Geophysical analysis of an area affected by subsurface dissolution – case study of an inland salt marsh in northern Thuringia, Germany

Abstract. The subsurface dissolution of soluble rocks, also called subrosion, can affect areas over a long period of time and pose a severe hazard. We show the benefits of a combined approach using P-wave- and S_H-wave reflection seismics, electrical resistivity tomography, transient electromagnetics, and gravimetry for a better understanding of the subrosion process. The study area, ’Esperstedter Ried’ in northern Thuringia, Germany, located south of the Kyffhäuser hills, is a large inland salt marsh that developed due to dissolution of soluble rocks at approximately 300 m depth.We were able to locate buried subrosion structures, subrosion zones, faults and fractures, and potential fluid pathways, aquifers and aquitards based on seismic and electromagnetic surveys. Further improvement of the subrosion model was accomplished by analyzing gravimetry data that indicates subrosion-induced mass movement as shown by local minima of the Bouguer anomaly for the Esperstedter Ried. Forward modelling of the gravimetry data, in combination with the seismic results, delivered a cross section through the inland salt marsh from north to south. We conclude that the tectonic movements during the Tertiary, which led to the uplift of the Kyffhäuser hills and the formation of faults parallel and perpendicular to the low mountain range, were the initial trigger for subrosion. The faults and the fractured Triassic and Lower Tertiary deposits serve as fluid pathways for groundwater to leach the deep Permian Zechstein deposits, since subrosion is more intense near faults. The artesian-confined salt water ascends towards the surface along the faults and fracture networks, and formed the inland salt marsh over time. In the past, subrosion of the Zechstein formations formed several, now buried, sagging and collapse structures, and, since the entire region is affected by recent sinkhole development, subrosion is still ongoing. From the results of this study, we suggest that the combined geophysical investigation of subrosion areas can improve the knowledge of control factors, risk areas, and thus local subrosion processes.