Looking into the earth with seismic waves

Seismic methods show distribution of groundwater in depth

3D model combined from the interpretation of geophysical methods: Scene to the EU project "Buried Valleys" (BurVal) to illustrate the combined use of geophysical methods in the exploration of Quaternary glacial gullies. © GGA Institute
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Earthquakes not only cause damage but also do their job well. By measuring the energies released, scientists can learn more about the inner structure of the earth. The seismic waves can even be generated artificially and now form an important research method in geophysics. It is used wherever geological structures and depositional conditions are complicated and unclear. Particularly in the search for raw materials such as crude oil and natural gas but also for subsoil investigations seismics is used.

"The seismic process exploits the elastic properties of the rock, which make the passage of a pressure wave possible in the first place, " explains Helga Wiederhold of the Institute for Geoscientific Community Tasks (GGA Institute) in Hanover. "If the seismic wave meets other rock properties as it passes through the rock, it will be refracted and reflected. By interpreting these reflection signals, we can draw conclusions about the geological structure of the subsurface, "explains Wiederhold, who heads the research area" Groundwater Systems "at the GGA Institute.

Ice ages characterize groundwater distribution

Seismic section (above) and interpretation (below): the glacial channel, filled with quaternary sands and clays, cuts 350 meters deep into tertiary sediments. Groundwater-conducting sand layers in yellow, groundwater-inhibiting layers in blue. © GGA Institute

As part of the "Groundwater Resources in Buried Valleys (BurVal)" project, researchers use seismic technology to examine a very special raw material: water. The studies focus on ice age gullies, which still have a significant influence on groundwater conditions between Cuxhaven and Bremerhaven or in the metropolitan area of ​​Hamburg. For the protection of groundwater and the drinking water supply the knowledge about its course, the sediment filling and the connection to the surrounding rock is enormously important.

UntersuchenThe channel shape and its extent down to a depth of almost 400 meters is examined with the aid of seismic measurements, Wieder explains Wiederhold. The additional combination of geoelectrical processes and "correct" drilling makes it possible to differentiate fairly well between groundwater-conducting layers such as sand or groundwater-inhibiting layers such as clay. In addition to basic research This is above all a very practical meaning: on the basis of this data, the vulnerability of the groundwater resources in the gutter to pollutant inputs can be calculated.

New measuring technology in the trial

Shear wave vibrator GGA Institute

Another focus of work at the GGA Institute is currently the development of new seismic metrology. For example, a hydraulic vibrator is currently in the test phase. This stimulates so-called shear waves (S waves), which can penetrate up to 500 meters deep into the earth. Currently, the so-called compression waves (P-waves) are normally used for seismic exploration, which fundamentally differ in their vibration behavior from the shear waves. display

The great advantage of shear waves due to their lower speed is their higher resolution of the subterranean layers, Wiederhold explains. As soon as the new process has matured, a combination of P and S waves would be conceivable. This would then also allow the derivation of those formation parameters that are responsible for the elastic wave propagation as well as the hydraulic features of a rock, Wieder Wiederhold dares to look into the future of seismology.

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GGA Institute

Project BurVal

(Helga Wiederhold / Institute of Geosciences Joint Projects (GGA-Institute), 08.06.2007 - AHE)