Examination of a Karstic Cave with Complex Geophysical Methods in North Hungary

The On-Site Inspection (OSI) under the Comprehensive nuclear-Test-Ban Treaty (CTBT) shall be conducted in the least invasive manner possible, consistent with the efficient and timely accomplishment of the inspection mandate. The active seismic and geoelectrical surveys to search for and locate underground anomalies, including cavities and rubble zones caused by underground nuclear explosion belong to the approved inspection techniques. The field testing of surface geophysical methods was carried out in Felsöpetény, North Hungary between 2012 and 2014. 3D active seismic and 2D geoelectrical measurements were performed over an abandoned underground clay mine, which is connected to an oval shaped cave of karstic origin of 30m diameter situated 70m below the surface. The cave was carved in the Dachstein Limestone by hydrothermal activity during the Pleistocene close to the overlying sediments where the opening of the karstic cave could happen. A weak zone of lower velocity above the cavern was revealed by the seismic refraction method. The observed low velocity anomaly on the seismic tomography profile and the characteristic high resistivity on the geoelectrical 2D profile appear at the position of the cave.