Three-dimensional numerical modelling of geo-electromagnetic induction phenomena
Date
2018-07-11
Authors
Pu, Xing-Hua
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Abstract
A finite difference algorithm for solving the forward modelling problem of geo-electromagnetic induction in three-dimensional structures has been developed in this thesis. Novel features of the method include the incorporation of a thin sheet of anomalous conductance at the surface of an otherwise quite general three-dimensional structure in which the anomalous region is allowed to approach two-dimensional configurations at infinity; the use of magnetic rather than the electric field components for obtaining the solution; the use of integral boundary conditions at the top and bottom of the model; and the application of new cell-integral finite difference equations to the main body of the model.
The algorithm has been tested for synthetic models against results delivered by existing two and three dimensional modelling programs which are already well established. The results are found to be very satisfactory. Applications of the algorithm have been shown for two cases. First, the dependence of the induction vectors on the period ranging from 10 to 10000 s has been studied for a model with two perpendicular lateral conductivity contrasts; the directions of induction vectors vary from site to site reflecting the combined effect of the two perpendicular contrasts. In the second case, the distortion effect due to small surface inhomogeneities over a buried 2D anomaly was studied using induction vectors and difference vectors. There is evidence of mutual coupling in a certain region which invalidates a simple subtraction of the vectors to reveal the form of the buried anomaly, but elsewhere the procedure appears to be quite valid.
Since surface anomalies can be simulated by an anomalous thin sheet over the general 3D structure, it is suggested that this algorithm could be very useful for testing the validity of existing schemes for impedance tensor decompositions used in MT studies when surface anomalies are thought to be distorting the real data.
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Keywords
Electromagnetic induction, Electromagnetism, Geomagnetism