Electric Resistivity Tomography (ERT) Measurements in Area of an Old Salt Mine

A large number of old salt exploitations are presently abandoned in Romania as a consequence of mine flooding, determining in many cases active subsidence and collapse. The geophysical study carried out in 2012 at Ocna Dej salt mine had as main objective the detailed illustration by means of ERT and VES resistivity data the actual extent of underground voids. The ERT geophysical measurements were executed above the area of the old Iosif Mine, using a multi-electrode resistivity meter (SuperSting R1- AGI). Two ERT profiles 270m long (D1 and D2) have been measured using the Schlumberger, the Wenner-Schlumberger and the Wenner arrays. According to the resistivity distribution the vertical shaft sector was divided in 3 layers:a) a high resistivity layer due to recently added filling material; b)a very low resistivity zone represented by the upper part of an old filling material, impregnated with salty water originating from the salt lens roof and c) an intermediate resistivity layer at the bottom due to more compacted filling material. ERT method proved to be a powerful geophysical technique in locating and illustrating in detail underground voids in mined salt bodies, its higher resolution allowing interpretations for both geotechnical and hydrogeological use.