Subrosion is a serious risk, particularly in urban areas. The term subrosion describes water-soluble components (salt, for example) leaching out of rocks. The resulting loss of mass can lead to instabilities or even cavities in the subsurface. On the surface, these processes are manifested by the ground slowly subsiding or suddenly slumping, creating what is referred to as a sinkhole. Subrosion can be expedited by faults in the subsurface and disruptions in the sequence of strata that can become pathways for water.
In recent years, subrosion processes in the form of the associated, sometimes catastrophic effects and events have increasingly come to the attention of the general public. The most obvious form of subrosion is a sinkhole: the sinkholes in Schmalkalden and Tiefenort (Thuringia) in 2010 represent two of the most striking examples in Germany due to the unforeseen collapse events and associated damage. They illustrate that the existing understanding of processes and thus the predictive capacity of such events is insufficient so far.
However, the example of the Leaning Tower of Bad Frankenhausen, Germany, also illustrates that subrosion encompasses far more than just the superficially visible sinkholes. The inclination of the tower is due to subsurface leaching and cavity formation.
Regions in northern and central Germany where salt structures occur are especially prone to subrosion.The Quickborn region in Schleswig-Holstein is characterised by a rising salt dome that has led to sinkholes and sinkholes in the urban area over the last twenty years. With the measuring system developed at LIAG from a mini-vibrator source (Elvis VII), a growing number of seismic shear wave profiles are being measured and thus courses of the rising salt structure can be determined more precisely; likewise, regions more or less influenced by the salt rise can be distinguished at LIAG. When identifying areas prone to sinkhole formation, sections in which the salt structure occurs in the near-surface area in connection with disrupted or shifted strata are of particular interest, as water pathways can develop at these areas. Borehole logs are employed to support the interpretation of the geological situation.
For several years now, LIAG has operated a gravimetric monitoring network in two regions of Germany – Hamburg Flottbek and Bad Frankenhausen (Thuringia). As the processes are slow and the changes small, the time series must be accordingly long. For the first time, we succeeded in detecting the loss of mass caused by subrosion based on oberservations of temporal changes in Earth’s gravitational field. The monitoring activities have been continued, motivated by these positive results. The examples mentioned above show the focus of our current research in the field of geohazards: improving the imaging of present conditions of the various research objects and establishing new methodological approaches and time series.