Discrimination of ULF signals from an underground seismogenic current

Abstract

AbstractA numerical model has been elaborated to calculate ULF electromagnetic fields in the ground-atmosphere–ionosphere system created by an underground horizontal current source of a finite length. The modeling has enabled us to examine in detail characteristic features of ULF response to an underground large-scale emitter that may be used for a search of electromagnetic earthquake precursors. The most promising features that might discriminate signals from an underground source and from a magnetosphere-ionosphere source are (a) the apparent impedance of the electromagnetic field derived from simultaneous observations of horizontal magnetic and electric fields, and (b) the ratio of vertical and horizontal magnetic component amplitudes. Besides that, the amplitude-phase gradients of signals from an underground source differ significantly from those of the magnetospheric source. For the same magnitude of horizontal magnetic disturbance on surface, an underground current source produces in a borehole a much larger vertical electric field Ez than a magnetospheric source does. At the same time, some properties, such as the ratio between the vertical and horizontal electric components, are shown to be ineffective. However, all these differences with ionosphere-magnetosphere source reveal themselves only in a vicinity of lithospheric source (< 30 km for depth 20 km). Graphical Abstract