Analysis On Three-Dimensional Geoelectric Field Induced In Complicated Conductivity Structure During Geomagnetic Storm By Finite Element Method

The three-dimensional (3D) conductivity structures of the Earth have crucial impacts on the induced geoelectric field during geomagnetic storms. To investigate the principle and law which govern the induced currents flowing in different conductivity structures and the corresponding impacts on the geoelectric field is the important prerequisite to calculate geomagnetically induced currents (GIC), to evaluate the effects of space weather on man-made conducting systems and to forecast disaster accurately. However, conductivity structures without lateral variations representing the Earth are still the most widely used models in geoelectric field research area. In this thesis, the mechanism of geoelectric fields affected by the complicated conductivity models during geomagnetic storms is analysed. The main contents and results achieved are as follows.Finite element method (FEM) is presented to be applied to calculate geoelectric field during geomagnetic storms, due to the disadvantages of analytical methods which can be only used for one-dimensional (1D) conductivity modelling. The geoelectric fields in two dimensions under the circumstance of 1D structure are calculated both by FEM and analytical methods for comparison. Several topics such as boundary conditions, element sizes, meshing strategies are discussed to evaluate the availability of FEM for modelling geoelectric fields.For the two-dimensional (2D) structure with conductivity varied along one horizontal direction and depth, geoelectric field is calculated by FEM under the assumption of infinite sheet current for source model. The proximity effect and skin effect due to the conductivity variations are shown. Source effect then can be taken into consideration for more realistic cases. It can be seen from the results that calculating geoelectric field in piecewise 1D layered structure by analytical methods will introduce large errors, while FEM can be applied easily.In three-dimensions, an approach to model complicated conductivity structure for 3D geoelectric field calculation is proposed, whose main feature is that only Earth region is needed to be modelled. The electromagnetic field in the Earth can be solved by applying geomagnetic field observed at the Earth’s surface as the boundary conditions. This approach can reduce the computation time and increase the efficiency. This approach is applied for study the 3D distribution of induced currents and their influences on geoelectric fields. The geovoltages are also obtained by integrating the geoelectric fields along specific paths.Based on the results mentioned above, two engineering cases are analysed. In the first case, magnetotelluric method (MT) results are applied for representing the Earth conductivity structures in Guangdong province. The geomagnetic field data from Guangzhou observatory is set as boundary conditions and 3D geoelectric field is calculated by FEM. Then geovoltages between several substations are obtained. Similarly, in the second case, conductivity structure in North China is fulfilled by MT results. Geoelectric field are calculated where the geomagnetic field are observed from Jiayuguan observatory, and geovoltages between different planned substations are analysed combined with conductivity distribution. These results can be used from the perspective of disaster prevention or geomagnetic storms forecasting. Recommendations on substation planning and site selection are proposed.