Explore fresh water resources with electric power

Geophysics distinguishes saltwater and sedimentary sediments

Aerogeophysical measuring system of the BGR (on the left the measuring device for the electromagnetics) © GGA-Institut
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Access to clean drinking water is one of our livelihoods and is therefore of great importance not only in dry areas but also in our latitudes. However, there is often a lack of clarity about the true thickness and extent of underwater groundwater resources. However, these can be measured across the board using geophysical investigation methods. The geoelectric makes use of the differences in the specific electrical resistance between the rocks.

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"For example, the freshwater-rich sandy and pebbly sediments that are of interest for drinking water have higher electrical resistivities than saltwater-bearing areas, " explains Michael Grinat from the Institute of Geoscience Joint Projects (GGA Institute). "Toned deposits, which may cover these layers and protect them from contamination from the earth's surface, have lower specific resistances." Accordingly, the structure of the groundwater system can be measured relatively well.

Saltwater border in sight

For example, the GGA Institute has been conducting geophysical surveys in the area between Cuxhaven and Bremerhaven for several years. Here, by way of example, questions about the structure of the groundwater system and the development over time, in particular the salt / freshwater limit, are to be clarified.

"This area is particularly interesting, as different landscape types such as Geest and Marsch interact with the North Sea and the mouth of the Elbe, " says Grinat. "It comes both to the ingress of salty seawater inland as well as to freshwater discharges into the Wadden Sea. In addition, in the underground filled gutter structures of the Elster Ice Age, which have been studied as part of the EU-funded project "Groundwater Resources in Buried Valleys" already more intense. "Display

Electricity in the underground

To analyze the groundwater system, the researchers make use of, among other things, the different specific electrical resistances of salt and fresh water. The classic geoelectrics require two electrodes, via which a current is fed into the ground at the earth's surface. In addition, there are two probes (also on the earth's surface) between which the electrical voltage generated by the flow of current in the underground is absorbed. Using model calculations, we can then conclude on the specific resistance at different depths and thus on the structure of the subsoil, "explains Grinat.

The depth of evidence ultimately depends on the distance between the current electrodes and increases with it: computer-controlled multi-electrode systems are used today to explore depths of 50 100 meters. These allow statements about the course of the resistivity both along the designed measurement profiles and at different depths. However, the scientists at the GGA Institute also carry out the classic geoelectrics measurements described above with electrode distances of up to ten kilometers, enabling them to study the subsurface to a depth of about two kilometers.

Electromagnetic survey from the air

Aerogeophysical measuring system of the BGR (on the left the measuring device for the electromagnetics) GGA Institute

In addition to geoelectrics, geophysicists are also using electromagnetics, which exploits the induction effect. Information about the resistivity at different depths results from the use of different measurement frequencies, says Grinat. Higher frequencies penetrate less deeply into the ground than deeper ones. The big advantage: The measuring systems usually only consist of one transmitter coil and one receiver coil, which do not have direct contact to the underground need. In addition, for large-scale investigations, they can also be mounted under a special helicopter, for example the Federal Institute for Geosciences and Natural Resources (BGR).

Further fields of application

However, this means that the application possibilities of the geoelectric and electromagnetic methods are far from exhausted. In addition, they also have a great significance in the exploration of landfills and contaminated sites. But geophysical know-how is also used in modern agriculture, soil science and archeology, "Grinat reports.

(Michael Grinat - Institute for Geosciences Joint Projects (GGA-Institute), 22.06.2007 - AHE)