Spectral element method for very-low-frequency normal mode modeling in layered fluid-solid ocean waveguides

Abstract A Spectral Element Method for Normal Modes is introduced for obtaining the underwater multicomponent acoustic field and seismic wavefield solutions for sound propagation at very low frequencies in layered fluid-solid ocean waveguides. Using fluid acoustic pressure and elastic displacement as basic variables, the method discretizes the modal equation in the depth direction and transforms the dispersion equation into a linear eigenvalue problem, directly yielding the full-depth modal functions and the multicomponent wavefields in both the water and the seabed. The proposed method computes modes that agree well with KRAKENC and ORCA. The corresponding wavefields, including the seismic field in the layered elastic seabed, match both KRAKENC and OASES to within 1 dB in the far field. Only near-field deviations from OASES remain, mainly because leaky modes are ignored. Time-domain waveform comparisons further confirm the effectiveness of the method and demonstrate its ability to capture Scholte wave propagation near the seabed interface. In addition to the waves field, the method provides the elastic normal modes, which offer physical insight into the layered elastic seabed. The method is suitable for studying very low frequency multicomponent acoustic and seismic wavefields near the seabed.