The paper considers methodological issues of solving the inverse problem of electromagnetic sound, based on the results of measuring the low-frequency magnetic field from two orthogonal antennas located on the Kola Peninsula in areas with different conductivity of the lithosphere. The polarization characteristics of the tangential magnetic field components are determined. They are compared with theoretical calculations using a plane-layered model of a wave propagation medium with homogeneous layers. Comparison of the theoretical calculations with the experimental data on the major axis of the polarization ellipse yields results consistent with existing concepts of the conductivity of the Kola Peninsula. Differences in the direction of the ellipse and ellipticity require complication in the model: the inclusion of faults in the lithosphere, as well as the anisotropy of the structures forming it.

Based on the experimental studies on measuring the controlled source signals in the near zone under different geophysical conditions, it is established that the amplitude of the field experiences variations in the lower part of the ELF band and at lower frequencies. At the same time, variations in the VLF range are absent. For identifying the factors responsible for this peculiarity in the behavior of the field, excitation of the ELF and lower frequency electromagnetic field in the Earth-ionosphere waveguide with different conductivities of the Earth and the ionosphere is considered. The theoretical calculations are proposed showing that at low conductivity of the Earth, the effect of the ionosphere in the near zone can be significant.

AbstractPolarization characteristics of the field of an on-Earth emitter located at the Kola Peninsula are experimentally measured at a distance that is no greater than the height of an effective ionospheric waveguide in the FENICS-2014 experiment. Variations in the field amplitude and orientation of the major axis of polarization ellipse are observed at lower frequencies upon significant changes of the K index of geomagnetic activity. Polarization characteristics of the horizontal component of magnetic field calculated with allowance for the ionosphere and two-layer Earth structure prove the observed sensitivity of the ultralow- and lower-frequency filed in the near-field zone to the state of ionosphere at lower conductivity of underlying medium. Theoretical results are compared with the experimental data. The results are important for deep sounding of the Earth and monitoring of ionosphere with the aid of controlled low-frequency ground sources.

Polarization characteristics of the field of an on-Earth emitter located at the Kola Peninsula are experimentally measured at a distance that is no greater than the height of an effective ionospheric waveguide in the FENICS-2014 experiment. Variations in the field amplitude and orientation of the major axis of polarization ellipse are observed at lower frequencies upon significant changes of the K index of geomagnetic activity. Polarization characteristics of the horizontal component of magnetic field calculated with allowance for the ionosphere and two-layer Earth structure prove the observed sensitivity of the ultralow- and lower-frequency filed in the near-field zone to the state of ionosphere at lower conductivity of underlying medium. Theoretical results are compared with the experimental data. The results are important for deep sounding of the Earth and monitoring of ionosphere with the aid of controlled low-frequency ground sources.

Excitation of electromagnetic fields at extremely low and lower frequencies in the near-field zone of the Earth–ionosphere waveguide is considered. Variations in the field amplitude in the lower part of the extremely low frequency (ELF) range and lower frequencies are experimentally determined under different geophysical conditions in the absence of variations in the ultralow frequency (ULF) range. The effects related to such variations are analyzed with the aid of theoretical calculations that show significant effect of ionosphere in the near-field zone at relatively low terrestrial conduction.

Влияние ионосферы на возбуждение электромагнитного поля диапазона КНЧ и более низких частот в ближней зоне© Е.Д

We study the super-low-frequency field of a grounded horizontal electric dipole on a high-latitude submeridional path having a length of up to 1000 km. Unlike most of the earlier works, this paper considers the case of wave propagation along the dipole axis, where the main component of the magnetic field is radial. The frequency dispersion of the ground conductivity under the source, which determines the coefficient of excitation of the Earth-ionosphere waveguide, is found from the measurements of the field at a distance of three heights of the ionosphere. Typical parameters of propagation of super-low-frequency waves are refined allowing for specific features of the path used in the experiment. The significance of allowing for ground conductivity as the radiation propagates along low-conductivity regions is demonstrated. The regions where one of the magnetic-field components is predominant are calculated in a polar coordinate system linked to the source, as well as the regions where the both components should be allowed for. The theoretical possibility of determining the phase velocity of the wave based on the ratios of the magnetic-field components measured at longer lengths is noted.

Propagation of the artificial electromagnetic waves with frequency of 82 Hz in the Earth-ionosphere waveguide was observed during the solar eclipse on both partially and totally obscured high-latitude paths with a length of 450-1200 km. Field excitation was monitored by the reference measurements in the near zone of the transmitter, which are free of the ionospheric influence. It is found that the amplitude of the field at the remote points varied depending on the solar illumination and solar elevation angle. We suppose that this effect was probably caused by the increase in the effective height of the ionospheric D layer, just as it was previously observed in VLF. The obtained results demonstrate the response of the propagating extremely low frequency (ELF, 3-300 Hz) wave to the change in the ionospheric boundary. This effect has been for the first time observed in this frequency range during a total solar eclipse.

