supply artificial currents to the ground.
Method with artificial power often use four dot arrays (two electrodes A, B to the power supply, two probes M, N for potential measurement), since the occurring at the electrodes only in this way, contact resistance can be eliminated. When arranging the electrodes in a line (e.g. power supply through the outer electrodes, measurement on the inner electrodes = probes) there are various options, e.g. B .:
- According to the Wenner method : All electrodes have the same distance from one another,
- according to Schlumberger method : the current electrodes have a greater distance than the potential probes,
- Dipole-Dipole : The voltage and current electrodes each form a dipole at a greater distance from one another,
- Pole-dipole forward and reverse: The voltage electrodes form a dipole, one current electrode is at a greater distance from the voltage electrodes, the second current electrode is at infinity.
The Wenner method is well suited for mapping changes in conductivity over a larger area, the Schlumberger method is mainly used for probing - for deep exploration. The dipole variants, on the other hand, offer better resolutions of conductivity contrasts, especially for smaller structures. The pole-dipole method, in combination with a forward and reverse variant, is particularly useful for mapping borders - e.g. B. in fault zones - can be used where the conductivity changes due to water. In practice, contact resistances are problematic, so that the voltage differences to be measured can sometimes be very small. The selection of the configuration that is suitable for the measurement task is a decisive factor in the subsequent information options for the measurement results.
The measured value in geoelectrics is the apparent specific resistance in .
application
Geoelectrics are mainly used in groundwater exploration , the search for contaminated sites, but also in archeology for tracing and mapping former settlements and other historical or prehistoric structures. Geomagnetic exploration is often used here as an alternative or in addition to geoelectrical exploration .
Basically, two main objectives are pursued: The exploration , which provides the structure below the exploration point with increasing depth and the mapping , which shows the areal structure in the selected focus depth . Newer multi-electrode methods allow the simultaneous probing and mapping of a limited area and are also referred to as geoelectric tomography.
By using inversion methods, the real specific resistance can be calculated from the recorded measurement data. a. enables the depth of the structures found to be specified. The calculation algorithms often use the finite difference or finite element methods.
Web links
Individual evidence
- ↑ Geoelectrics - making structures in the underground visible, ( Memento of the original from June 7, 2017 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Portal "planeterde", accessed on January 7, 2016.
