Douchko Romanov

Cave detection using multiple geophysical methods: Unicorn cave, Harz Mountains, Germany

Unicorn cave in the southern Harz Mountains of Germany is a show cave in dolomitic rocks of the Zechstein Formation. The cave’s trunk passage is interrupted by larger rooms. The overburden is only around 15 m. The passages are filled with sediments that can be up to 50 m thick. We used gravimetry and electrical resistivity imaging over the cave area to identify the subsurface voids and the extent of the sediment infill of the cave passages. Our choice of methods was based on several conditions unique to the cave: (1) well surveyed, (2) shallow overburden, (3) large air-filled passages, and (4) thick sediment cover, concealing true passage size. Using the cave survey as an initial model for the subsurface structure, we successfully identified the air-filled cave with both methods. We then inferred the thickness of the sediment infill by forward modeling and identified a possible southward continuation beyond the currently explored passages.

Deep conduit flow in karst aquifers revisited

Caves formed in soluble rocks such as limestone, anhydrite, or gypsum are efficient drainage paths for water moving through the aquifer from the surface of the host rock toward a resurgence. The formation of caves is controlled by the physical solution through dissociation of the host rock by water or by the chemical solution through reactions of the host rock with water enriched with carbon dioxide. Caves as large underground voids are simply the end-member of secondary porosity and conductivity characterizing the aquifer. Caves and their relation to a present or past base level are found both close to a past or present water table (water table caves) and extending far below a past or present water table (bathy-phreatic caves). One explanation for this different speleogenetic evolution is the structural control: fractures and bedding partings are preferentially enlarged around more prominent faults, thus the fracture density in the host rock controls the speleogenetic evolution. This widely accepted explanation can be extended by adding other controls, e.g., a hydraulic control: as temperature generally increases with depth, density and viscosity of water change, and particularly the reduction of viscosity due to the increase in temperature enhances flow. This hypothesis was proposed by Worthington (2001, 2004) as a major controlling factor for the evolution of deep bathy-phreatic caves. We compare the efficiency of structural and hydraulic control on the evolution of a cave passage by numerical means, adding a third control, the chemical control to address the change in solubility of the circulating water with depth. Our results show that the increase in flow through deep bathy-phreatic passages due to the decrease in viscosity is by far outweighted by effects such as the decrease in fracture width with depth due to lithostatic stress and the decrease in solubility with depth. Hence, the existence of deep bathy-phreatic cave passages is more likely to be controlled by the structural effect of prominent faults.

Examples and Case Studies

The presented examples and case studies illustrate specific applications of adaptive management of water resources in the region of Basel, Northwestern Switzerland. Such concepts together with the setup of tools and process-oriented experiments allow testing hypotheses. The applied methods facilitated us to fill several gaps of knowledge of subsurface processes. The examples focus on questions with practical as well as research. Most topics are relevant for urban areas and the sustainable use of subsurface resources in general.