An integrated geophysical and geological interpretation of the area around the Unicorn Cave (Southern Harz Mountains, Germany)

The Unicorn Cave, in the southern Harz Mountains in Germany, is a natural cave that developed in dolomitic host rocks of the Permian Zechstein period. The cave is located on a plateau, with a shallow overburden of 10-15 m. The cave is essentially a north-south trending sequence of shallow passages, interrupted by large rooms. The cave terminates to the south in a collapsed room, providing natural access through two daylight openings. To the north, shallow tunnels that are drowned in sediment, terminate the cave. We have mapped the known and accessible cave with a LIDAR scan, which provides a detailed model of the cave's interior, and allows the unravelling of the structural control on the development of the cave. With geophysical surveys (gravity, electric resistivity imaging, ground-penetrating radar, shear-wave reflection seismics) above and in the cave, we investigated the substantial sediment infill of the cave, mapping material properties, such as density, electrical resistivity, electrical permittivity, and shear modulus. Based on the indirect geophysical signals obtained above the cave, we have mapped the terrain south and north of the known cave with the same geophysical techniques. We have identified the hitherto-unknown southern extension of the Unicorn Cave using geophysical methods, which we then proved by drilling. The boreholes pass through an air-filled passage with large sediment infill.