Mapping and modelling of collapse sinkholes in soluble rock: The Münsterdorf site, northern Germany

Münsterdorf is a small village in the north of Hamburg, located along the northern rim of a salt diapir. The Permian rocks are uplifted and overlying rocks such as the Cretaceous limestone, normally at several kilometers depth, have been pushed close to the surface. In Münsterdorf, the Cretaceous limestone can be found at around 20 m depth, and about 2 km further south, Cretaceous limestones are quarried in a large open-pit mine.

Since 2004, collapse sinkholes have formed on a sports field in Münsterdorf, with a frequency of about 1 every two years, about 3–5 m in diameter and 3–5 m deep. The collapse sinkholes do not reach the underlying limestone, but seem to be related to accelerated dissolution in that formation. Above the Cretaceous limestone, Quaternary gravels and glacial tills provide a non-soluble, but permeable and heterogeneous cover of about 20 m thickness.

We have mapped the sports field and its vicinity with gravity (GRAV), electrical resistivity imaging (ERI), and ground-penetrating radar (GPR). The ERI profiles indicate a significant change in the surface layer, from thin and irregular in the northern part to thicker and more homogeneous in the southern part of the sports field. GPR profiles confirm this result.

We numerically model the evolution of flow and porosity in the Cretaceous limestone to estimate the evolution time of subsurface voids in the limestone, and we discuss the potential cause of the sinkhole formation and its sudden onset in light of the hydraulic boundary conditions.