Methods of active geophysical monitoring with the use of powerful seismic vibrators play an important role in the investigation of changes in the stressed-deformed state of a medium in seismic prone zones for problems of the seismic hazard prediction. During the last three decades, this scientific field has been actively developed at institutes of Siberian Branch of the Russian Academy of Sciences. In this period, experimental systems for the active monitoring of a medium have been made up. They include powerful (vibrational) sources with a vibrational force of 100 tons in the frequency range from 5 to 15 Hz, computer control systems, mobile specialized complexes for the precise recording of vibrational seismic signals, such as VIRS-M, VIRS-K, and ROSA, and computerized data processing systems.
A method for the active monitoring of a medium with the use of wide band sweep signals and narrow band harmonic signals radiated by seismic vibrators has been developed. In order to determine the sensitivity of the active monitoring system, some experiments to detect the influence of the Earth's crust tidal deformations (of order 10−7) on seismic wave velocities have been performed. A 100-ton seismic vibrator and recording systems were located at a distance of 356 km. The radiation sessions of harmonic and sweep signals were repeated every three hours during four days. This made it possible to construct time series of variations of the signal amplitudes and phases and wave arrival times. Both 12-hour and 24-hour periodicities correlated with the Earth's tides were detected in the spectrum of variations of the recorded signals. The experiment has shown that the active monitoring system makes it possible to detect relative variations of the seismic wave velocities of order 10−5-10−6 in an area of 300-400 km around the source. This allows direct monitoring of the state of stresses in an area of 100 thousand km2 to detect regions and phases of the critical stress as an earthquake precursor.
In recent years, the active monitoring of lake Baikal region has been started. It uses vibroseismic interferometry method, based on the seismic sounding of the region by powerful seismic vibrators with a long-time radiation of narrow band harmonic signals. Changes in the stressed-deformed state are determined through variations of the amplitude-phase characteristics of stationary wave fields, which are excited in the medium due to a long-time radiation of harmonic signals of constant frequency from the vibrator. The method of vibroseismic interferometry has a high sensitivity to the time changes of the parameters of the medium in the case of large-scale observations. A peculiarity of the experiments in Baikal region is the simultaneous use of data from regional seismic stations and from mobile recording complexes.