Geophysical Detection of Underground Voids - Ocna Dej Salt Mine (Romania)

Active subsidence followed by collapse of shallow geological structures occur in areas with old abandoned mining works in salt bodies, due to the dissolution processes developed in salt by fresh ground water.

The active subsidence started beneath a new building in the salt mine Ocna-Dej area was not possible to be stopped using geotechnical and hydrogeological techniques. Detailed resistivity and gravity investigations were employed to detect the underground cause of the building subsidence.

VES data succeeded to illustrate the location of large underground voids in the salt body along faults or in places where the exploitation chambers roof or pillars were damaged by dissolution processes. The gravity data were interpreted in the new frame offered by detailed resistivity imaging, contributing to the evaluation of collapse hazard in the Ocna Dej salt mine area.

The interpretation of the gravity and resistivity data showed that the highest hazard for collapse in the Ocna Dej salt mine is in areas where the walls separating the bell-shaped and rectangular exploitation chambers were already dissoluted, resulting larger underground voids, and along faults crossing the salt body on its entire vertical development, with very active dissolution processes.