Joint propagation characteristics of acoustic and electromagnetic waves in shallow ocean

Abstract

Most underwater communication and sensing systems use acoustic waves exclusively because of their low loss propagation characteristics. In this work, we aim at joint processing of sensor array data for acoustic and electromagnetic/optical waves for estimating constitutive parameters of a layered shallow ocean model. Key acoustic propagation media characteristics are sound speed and density profiles, whereas permittivity and conductivity are key characteristics for electromagnetic propagation. Many approximations to the modelling of wave propagation have been used: analytical (e.g. Ray Methods, Normal Modes) and numerical (Finite Elements, Finite Differences) [1]. We use the finite element method to compute the sound pressure field in response to an isotropic point source emitting a sinusoidal spherical wave in a shallow ocean with sound speed c = 1500m/s, water depth d = 130m at a frequency of f = 170Hz. Initial results for acoustic propagation are obtained with a commercially available three-dimensional finite element solver (Iterative Solver Shifted Laplace) using the Vienna Scientific Cluster (VSC) as numerical platform. This method is computationally expensive, therefore the calculation at the VSC. The finite element solver is used for finite volume calculations; a problem with the ocean. Through boundary conditions, such as the Perfectly Matched Layer (PML), acoustic propagation calculations and approximations are nevertheless achievable. The PML setup allows one or two modes with a specific wave number to leave the modeling domain with minimal reflections. We specify the size of the modeling domain to be 130 m in length times 130 m in width times 130 m in height and the maximum tolerable element size λ/5 ≈ 1.8 m. The complete mesh consists of 6991923 domain elements, 76460 boundary elements, and 900 edge elements. The boundary conditions for this model: The sea sur- face is simulated by the Dirlichlet boundary, which satisfies p = 0. The seabed is simulated by the Neumann boundary, which satisfies dp/dn = 0. We deal with the open boundaries, with the PML ap- proach [2]. The numerical result for the sound pressure field is visualised in Fig. 1 and compared to the normal mode approximation.