Determination of periodicities of low-frequency deformation processes in electromagnetic radiation variations in the Ural Superdeep well

Abstract

The electromagnetic radiation (also called EMR) from rocks is studied by a few methods. The EMR reg� istration method is widely used for studying physical- mechanical characteristics of rocks and rock breakage processes, for modeling of earthquake preparation and earthquake precursors (1), as well as for estimation of stress patterns in a rock massif (2). In the insitu rock massif, the estimation of the stressdeformation pat� tern of the rock massif and control over the process of rock breakage are conducted using the EMRbased data (3). These studies, conducted in underground mines, are used for the solution of tasks of engineering geology. There are results of the study of electromag� netic radiation during the preparation of a seismic event (4). Moreover, the EMR was considered in a study of the Earth's dynamics (5, 6). Study of the EMR in superdeep wells has still not been conducted. As is shown in this work, the electro� magnetic radiation in the wells is extremely sensitive to stress pattern variation and connected with zones of fractured rocks. This work represents the results of the EMR measurement in the bare hole of the Ural Super� deep well using equipment developed at the Institute of Geophysics of the Ural Branch, Russian Academy of Sciences. The amplitude-frequency characteristics of the EMR responses made it possible to determine the dominant periodicities (4-20 s). Electromagnetic radiations are associated with the formation of new discontinuities in the geological environment. The remarkable properties of these dis� continuities are as follows: unevenness, degree of roughness and fractalness determining the heteroge� neity of the microstructure and macrostructure of dif� ferent rock bodies, ranging from monocrystals to rocks. It is known that a fracture does not extend reg� ularly, as a rule; it pulsates with the velocity, varying in magnitude and direction, especially during branching. Moreover, from the surface and the fracture tip, elastic waves propagate. As a result stress relaxation in the geological environment takes place. This process can take place in any regime of the growth of fractures. However, the fracture can be stopped near a barrier in the form of some heterogeneity. The stresses in the fracture tip increase, and after the breakthrough the radiation impulses take place. Impulsive transforma� tions happen at a steep change of the fracture trajec� tory. The dynamics of fracture development is accom� panied by generation and relaxation of charges gener� ating electromagnetic impulse radiation. A fracture, as a mechanical transducer, is usually represented as an electric dipole with two different ori� entations: normal to the fracture plane and along the strike. The dipole oriented normal to the fracture plane can be formed by two mechanisms: discharges between fractal surfaces with alternating mosaic distri