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The objective of this work is to support SERDP in modeling and physical interpretation of acoustic response of munitions to sonar in realistic ocean environments, particularly those that would result from unexploded ordnance (UXO) Remediation Systems deploying acoustic sensors. This effort will focus primarily on the Multi-Sensor Towbody (MuST) system but can easily be used for other systems.

In modeling the response of UXOs to sonar, the project team plans to use the Coupled Finite Element/Boundary Element (CFEBE) model, which is a combination of finite element method to model the vibrations of the target, and the boundary integral method to model the acoustic field in its surrounding medium. The coupling between the incident acoustic signal and the resulting structural vibrations of the target is treated self-consistently, which results in a numerically exact model that can account for all physical effects such as multiple scattering, shadowing, the presence of boundaries/interfaces and other targets. Even though the model is capable of computing scattering from UXOs in any environment, a two half-space environment composed of a water half space and a bottom half space can account for most of the physical effects seen in the measurements. This type of environment can most efficiently (albeit with simplifying assumptions) be modeled by Target-In-the-Environment-Response (TIER) model, developed by Applied Physics Laboratory-University of Washington (APL-UW) (S. Kargl). Currently, APL-UW utilizes a hybrid axially symmetric Finite Element/Propagation model to provide the free-field scattering amplitudes, and thus is limited to only axially symmetric targets. An important part of this project is to provide TIER free field scattering amplitudes for general 3-dimensional (3D) (non-axially symmetric) targets. Additionally, CFEBE will be used to validate the TIER model, help improve its performance and interpret physical effects that result from scattering from complex targets in complex environments, all of which require a high-fidelity model.

The most immediate benefit of this work is providing the ability to model the response of UXOs to sonar in a realistic ocean environment. This ability is crucial in predicting sonar performance, analyzing measurements and designing experiments. The most important features of the approach are: 1) The method is inherently 3D, meaning that it can compute scattering from arbitrarily complex targets in arbitrarily complex environments without any approximations. 2) The method provides an efficient way of computing the acoustic color since it requires a matrix inversion for each frequency, but not each angle. Most models must solve a full finite element problem for each frequency and each angle of incidence. 3) Since this method computes the target impedance matrix in vacuum, the same impedance matrix can be used in any environment, so changing the environment for the same target does not require a full finite element solution. 4) By projecting the impedance matrix onto the surface nodes, this method reduces a finite element problem to a boundary element problem with far fewer unknowns. This reduction in the number of unknowns enables the method to solve a 3D problem with ease. 5) Due to its modular nature, the method easily lends itself to parallel processing, including graphics processing unit processing. 6) It is capable of computing scattering from multiple targets in any environment.