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