ИСТИНА

A direction finding techniques for natural ultra low frequency (ULF) electromagnetic (EM) sources are of a special interest in searching of EM precursors of impending earthquakes. Moreover, knowledge of the source location of the ULF EM signals is of paramount importance when discussing possible sources mechanisms of these signals. In this study the modern concepts of the finding of ULF source signal location are analyzed and compared. The study is begun with traditional radiowave methods based on detection of lag time or phase difference between signals recorded at different points. These methods seems to be not effective in ULF region due to a strong dissipation and dispersion of EM signals in conducting layers of the Earth’s crust. In such a case the ULF-field propagates in the conducting rock according to diffusion law that causes both an expansion of the signal front and decrease of its magnitude. Next problem to be analyzed is how to distinguish a weak EM signal possibly related to the seismic activity from the background of natural ionospheric and magnetospheric noise and man-made interference. W network of ground-based three-component magnetometers may be of interest at this point in future geophysical experiments. The influence of large scale background noise can be thus eliminated by subtraction of the data obtained by different magnetometers located not far from each other. A technique based on the analysis of amplitudes and spatial derivatives of the magnetic perturbations is discussed. Comparing these derivatives with theoretical formulae makes it possible, in principle, to find the ULF source parameters such as the distance and amplitude. This method of attack seems to be more perspective despite this method has been developed only for the magnetic-dipole type source. In the remainder of this review the polarization ellipse (PE) technique is discussed. The magnetic-dipole vector of the source can, in certain situation, be at the PE plane. A special orientation of this vector is determined by the intersection line of two planes measured at different points. Predominant orientation of magnetic dipole is assumed to be indicative of tectonic activity in the focal zone. The field of application of the PE technique is discussed. From the review of the problem it may be deduced that a great deal needs to be done for solving the problem of direction finding for underground ULF EM source. It is necessary at this point to achieve understanding of the underlying source mechanisms of the ULF field.