Efficient Forward Modelling of Electromagnetic Surface Impedance for Coal Seam Assessment

Abstract

The two-dimensional impedance method is used to calculate the electromagnetic surface impedance above subsurface structures at very low frequencies (VLF). The method was derived from Faraday?s and Ampere?s Laws and results in a single matrix equation where the right hand side corresponds to the source field introduced into the model as a fixed magnetic value. The left hand side corresponds to the impedance matrix determined by discretizing the solution space into pixels bounded by lumped impedance elements with values determined by the electromagnetic properties of the local media. Due to matrix sparsity and very large matrix dimensions, an iterative solver with a preconditioning technique was used to improve the speed, size and convergence of the solution. The improved method has been applied to the analysis of a coal seam with various structural anomalies and line of oxidation (LOX) along a line defined by 500 m with 0.5 m resolution. This paper reports a number of likely coal-seam scenarios relevant to surface mining operations.