Numerical Modelling of Underwater Electromagnetic Propagation

This paper considers the application of numerical techniques to modelling large 3D Extremely Low Frequency (ELF) electromagnetic problems in the time and frequency domains. The environment model consists of three material layers: air, sea and seabed. To represent a coastal region, the sea region is shallow with a variable depth and includes a sloping seabed. The electromagnetic source is a harmonic, horizontally polarized plane wave at a frequency of 100 Hz. The model is solved using several numerical methods for ease of validation. This is because no analytical solution exists and experimental methods are expensive. The numerical techniques used were a time domain Alternating Direction Implicit Finite Element Method (ADI FEM), a commercial Transmission Line Modelling (TLM) package and a commercial frequency domain Finite Element Method (FEM) package. All three methods produced solutions with a high degree of agreement. It was found that the TLM method produces good results with an extremely coarse mesh and generally it is easier to get a reliable solution for this type of problem using TLM compared with FEM. The main advantage of using ADI FEM was that problems with more elements could be solved. Key-Words: Electromagnetics TLM FEM Underwater Propagation Scattering