Storing nuclear waste is a world-wide problem. A few years ago, the German government decided to search for a permanent nuclear storage location. The waste is divided into categories depending on radiation and origin. A geologically stable repository and leak-proof waste containers are required. Storage depths should be more than 300 m. In Germany claystone, salt and crystalline rocks have been identified to be potentially capable for waste storage.
Example:
Seismic tomography can be used to map the structures between boreholes at high resolution. The example shows a tomogram measured a few hundred meters below ground and at a distance of more than 100 m. This tomogram should demonstrate the capability of the method to map geological structures and to contribute to a wider knowledge base for planning repositories.
Crosshole seismic tomogram
The disposal of radioactive waste is planned in deep underground rock layers. Suitable host rocks must have special properties, such as low hydraulic permeability, high mechanical stability and high sorption capacity for radioactive particles. Salt formations are often considered for this purpose. However, they can face challenges like instability, water intrusion, radioactive leaks, or unknown underground shapes. Seismic methods help to image the subsurface and reduce these risks.
Example:
Geophysical investigations were carried out to assess the integrity and stability of the rock and to image the interior and extent of the storage chambers. Seismic methods applied included seismic tomography and crosshole testing, with P- and S-wave measurements conducted between the tunnel and a horizontal borehole, as well as between two horizontal boreholes. The results showed that the salt rock is less homogeneous than previously assumed, but both P-wave and S-wave tomography were successfully performed in the horizontal wells.
Tomograms show the distribution of P- and S-wave velocitites in the salt mine between tunnel and borehole and between two horizontal boreholes