Ultra-Low Frequency Electromagnetic Wave Analysis in Ocean-Ionosphere Based on ADI-FDTD Combined with SIBC

In this communication, the alternating-direction implicit finite-difference time-domain (ADI-FDTD) method to efficiently simulate ultra-low frequency (ULF) electromagnetic wave propagation in complex marine environments for underwater target detection. The modeled domain encompasses the Earth-ionosphere waveguide, seafloor, and crustal layers (sediments, volcanic rock, hydrothermal deposits, and polymetallic nodules), with non-uniform ocean conductivity accounting for salinity effects. To analyze ULF wave attenuation in the ocean-ionosphere waveguide, we propose an ADI-FDTD method enhanced by a surface impedance boundary condition (SIBC). This approach reduces computational costs while preserving accuracy in modeling crustal and seafloor interactions. The iterative formulation of the SIBC-enhanced ADI-FDTD algorithm is derived and deviates from the traditional tridiagonal matrix form. The Fourier method is employed to evaluate and validate the stability of the proposed algorithm. Various scenarios were examined and characterized, revealing that the proposed method improves computational efficiency compared to the traditional FDTD method, without compromising accuracy in simulating ULF phenomena and large-scale models. Furthermore, GPU parallel computing schemes are employed to further enhance computational efficiency.