Superdeep Sounding of the Lithosphere in the Baltic Shield Using Industrial Electric Power Lines

摘要

In this communication, we describe the method for, and present the first results of, the superdeep electromagnetic sounding of the lithosphere using industrial electric power lines (EPL) at a distance of up to 504 km from a source. The method is based on the application of two mutually orthogonal EPLs (109 and 89 km long) and an alternating current generator (up to 100 kW) within a frequency range of 0.325-166.7 Hz. This method combined with magnetotelluric sounding (MTS) allowed us to study the electroconductivity of the lithosphere at a depth of 10n to 100-200 km. The urgency of this work is dictated by the debatable character of the problem concerning the deep electroconductivity of the lithosphere. Many researchers have pointed out the general increase of longitudinal conductivity to tens and hundreds of Siemens at a depth of 10 to 30-40 km due to the presence of fluids. At the same time, petrological data suggest that water does not conduct electricity under conditions of steadily cooling shields and platforms, because it is incorporated in lattices of minerals of deep-seated mafic and ultramafic rocks. In this connection, researchers have questioned the occurrence of free fluids at depths, as well as the extent to which the magnetotelluric data adequately reflect the electrical properties of the subsurface zone. It should be added that the presence of free fluids in the middle and lower crust is limited by a rapid increase in lithostatic pressure that leads rocks to the plastic state and squeezes out the liquid phase toward the surface.