Simulation of acoustic scattering from an aluminum cylinder near a rough interface using the elastodynamic finite integration technique

We present calculations of acoustic scattering from an aluminum cylinder near a rough interface computed using the elastodynamic finite integration technique (EFIT): a time-domain numerical method useful for pulse propagation in inhomogeneous fluid-elastic environments. These calculations are relevant to the modeling of underwater acoustic scattering by objects near the ocean seafloor in the low-frequency structural-acoustics regime where penetrability of both the object and seafloor are important. The generality of the EFIT allows for the inclusion of stratified seafloors with rough interfaces and volume inhomogeneities such as shells or rocks. Non-reflecting computational boundaries are implemented using a recursive convolution time-domain form of the perfectly matched layer (PML). The scheme and examples discussed are in two space dimensions for computational simplicity. The explicitness of the scheme (unknowns only depend on spatially local values at previous time steps), however, allows for straightforward parallelization by decomposing the domain which is efficient for three-dimensional problems. We first examine the relationship between source geometry and bottom penetration for grazing angles below the critical angle for a fluid-fluid interface similar to a water-sand interface in the ocean. Ensemble averaged bottom penetration is then computed for a statistically rough power-law interface, and comparison is made with the flat-interface case. The aluminum cylinder is then introduced at variable height relative to the fluid-fluid interface, and backscattering is computed for both sub and supercritical incidence angles. Separation of interface reverberation and cylinder echo contributions to the total backscatter is made to demonstrate the importance of roughness. The EFIT is demonstrated to effectively capture the enhancement of bottom penetration and object backscatter for subcritical incidence angles for a buried object under a rough interface. We also consider an example of scattering in the presence of a rough interface and small randomly distributed subsurface inhomogeneities to demonstrate how different environmental factors can influence an echo from an object.