The results of the experiment on studying the dynamics of the electromagnetic field (EM) generated by the stationary controlled ULF-band source during 30 days on the Baltic crystalline shield are presented. Diurnal variations in the EM fields and slow variations in the surface impedance with a period of about 14 days are revealed. The diurnal variations in the fields are mainly due to the fluctuations in the ionospheric parameters caused by the changes in the daytime ionization of the ionosphere by solar radiation. By comparing the harmonic component with a period of about 14 days, which was established in the time series of surface impedance, with the slow tidal deformations of the Earth’s crust, we revealed the correlation between the EM variations and tidal processes in the Earth. The estimates for the probable changes induced by tidal deformations in the structure and conductivity of the underlying medium are obtained by modeling.

Описан эксперимент по измерению фаз трех компонент напряженности магнитного поля мощного контролируемого источника излучения сверхнизкочастотного диапазона вдоль двух ортогональных направлений на разломной тектонике. В ходе эксперимента обнаружены резкие изменения фазы величиной около 180° между некоторыми точками измерения. Установлены их связь с токопроводящими геологическими разломами и, соответственно, возможность локализации таких разломов.

The experiment on phase measurements of three components of a ULF magnetic field generated by a high-power controlled source in the region dominated by fault tectonics is described. The measurements were carried out along two orthogonal directions. It is established that the phase sharply varies by about 180° between some measurement points. The phase jumps are found to be confined to the conductive geological faults, which opens the possibility of locating the faults using the phases of ELF magnetic fields.

The results of the experiment to study the dynamics of the electromagnetic field of artificial source ELF-range during period of 30 days are presented. Variations of the field with periods of several hours to several days are found. Observations during the sidereal period of the Moon show that field variations are inphase with the tidal deformations of the Earth's crust along a vertical axis. These variations have considerably higher amplitude than daily variations and may be associated with large-scale tidal processes occurring in the Earth's crust. Field variations with periods of less than a day are most likely caused by ionospheric processes.

The amplitudes of variations in the magnetic and electric fields at extremely low frequencies were studied on the Kola Peninsula during a five-day-long experiment under different geophysical conditions. These studies demonstrated that the influence of the ionosphere is distinguishable at frequencies of <10 Hz and the structure of field variations reflected by the ionosphere is similar to that of the transverse wave. It is established that the ratio between semimajor axes of polarization ellipses of the electric and magnetic fields is independent of the state of the ionosphere and is likely determined by the deep structure of the crust beneath the site of the experiment.

The results of measurements of the artificial electromagnetic signal in the range 0.1–10 Hz on the Spitsbergen archipelago are presented in this paper. The signal was emitted by a horizontal antenna located on the Kola Peninsula. Resonance spectra structures (RSSs) caused by the influence of the ionospheric Alfvén resonator (IAR) were found in the artificial signal. The calculations of the field performed using the method of a two-dimensional telegraph equation for an anisotropic inhomogeneous model of the ionosphere allowed us to explain the main properties of the observed artificial RSS signal. Special attention was focused on the choice of the ionospheric model and calculation of the reflection coefficient for a plane wave normally incident on the ionosphere. The needed profiles of the ionospheric parameters were selected on the basis of the Upper Atmosphere Model (UAM) and IRI2007 model, while the atmospheric parameters were specified on the basis of MSIS-E-90. The advantages of the UAM model compared with the IRI for the description of the experimental results in a high-latitude region are shown in the paper.

The results of the first experimental reception of electromagnetic signals, emitted by the ground antenna at frequencies of 1–10 Hz in the transition zone in the case when the distance from the transmitter is comparable with the equivalent waveguide height, are presented. The works were performed episodically in 2006, in different seasons and at different time of day, under quiet geomagnetic conditions. A pronounced effect of the state of the ionosphere on the received signal value was found out at distances about 80 km from the transmitter; in this case the seasonal manifestations were more substantial than the daily ones. The obtained results indicate that it is necessary to take into account the effect of the ionosphere on the wave amplitude in the transition zone, when the electromagnetic sounding of the Earth’s structure is performed at frequencies of 1–10 Hz, and that it is reasonable to use artificial waves in this band to perform quasi-vertical sounding of the ionosphere.

The experimental measurements show that the state of the ionosphere pronouncedly affects the field of the extremely low frequency (ELF) band in the transition zone. The measurements were performed on the Kola Peninsula at different times of day and in different seasons of 2006. The observed seasonal variations in the results are larger than the diurnal variations. The measured field strength does not tend to the calculated statistical value with decreasing frequency if the effect of the ionosphere is ignored. The possibility of using quasivertical radio sounding of the ionosphere in the ELF band to study the structure and electric parameters of the ionosphere is discussed.