Based on the conventional finite-difference time-domain (FDTD) method, a novel dual-meshed technique is presented to deal with the underwater detection problems applying in low frequency electromagnetic wave. A transformation surface connecting the coarse cell with the fine cell is implemented by applying a total-field scattered-field source (TSS) technique, which is carried out by two-step FDTD simulation. The ratio of a coarse cell size to a fine cell size can be set as an arbitrary integer, such as N=10. Moreover, it is illustrates that non-physical reflection fields from the TSS surface are avoided by introducing the TSS surface. We have derived, in detail, the update equations of fields on grids of the TSS surface. Three cases of dealing with different underwater electromagnetic problems are discussed. Numerical results show that by analyzing the magnitude and phase of scattered fields from obstacles underwater we can distinguish the category of the obstacles which belongs to either a high resistivity body or a low resistivity body. Therefore, the proposed method provides us an effective tool for analyzing the electromagnetic response of materials underwater.
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