The EURISGIC project (European Risk from Geomagnetically Induced Currents) aims at deriving statistics of geomagnetically induced currents (GIC) in the European high-voltage power grids. Such a continent-wide system of more than 1500 substations and transmission lines requires updates of the previous modelling, which has dealt with national grids in fairly small geographic areas. We present here how GIC modelling can be conveniently performed on a spherical surface with minor changes in the previous technique. We derive the exact formulation to calculate geovoltages on the surface of a sphere and show its practical approximation in a fast vectorised form. Using the model of the old Finnish power grid and a much larger prototype model of European high-voltage power grids, we validate the new technique by comparing it to the old one. We also compare model results to measured data in the following cases: geoelectric field at the Nagycenk observatory, Hungary; GIC at a Russian transformer; GIC along the Finnish natural gas pipeline. In all cases, the new method works reasonably well.

Statistics of geomagnetically induced currents (GIC) in the European high-voltage power grids based on 1-min geomagnetic recordings in 1996-2008 and on

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The term "null array" is introduced for those electrode configurations where the measured potential difference is zero above a homogeneous half-space when using a measuring dipole M 0 N 0. Different types of null arrays (three-electrode, Schlumberger, and dipole axial/equatorial null arrays) and their corresponding traditional arrays are studied. It was shown in a field study carried out in a karstified limestone area covered by thin sediments that it is possible to obtain geologically meaningful results with null-array techniques. The main features of the null-array data are as follows. (1) Nullarray data appear to be more spatially variable than the classical data. The spatial variability provides information about the presence of karstic fractures in the subsurface; (2) The null-array anomalies caused by nearly vertical karstic fractures in the limestone basement do not decay with depth as quickly as the classical array anomalies. (3) The strike direction of the fractures is much less ambiguous than that found by using classical arrays. Nevertheless, the depth variation of the basement is more reliably observed in geoelectric anomalies obtained using traditional arrays. Therefore a joint use of classical arrays and their corresponding null methods is recommended, because the combined methods provide more information about the subsurface structure.

An inversion algorithm for multistation Schumann resonance measurements is presented and tested in this paper. The location and intensity of the lightning activity is estimated in absolute unit (C2 km2/s) from the Schumann resonance electromagnetic field components measured at distant observation sites on the globe. We summarize the main steps of the forward modeling for single and multiple sources and describe the main characteristics of the applied inversion. The applicability of the inversion algorithm is tested via synthetic data.

SUMMARY While traditional geoelectric array configurations, such as the Wenner–Schlumberger or the dipole–dipole, can provide very good images of 1-D or robust 2-D structures, they are not sufficiently sensitive to those inhomogeneities that have a small effect on the surface electrical potential distribution. The detection and description of such inhomogeneities become possible by applying quasi-null arrays, which provide very small (close to zero) signals above a homogeneous half-space. The imaging properties of the members of an array series containing such arrays, the so-called γ11n arrays (n = 1–7), are demonstrated and compared to those of the most popular traditional arrays. Although the field applicability of the quasi-null arrays has been heavily questioned, it was demonstrated by our quasi-field analogue modelling experiments. The quasi-field tests also validated all of the numerical modelling results as follows: (1) many or all of the γ11n arrays were able to detect prisms and vertical sheets located at depths larger than those detectable by traditional geoelectric arrays, including the optimized Stummer configuration; (2) the horizontal resolution of the γ11n arrays proved to be better than the horizontal resolution of traditional arrays; (3) with n increasing, the γ11n arrays proved to be less sensitive to 1-D, but more sensitive to 2-D bodies. In case of high n values, the γ11n arrays may even be entirely insensitive to any 1-D structure. On the basis of the quasi-field experiments, γ11n arrays are expected to be very efficient to indicate bodies, or variations in time that only have a small impact on the surface electrical potential distribution (e.g. caves, mines, tunnels, tubes, cables, fractures, dykes), or small changes in the subsurface conditions (monitoring of dams or waste deposits). Data acquisition by both a traditional and a γ11n array, individual inversion of their data, and a joint interpretation of the results are recommended to obtain both a robust image and fine details of the subsurface.

[1] It is known that second-order magnetic phase transition, the transition between ferromagnetic (ferrimagnetic) and paramagnetic states of the material at the Curie temperature, is accompanied by a sharp (theoretically infinite) enhancement of the magnetic susceptibility. A second-order magnetic phase transition within the Earth (usually at mid-crustal depths, depending on geothermal conditions and on the type of magnetic material) is assumed to produce extremely high susceptibility zones of a thickness of a few hundreds of meters. Such strongly magnetized zones may be sources of well-known but not-yet explained geomagnetic anomalies, and at the same time, they may produce complicated electrical conductivity anomalies, as well. The second-order magnetic phase transition should be taken into account as one of the possible sources of geomagnetic and magnetotelluric anomalies.