Fiftieth Anniversary of the Kola SG-3 Superdeep Borehole

The results of a comprehensive study of electrical properties of the upper part of the Earth's crust according to the data of lateral resistivity logging of the Kola SG-3 superdeep borehole and the results of deep and superdeep sounding using controlled sources and electrical profiling on the daylight surface are presented. The possibility of extrapolation of the Kola SG-3 temperature field to a depth below the bottomhole, up to the Moho discontinuity, taking into account the data MHD sounding with the Khibiny source and with the use of industrial power transmission lines (FENICS experiment) is considered. The results of frequency electromagnetic soundings with controlled sources are analyzed, and their geodynamic interpretation is performed together with the data of drilling the SG-3 borehole. The geological and geophysical data are interpreted in the Kola SG-3 section and on the daylight surface to search for homologs and define the nature of the intermediate conductive layer of the dilatancy-diffusion nature (DD layer) discovered in the Earth's crust of the Baltic Shield at depths from 2-3 to 7-8 km. Based on the latest results of remote deep sounding (the Murman-2018 experiment), it is concluded that, in the vicinity of Kola SG-3 at a depth of 10-12 km, there is a boundary of "impenetrability" for direct current (for the galvanic mode). The geodynamic interpretation of the data is carried out taking into account the model by V.N. Nikolaevskii (1996); it has been suggested that the new boundary most closely corresponds to the brittle-to-quasi-plastic transition area of the Earth's crust. The results are compared to the log data of the SG-3 borehole and the data of superdeep drilling.